Friday, 16 December 2011

Electronic First Curtain

One of the nifty features of the A77 (and other cameras like the NEX-5n, A65 and NEX-7) is the option of using Electronic First Curtain (EFC) shutter mode. This eliminates the need to use a mechanical shutter to cover the sensor prior to exposure. In a live view camera, usually the sensor needs to be covered so that the pixels can be reset to zero before the exposure. EFC can accomplish this without requiring the sensor be covered. This has been available in some previous consumer DSLRs but the A77 is the first to really capitalise on its advantages due to the SLT design.

To see how EFC works, we should first understand how a normal shutter works. The purpose of the shutter is to allow light to fall onto the sensor for a specified period of time. In a traditional (non-liveview) camera, the sensor starts off covered by the first curtain, typically a set of interleaved metal blades. As the sensor is not exposed to light in this mode, it is a simple matter to reset it to accept the exposure and record the image. At the beginning of the exposure, the first curtain begins to slide down, exposing one edge of the sensor. As it uncovers the sensor, light falls on the pixels and creates charge. The edge of the first curtain continues to draw across the sensor until the whole sensor is uncovered. After the first exposed pixels have received the proper exposure, the second curtain now starts to move in the same direction as the first curtain to cover the sensor. The charges accumulated in each of the pixels are readout and converted into a digital image. Both first and second curtain then return to their original starting positions keeping the sensor covered, ready for another exposure to be taken.

For even exposure, the first and second curtains should traverse the sensor at the same speed. The delay between the motion of the first and second curtain determines the exposure time. On a professional camera, it take roughly 1/250th of second for the curtains to travel across the height of the sensor. This is called the flash-sync speed as this is the fastest shutter speed you can use for even flash exposure. The whole sensor must be uncovered at the point at which the flash is fired. At higher shutter speeds than flash sync, the region of the sensor which is exposed at any one moment is a slit between the edges of the first and second curtains. Faster sync speeds are difficult as this would require accelerating and decelerating the mechanical blades of the curtains beyond what is easily accomplished given the engineering constraints. Here's a link to a video explaining this.

In a liveview camera, the sensor starts off exposed to light. When the shutter is depressed, normally the first curtain rises from its stowed position to cover the sensor, the sensor is then reset (all the charges in the pixels are drained), and then the sequence as described above is triggered except that at the end, only the second curtain retracts so that the sensor is again exposed for liveview.

EFC eliminates the need for the first curtain to rise to cover the sensor before pixel reset. The pixels are design so that they can be reset without having to be in darkness. When the shutter is depressed and exposure initiated, rows of pixels of the sensor are reset beginning from the top. This line of reset pixels travels down the height of the sensor, just as a first curtain traversing the sensor reveals successive rows. As soon as a pixel is reset, it starts accumulating photocharge. After the proper exposure time, the second (mechanical) curtain then begins its journey across the sensor to shut off light and the image can be read out.The second curtain now returns to the top exposing the sensor for liveview.

The key point is that the start of exposure for any pixel can be electronically and independently controlled, and only the end of the exposure is controlled by a mechanically moving shutter blade. There are particular advantages to this, especially in a pellicle mirror (SLT) camera. Eliminating the moving mirror improves sharpness by avoiding mirror slap, but the first curtain would then be the dominant source of vibration. EFC eliminates this source of unsharpness as well. The sound of the shutter is quieter as there are less moving parts. And the time between the shutter button being depressed and the start of exposure can be shortened as well.

However, there are further potential benefits which could be exploited in future models. Flash sync speed could be effectively halved. EFC could start row initialisation from both sides of the sensor simultaneously, meeting in the middle, at which the whole sensor is exposed and the flash could be triggered. Then both mechanical curtains could move so that they also meet at the middle. Since they have to only travel half as far, the time it takes to cover the sensor is also half leading to a shorter flash sync time. The exposures of the top and bottom halves would have to be matched quite accurately (making sure the traversal speeds were the same), and the overlap region in the centre of the frame would also have to be designed to give even exposure.

A more extensive use of EFC is to do per-pixel exposure control. If each pixel can be reset independently, then the time it accumulates photocharge can be also be controlled individually. Let x be the distance from the top edge of the sensor (where the second curtain starts to cover the sensor) and v be the second curtsin traversal velocity. If at time t_0 the second curtain starts to move, the pixel will be covered at time t_x=t_0 + x/v. To obtain an exposure time of t_e, the pixel needs to be reset at time t_x-t_e. This choice of exposure can be done independently for all pixels.

