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.