In a liveview camera, since the camera has a realtime high resolution exposure map of the scene, it can calculate suitable per-pixel exposure times in order to compress the dynamic range of a scene, for instance using a short exposure time to capture the sky and a longer exposure for areas in shadow. This will require specific pixel reset capabilities of the sensor, current ones may only be able to reset pixels on a row by row basis. However, even row by row exposure compensation could give you an in-camera neutral grad filter (albeit only for landscape orientation).

Saturday, 29 October 2011

A77 Does Street

I had my first proper outing with the A77 to test it as a street camera together with the 16-50mm/2.8 and 35mm/2. I'm still getting to grips with the A77, and the weather was pretty miserable today, lots of damp Glasgwegians. Lighting was gloomy and flat. I just wanted to test the responsiveness of the A77 in rapid and spontaneous street/candid shooting rather than testing noise performance, dynamic range or resolution. Those can come later when I shoot some landscapes and panoramas.

My general impressions (coming from the A700 and A580):
  • The weather sealing of the A77 and the 16-50mm came in handy, no worries about shooting in the rain. The 35mm/2 also withstood the odd shower or two.
  • The A77 autofocus is fast and accurate, especially with the 16-50mm. Much better than the A580 and at least as good as, if not better than the A700. I can quickly point the camera in the direction of the action and squeeze off shots and most of them will be in focus, even at f/2.8 or f/2. Zone AF was very handy, especially when shooting from the hip. It's a nice intermediate between spot AF and wide AF.
  • The AF finetune was invaluable in resurrecting the use of one of my 35mm/2 which horribly front-focusses on both the A700 and A580 (my other 35mm/2 RS if anything backfocuses slightly). This is my "beater" 35mm/2, old style lens which I picked up cheap ages ago. The 16-50mm seems to be very accurate out of the box.
  • I took most of the shots from the hip, no framing at all. The 16mm end of the zoom came in handy, though you have to get close to your subject to fill a reasonable portion of the frame.
  • The exposure is excellent, hardly any blown highlights, conversely not much underexposure due to the sky either. This is much better than the OVF metering from the A700 and A580. The LiveView mode of the A580 gets close. I don't miss the AE pattern switch on the A700.
  • I'll need to get faster SD cards. My Transcend 16GB Class 10 cards work fine, just that review and deletion could do with being faster. I'll be ordering some UHS-I 95MB/s cards soon I think.
  • For waist-level shooting, the A580 LCD hinge design may be better. I'll have to get used to the A77 mechanism. With the LCD in the usual outward facing position, it's more fiddly to get it into a position for waist-level shooting. You have to lift it out and up over the viewfinder cup and flash shoe. It's easier to have the LCD in the closed position (facing the body) and then flip it down for waist-level shooting instead. I do like the ability to protect the LCD by having it face inwards. It also reduces the temptation to "chimp" emoticon - smile.
  • I generally did not find the EVF switching time a problem. It was fast enough for the times I did use it to frame. Priming the camera with a shutter half press as you bring the camera up helps.
  • The large 100% view of the EVF was much better in use than the A580 OVF for exact framing. I'll have to shoot back to back with the A700 to make a definitive conclusion but I suspect the A77 EVF will also prove to be better for composition as well.
  • The EVF brightness is still behind that of the OVF in daylight. It's still perfectly usable though. As light levels fall, the EVF gain makes it easier to see the scene and compose the frame.
  • The shutter is so much quieter than both the A700 and A580. It is so much less noticeable, even at close range. In contrast I could hear my shooting partner's K20 shutter from several metres away even on a busy street. Coupled with the quick AF, it was very easy to take several quick shots even as people passed right by without them noticing.
  • The controls are just what I am used to with the A700. I missed the second wheel on the A580, mainly to set exposure compensation. It makes a welcome return on the A77, however ironically I didn't have to touch it once since the exposure on the A77 is so good.
  • Having the buttons mostly on the right hand side of the body actually makes sense to me as I don't have to take my left hand away from supporting the lens. It's easier to use my right thumb to press the display, play, and delete buttons than it was to have to switch left hand grip. With a big lens like a 70-200mm/2.8 zoom, not having to change left hand position will be a boon.
  • The grip feels solid and comfortable. I have small to medium hands. The A77 was perfectly usable one handed with the 16-50mm attached, great for candid shooting where you want to look as if you are taking photos.
  • The shutter press on A700 to my fingers might just have the edge on the A77. The A700 shutter has slightly more distinction between a half and full press I feel.
  • I'm getting used to the layout. Last night on a short test run, I accidentally activated the movie button several times. Today, I managed to avoid this at all. It would ocassionally happen with the A580 but the added controls on the A77 makes it more likely to hit a wrong button when getting used to it.
  • I hardly ever looked at the top LCD. Perhaps it's because I'm so used to the A700 and A580 but I don't think it makes that much of a difference. Maybe I'll make more use of it when I get used to it
YMMV of course. The A77 has addressed most of the shortcomings of the A580 compared with the A700, mainly in terms of controls and AF responsiveness. There are issues which Sony should aim to address in firmware updates or the next generation:
  • Shutdown time is way too long. This is an issue when changing lenses and cards. The wake-up time from sleep seems to be quick enough that I have no worries setting the sleep timer for 10s to conserve power so this reduces the need to switch the camera on and off. I do hope they can reduce the shutdown time considerably.
  • Responsiveness could be improved. It's by no means unusable, but taking some of the remaining lag when changing settings would greatly improve the experience.
  • Iron out some remaining bugs. Last night, the camera started to refuse to AF unless I used DMF and approximately focused. I had to do a camera reset to get it back to normal.
  • Put an AF/MF indication in the finder like on the A700
  • Allow a setting for autoreview on the LCD only. In normal shooting, the EVF would be uninterupted but you could then check your shots on the LCD automatically. Bring back the zoom into the AF point in autoreview.
  • The ability to shut off the rear LCD LiveView by pressing the Disp button. Sometimes I'd like to turn it off quickly and shoot from the hip. Saves having to flip it around to face the body, or else dive into the menu and change the auto-switching from EVF to LCD.
  • Option to disable the Movie Button. Prevents accidental movies.
  • Add CDAF (at least for SAM/SSM lenses). This could be coupled with the AF finetune function for automatic adjustment of the PDAF system for different lenses.
  • A second card slot. The A580 has two. I find the second slot handy as a backup (even if it is MS).
  • I still miss the SteadyShot switch.
Street shooting with the A77 is fun and quite doable. It's not as inconspicuous as a small rangefinder to be sure, but since everyone seems to have a DSLR these days, it's not too difficult to fit in and take candids.

    Tuesday, 18 October 2011

    A77 First Thoughts

    I've just received my A77+16-50mm and have been playing around with it to get a feel for its handling and preliminary image quality. So far so good, the focussing seems to be reasonably accurate, especially under the terrible fluorescent lighting in my office which was a cause of considerable back-focus on the A580. I will have to test all my lenses and perhaps take a look at the AF Fine-Tuning.

    The 16-50mm lens is solid, fast focussing, and quite sharp. Comparing it with my well-worn Tamron 17-50mm shows that the Sony lens has better across the frame sharpness, possibly due to better control of curvature of field. It's the first SSM lens I've owned and I am quite impressed with how smoothly it focusses. Whether it is actually faster than the Tamron is something to test.

    The OLED XGA viewfinder is a delight to use. The resolution is such that I do not see any noticeable pixelation. I cannot say that the A580 viewfinder is better. The A77 has a larger, brighter, and much more useful viewfinder, especially in low light where the auto gain means you can actually see what you are taking. One can adjust all the settings on the camera without taking your eye away from it. The refresh rate seems to be pretty fast, even in indoor lighting. Seeing the previewed shot will doubtless be very useful when controlling exposure together with the histogram. I wear glasses and it is not problem to see the whole viewfinder image.

    The controls on the A77 remind me of what I missed from the A700 when I cross-graded to the A580. I was able to customise the wheels and buttons in the menu without having to open the manual. I've been able to set it up just as I did my A77. I still have to fine-tune the extra customisations, such as the cycling of the finder information, peaking sensitivity etc.

    The rear AF/MF button has made a comeback. This is so much more convenient to use than toggling the sliding switch on the A580. Together with the peaking function and the magnified view, using manual focus will be much quicker.

    The joystick is so much nicer to use than the pad on the A580, and similar to the one on the A700.

    The shutter sound is so much quieter than the A580. There is no thunk of the mirror. It is quieter than the A700 as well, though pitched similarly. The reduced shutter sound should help with taking photos in concerts.

    The body feels solid without being overly heavy. The size is similar to the A700 and A580 and the grip is well fitted to my hand. I welcome the return of the red AF illuminator

    Minor gripes

    The camera takes what seems to be an eternity to switch off. Hopefully a further firmware upgrade can reduce this.

    Being left-eyed, my nose gets a bit close to the joystick. This was the same with the A700.

    Lack of dedicated SSS button. I do miss this from the A700. Sometimes it is nice to be able to glance down to check whether SSS is one or off. Also, sometimes I will try shots with and without SSS if I'm not sure if the shot will benefit from it, so a switch certainly increases efficiency here. I also switch SSS on when taking a hand-held nadir shot. I've gotten used to the menu driven SSS control on the A580 so it's only a minor gripe but it would have been nice to see it return.

    Only one card slot. Even the A580 had two slots. It's useful having a spare card in the camera for emergencies. I use the second card in the A580 for taking storing non-critical photos which I don't want in my main archive. Saves having to sort out test shots.

    Things I'll get used to

    The button layout on the lower right hand side will take a bit of practice to get used to. The AF/MF  and AEL buttons are where I am used to them being on the A700. The joystick is similarly placed. However the DISP, Play, Fn and delete keys are situated differently so they will require some adjustment.

    Playback on the EVF. Instead of taking my eyes from the viewfinder, I am trying to review shots on the EVF. This also means I can keep the LCD turned into the body keeping it protected. I'd only bring it out for taking high/low/awkward shots where I need to hold the camera away from my eye.

    Features to try

    Peaking. This looks to be a great feature for MF, and touching up focus using DMF. It makes the STF 135mm very usable.

    AF Fine Tune. I've managed to get my 50mm/1.4 accurately focussed for the first time in 4 DSLRs (KM7D, A700, and A580 previously). I'll have to try it out with my tremendously front focussing 35mm/2.

    Object tracking. Could be good for sports.

    Face tracking. You can set the camera to recognise registered faces.

    Movies. Not really my cup of tea but I might as well see how this functions in case I need to do a spot of video.

    Preliminary Conclusion

    The A77 is a worthwhile successor to the A700. The pellicle PDAF mirror may be at an emotional level not completely satisfactory but in terms of implementation, the A77 has made good use in showing the advantages of the concept in giving a solid, responsive camera capable of serious photographic work.

    The sensor does deliver increased resolution over the A580, and with careful post-processing it should give comparable results in low light at the same output resolution. I'm not looking to blow up ISO6400 shots to A2, if they can print at A4 respectably then that's good enough for me.

    The handling of the body is great, certainly for someone coming from the A700. It's not too large or heavy yet feels solid enough to balance a 70-200mm/2.8 zoom. The return of the second control wheel, extra buttons and the joystick have made the A77 as much a joy to use as the A700.

    Sunday, 24 July 2011

    Ergonomic Comparison A700 vs A580: Lessons for the A77

    With the rumoured A77 gaining more solidity every day, there has been much discussion over its looks, especially the ergonomics of the button and switch layout. The A77 has to live up to some lofty expectations, it has to stand in the shoes of the Dynax 7D and the A700. The a700 was a clear evolution of the 7D layout, losing one knob, eliminating the dials, adding extra buttons, but still retaining the main button grouping for the right hand. If we now look at the purported images of the new A77, we see a further iteration of the control layout which consolidates the trends seen in Sony alpha cameras.

    Front of rumoured A77, credit Pyoshin at

    Rear of rumoured A77, credit Pyoshin at

    I have been lately been using the A700 and A580 together on shoots. The A700 is usually loaded with the Tamron 70-200mm/2.8 lens and the A580 with a shorter focal length prime or zoom. This means I can take a mix of wide/normal shots with tele shots without lens changing, especially important for covering events like weddings. Shooting in the way means that I have gotten to appreciate the ergonomic differences between the two cameras. The A77 images show that the layout has strong resemblance to both the A77 and A580 so this post will reflect on how the both work in practice. This is all my personal experience, it may not apply to anyone else so it is best to handle and work with any camera for a reasonable amount of time before declaring its worthiness.

    With the very noticeable absence of a replacement model for the A700, the advanced alpha APS-C shooter has been left in a bind. The choice is between stepping up to the A900/850 full format cameras, soldiering on with the A700 for as long as possible, or else find an alternative body to use. The A580 for me meets enough of the features for an A700 substitute to use. It is clearly not in the same ecological niche as the A700 but its mix of features, especially the performance of the 16MP sensor, is extensive enough to act as a proxy in many cases. The main differences are to do with the ergonomics, simplifying in some cases, but also extending it in others to accommodate new features such as live view and movies. I will concentrate on comparing the features of the A700 rather than any new functionality of the A580.

    The first thing to notice when picking up both cameras is the different on/off switch placement. The A700 has a slider switch on the top left back, the A580 a rotary switch surrounding the shutter release. The A700 placement is the same as for the Dynax 7D and the Dynax 7 film cameras. The A580 placement reflects that of all recent alpha models. After getting used to the A580, I have to admit that this position is quite handy in fact. The design of the switch doesn't get in the way of the shutter or the front control wheel. It's easy to switch on the camera in one motion when taking the camera out. Relocating this function to the right hand actually makes sense when carrying large/heavy lenses as you don't have to remove the left hand from supporting the lens to operate the camera. This basic design decision is one which is increasingly carried over in subsequent cameras.

    Next, the A700 has a dual control wheel setup. I have set the option for the front wheel to control aperture (I shoot in A mode 99% of the time) and the rear wheel to control exposure compensation. This works extremely well, I can quickly change exposure compensation without going into menus. The A580 lacks a rear wheel but instead has a +/- button which takes you into the exposure compensation menu. This is considerably slower and is far less satisfactory than the A700. Encouragingly the A77 images show a rear control wheel so hopefully it can configured the same way. With in-EVF histogram display, hopefully this will lead to far better and faster exposure control. The front control wheel on the A700 is better positioned, the A580 places it too far away from the shutter and at a more awkward angle, mostly due to the presence of the on/off switch. In use, it doesn't seem to be too much trouble to use the A580 front control wheel, but it could be better. The A77 seems to be halfway between the A700 and A580 in this regard.

    The A700 does feel better in hand than the A580. It is probably to do with the greater height of the grip, different materials, and the more sculptured thumb rest incorporating the rear wheel on the A700. The squeaky card door on the A580 does not help. That being said, the grip of the A580 has been perfectly adequate for me, it's just that the A700 just feels nicer to hold, especially with big lenses.

    The buttons I use most, aside from menu, display, and fn, are ISO and Drive. The A580 actually is better for me, being accessible by the shutter finger rather than the thumb (reaching over) on the A700. The button I mostly miss on the A580 compared to the A700 is the rear AF/MF switch. To switch into MF mode on the A580, one has to use the slider on the left lower side of the body, or else a lens switch if present. This slows things down a lot compared to the thumb press on the A700. The button makes a welcome return on the A77.

    The A700 uses a mini-joystick whilst the A580 uses a direction pad. The joystick just feels much better to use, though both are functionally equivalent. In shooting mode, its main function is to select AF points and the joystick gives a much more tactile method of selecting the points by pushing it in the desired direction. The direction pad requires a positional selection of the correct button instead. A handy feature of the A700 is that a central push of the joystick centers the AF point which is very handy when you need to switch to that very quickly, it is a much slower process on the A580 to first determine which point you have selected, and then push the right sequence of buttons to center the AF point.

    The metering mode rotary switch on the A700 is not missed. I tend to leave it on multi-segment metering, hardly ever in spot mode. Occasionally I will put it in centre-weighted but it doesn't seem to warrant a whole switch. Its lack on the A77 will not be an issue for me.

    One thing which I have missed is the SSS switch. Changing from hand-held to tripod use, switching the SSS off is easy, and vice versa. Also, depending on the lighting conditions and shooting circumstances, it is useful to be able to select SSS on a shot by shot basis. It will be interesting to see if the A77 will allow the use of one of the buttons as a toggle switch for SSS.

    The A77 seems to consolidate all except for the menu button on the right hand side of the rear. This actually makes some sense as in usual shooting position, the left hand is supporting the lens, especially if it is a large or heavy one. I have found that on the A580 it was much more convenient to review and delete shots on the fly when the 70-200mm/2.8 lens was mounted than on the A700 which required the left hand to come off the lens to activate the left mounted buttons. Else, I had to take the right hand off the grip to press these. Hence I am not too concerned about the A77 having most of the rear buttons on the right hand side.

    Straying a bit from pure ergonomics, the performance of the A700 still exceeds that of the A580 in certain areas, mainly AF. The A700 seems to be quickly and more positive than the A580, especially with the 70-200mm/2.8. The viewfinder is better (inevitable considering the QuickAF liveview system of the A580), and the lighting of the AF points is more visible. The PC sync socket on the A700 has saved me on occasion when I have forgotten to bring my hotshoe adapter to the studio. However, the standard mini-USB socket on the A580 is an improvement to the one on the A700 which requires a less commonly available cable. Switching card slots is more convenient on the A700, the A580 requires that the camera be turned off before the physical switch be operated, whilst the menu driven switch on the A700 is more convenient.

    In summary, there are distinct ergonomic evolutionary changes between the A700 and A580, not all of which are to the detriment of usability. Hopefully, in the design of the A77 this will put right most of the notable omissions in the A580 and improve on the ones already added.

    Wednesday, 20 July 2011

    The Photo Lounge

    I thought I'd try to post a bit more so I'm going to try to give a few examples of my photos together with some of my own thoughts. Photography is a highly subjective endeavour so I don't make any great claims to their brilliance. It is a useful exercise for myself to reflect on what I see in my own photography. I hope it may help in understanding the photographic process and how photos are "made".

    Glasgow 2011

    The first photo is a case of pure serendipity. I went out with some other members of the Glasgow University Photographic Society to do some street photography. As usual, the weather was a bit rubbish, but not raining which is always a plus. The generally grey atmosphere lends itself to black and white photography. It's been a long time since I shot a piece of film (though I have a few loaded film cameras lying in drawers) so I do black and white conversions in Lightroom. In this photo, the black and white tones reflect the grimness of the person's plight.

    I used the LiveView feature to hold my camera at waist level to take a few photos discreetly. The Sony implementation allows full-time phase-detect AF in LiveView so there are no issues with quick shots. It allows it to be truly useful in general shooting, rather than other DSLR versions which are really only good for static/tripod situations. I snapped off a few photos while surveying the square. I thought there might be an interesting shot in the sequence but it wasn't until I got back to the computer that I found the right frame.

    By sheer luck, I managed to capture the poster in the background with the girl in a police hat wagging her finger at the person. I was totally unaware of the background when I took the shot. If I had seen it, I might have tried to angle myself a bit more to the left so that the girl would be more to the left of the bench. But I think it works more or less as it is.

    Sunday, 6 February 2011


    One of my photos has been ripped off. I release most of my photos on the web under the Creative Commons license requiring that I be acknowledged as the photographer, no derivative works, and for non-commercial use only. Now, I am very happy to be asked for use beyond these licensing conditions and I am usually quite generous. I should feel privileged that someone found my photo useful, but it would not have taken much effort to have contacted me prior to its use, otherwise it is not only illegal but terribly rude.

    My Original (Bad scan of a print)
    Mutilated Version (Badly photoshopped)

    Sunday, 23 January 2011

    A580 Focus Shift under Fluorescent Lighting

    When I first acquired an A580, it displayed signs of back focus. I sent it in to Sony Service to get it adjusted. After getting it back, I tested to see if the problem had been fixed. Initial tests in daylight were positive, but when I tested it at home under fluorescent lighting, I still found substantial back focus problems. One of my favourite lenses is the 100mm/2 and this seemed particularly affected, but all my f/1.4 to f/2 lenses were rendered unusable wide-open under fluorescent lighting. Noticeable degradation of resolution at the point of focus is also evident in f/2.8 lenses due to focus shift towards the margins of the depth of field.

    Fig. 1: AF under incandescent illumination. A580 with 100mm/2 at f/2. Focus is acceptably accurate.

    Fig.2: AF under fluorescent illumination. A580 with 100mm/2 at f/2. Focus is behind the target leading to significant loss of resolution.

    I decided to dig a bit deeper into the problem to try to find out more about what causes the problem. Under daylight and incandescent (blackbody radiation) lighting, focusing was accurate. Under all kinds of fluorescent lighting, the focus was rear of the actual point. A working hypothesis is that the spectral lines in fluorescent lighting, coupled with longitudinal chromatic aberration in the AF light path, is what causes the phase-detect AF system to focus behind the subject.

    Prompted by a suggestion by David Kilpatrick, I printed out a coloured target against a black background to see what wavelengths of light are to blame for the mis-focus. The different pigments in my inkjet printer will absorb various wavelengths of light and hopefully from the different combinations of reflectance, it may be possible to broadly identify the spectral region of interest. All the following crops are from the A580 with the 100mm/2 at f/2 and Auto ISO (generally ISO1600), exported from Lightroom with only white balance and minor exposure changes.

    Fig. 3: Colour Target for AF Testing. Colours are supposedly (clockwise from top left) green, red, yellow, magenta, blue, cyan. The gamut of my printer in this particular paper is restricted hence the rather inaccurate colours.
    Under incandescent lighting, all colours were accurately focussed. Three target colours, green, cyan and blue, gave no problems under fluorescent lights, but red and yellow gave out of focus results. Magenta was also out of focus under fluorescent lights but not quite to the same extent as red and yellow.

    Fig. 4: Green target under fluorescent lighting, AF.

    Fig. 5: Magenta target under fluorescent illumination, AF. Though not as degraded as the red target, this is clearly not in focus.

    Fig. 6: Red target under fluorescent lighting, AF. Yellow target shows similar results.

    Fig. 7: Red target under fluorescent lighting, MF. Exposure difference due to using manual focus check Live View mode.

    Fig. 8: Red target under incandescent illumination, AF.
    From these results, one can surmise that it is a spectral peak in the yellow to red region of the spectrum which is causing a spurious displaced image in the phase-detect AF system which causes the backfocus. Backfocus is approximately 11 cm for a target at 3 m which indicates a shift of the image plane by approximately 140 microns. In comparison, the depth of field is only 3cm in front and 4 cm behind the subject, and the depth of focus is only 35 microns.
    Fig. 9: Green target under incandescent illumination, AF. This is even sharper than the comparative shot under fluorescent lighting.

    Under illumination by a continuous spectrum, a distinct image would not be be rendered at a different position, hence the accurate AF under incandescent lighting which has a lot of yellow/red light compared to green/blue. Under low pressure sodium street lighting (spectral lines at 589.0nm and 589.6nm), backfocus is very evident. However, backfocus is also seen under LED illumination which generally has a broadly continuous spectrum hence a more complete explanation is still required.

    In comparison, the A700 does not seem to display such a focus shift under different lighting. It is unfortunate that the A580 has this trait as the widespread adoption of fluorescent lighting (and increasingly LED) means that the A580 is handicapped in its use of large aperture lenses, especially in conditions where they are needed most. As the A700 AF system seems more robust to changes in illumination, Sony should incorporate those elements responsible for its reliable performance under a range of lighting into future models. Ideally, they should also develop an improved AF module for retrofit into affected A580 cameras.

    Friday, 14 January 2011

    Part II: How a camera works and takes a photo

    In this part of my guide for beginners, I'll explain how cameras form images.

    The main elements of a camera are the lens, and the sensor. The lens captures light, "processes" it, and then sends an image to the sensor. The sensor records the image falling onto its surface. The rest of the camera supports the these two main ingredients.

    The Lens

    The lens, very simply, takes light travelling along one set of paths and directs them along different paths. The two important attributes of a lens are how much light is gathered, and how much the light is bent. A lens will usually be specified by a set of numbers and characters, e.g. 100mm f/4 or 24-105mm/3.5-4.5.

    The first, how much, is the light gathering ability, or effective aperture of the lens. It is the effective size of the hole through which light can make it through to the sensor. The bigger the hole, the more light can be gathered in any given amount of time, hence photos can be taken "faster". This is why large aperture lenses are called "fast". The size of the aperture is usually given as a ratio of the focal length f (discussed below), e.g. f/4. This means that the diameter of the effective aperture is one fourth the focal length of the lens. Confusingly, the "/" is omitted, e.g. f4 or F4. A larger aperture is denoted by a smaller number (dividing the focal length by a smaller denominator gives a bigger aperture).

    The second attribute, how much the light is bent, is the focal length of the lens, e.g. 100mm. A long focal length lens bends light less than a short focal length lens. Only light coming from near straight ahead will end up bent enough by a long focal length lens to make it to the sensor, compared to a short focal length lens, hence a long focal length lens will have a smaller angle of view. This gives a magnified view of the scene. A short focal length lens will bend  light more, hence light coming in from a greater angle will make it to the sensor giving a wide angle of view.

    The angle of view of a particular focal length on a particular sensor size can be visualised easily using a rectangular cut-out of the same size of the sensor, and a piece of string the same length as the focal length of the string. By holding up the cut-out at the same distance from your eye (using the string to gauge the distance), the view through the cut-out will show the angle of view "seen" by the sensor looking through the lens. By bringing the aperture closer, the angle of view increases, putting it further away narrows the view.

    A zoom lens can vary its focal length, by moving lens elements around, either internally or by telescoping the whole length of the lens. On most lenses, this will also affect the effective aperture of the lens, e.g. 24-105mm f/3.5-4.5 means that the lens has an effective f/3.5 aperture at 24mm focal length setting, and f/4.5 at the 105mm focal length setting. A few lenses are designed so that the aperture stays constant throughout the zoom range, e.g. 70-200mm/2.8 will have a constant f/2.8 aperture regardless of the focal length setting.

    The aperture specifications discussed above refer to the maximum aperture that the lens is capable of. Usually, lenses can be set to close down the aperture, e.g. an f/4 lens could be set to f/11 for a particular shot. The minimum aperture is sometimes also quoted, typically in the range f/16-f/32, but this is usually less of a concern in most situations.

    The Sensor

    The sensor records the light falling upon it. In times gone by, this would have been a piece of film. Nowadays, the sensor is typically a piece of silicon upon which specialised electronic circuits have been etched.

    Film consists of a backing material (polyester, glass etc.) upon which an emulsion of silver halides would be coated. Silver halides are light sensitive, they react when exposed to light. This reaction can be made visible by processing with chemicals to develop the latent silver grains. The unexposed silver halide could be disolved away by fixer leaving the exposed silver grains as a negative record of the light falling onto the film. This negative could then be used to project an image upon photographic paper (which works essentially the same way as photographic film) so that you get a negative image of the negative which results in a positive image.

    A silicon CCD or CMOS imaging sensor operates using the same basic principle. When visible light falls on silicon, the light is absorbed and in the process, an electron is released. This free electron can then be collected in a photodiode in each pixel. The difference between CCD and CMOS is in the way that these collected photoelectrons are measured. In a CCD (charge coupled device), the electrons are transported, bucket-brigade like, to the periphery of the imaging area to special sense nodes where the total charge collected in each pixel is then measured. A CMOS sensor does not transport the electrons, but instead uses in-built transistors surrounding each pixel in order to measure the charge in situ.

    The charge in each pixel is measured by a charge-voltage transducer. By varying the gain of this transducer, the effective ISO or sensitivity of the sensor can be altered. The voltage is then digitised by a ADC (analogue to digital converter) to give a number which is then recorded by the camera.

    Taking a Photo

    We will assume that we are using a compact digital camera, there are minor variations for SLRs and other types of digital cameras. The photo taking process then proceeds as follows:
    • Composition. The scene to be taken is framed.
    • Focus. The camera will adjust the lens so that the objects of interest are in focus. This is achieved by moving elements of the lens, or the whole lens itself in relation to the sensor.
    • Metering. The camera evaluates how much light to let through the lens and strike the sensor. This is controlled by two factors, the working aperture of the lens and the length of time the shutter stays open. The camera may also determine an effective ISO of the sensor for use in the readout phase.
    • Aperture closure. The aperture is closed down to the working aperture determined in the above step.
    • Shutter opening. Depending on the camera, the shutter may have already been open in order for the composition, focusing and exposure steps above. But before the actual taking of the photo, the shutter will usually be closed in order to reset the sensor (make it blank). When the actual photos is taken, the shutter will open to allow the image forming light to fall upon the sensor.
    • Sensor exposure. The light gathered by the lens is directed onto the sensor. This liberates free electrons from the silicon which is collected in each pixel.
    • Shutter closure. At the end of the exposure, the shutter closes to prevent extraneous light reaching the sensor.
    • Sensor readout. The photoelectrons are now measured and recorded.
    To make better photos, the main elements to consider are composition, focus, aperture, shutter speed, and ISO. In the next section, we will see how these elements interact to affect the final photo.