|A2C2 (AFMA Array Color Chart) produced by FoCal for a Nikon D800E with Nikkor 24-70/2.8G lens at 24mm f/2.8 at 1.2m distance.|
I think the D800 is a great camera with that particular flaw Nikon will eliminate sooner or later, with or without ever making a comment about the issue (Nikon promised to keep me up to date about news coming from Japan but so far, none have been received which they wanted to share). So, customers of the D800 are well adviced to get their cameras fixed (by service) or exchanged until the issue is resolved. That's a better advice than to buy a different camera because there isn't any true competitor. The Nikon D600, Canon 5DmkIII or Sony A99 are nice but lack the medium-format alike resolution.
This is why I think it is important (even for Nikon) to have an as simple and as reliable as possible procedure to check for the AF array issue.
Rich, the founder of Reikan and author of FoCal thought the same and created the FoCal Multi Point Focus Test tool. Then, he and myself (aka LumoLabs) worked together to make the tool produce reliable and comparable results. This version (1.6) of FoCal is described here:
How to produce the A2C2 with FoCal Pro 1.6.0
In a nutshell, this is how it is produced:
- Open the "Multi Point Focus Test tool".
- Tick AFMA: From: -20 To: 20, using Count: 6 steps rather than 5.
- Tick Validate Focus Consistency to 2 percent rather than 6 %.
- Run the test and save the chart from page 3 of the PDF (or screenshot after the test is complete).
I call this color coded chart the A2C2 (AFMA Array Color Chart). Deviations (from the center AF point) much larger than 10 appear red or blue, around 10 appear yellow or cyan, smaller values are greenish.
An important point is that the A2C2 shall be interpreted for an f/2.8 lens (i.e., a lens with nominal aperture of f/2.8 -- an f/1.4 lens measured at f/2.8 does still not produce a comparable A2C2).
If you do the test, you are highly advised to read the full white paper with more detail and examples given:
The above paper contains examples for other cameras (Canon 7D, Nikon D3), other lenses incl. f/1.4 and measurements which translate the AFMA unit step into physical dimension (µm) of focus shift.
Interpreting the A2C2
I found out that the AFMA scale for Nikon is more fine-grain than for Canon. Moreover, I consider deviations of up to 10 to be tolerable (may be corresponding to deviations of up to 4 for Canon) in the A2C2. AFMA values can vary up to +/-3 for Nikon (which would probably only correspond to +/-1 for Canon) from run to run and around the optimum value, have no perceivable effect on sharpness.
I found (although with probably a significant systematic error) that an AFMA unit change corresponds to a shift of the focus plane which equals a change of the circle of confusion of 1/8 (Canon) or 1/24 (Nikon) of a full circle of confusion (20µm or 30µm). With an f/2.8 lens, 1/24 translates to a shift of focus of 3.5µm. Phase AF systems aren't more accurate than maybe 20µm in the focal plane, so it doesn't make much sense to fiddle around with AFMA values below 5. BTW, most contrast AF systems aren't really more accurate either. With the D800, phase AF can beat contrast AF ...
Therefore, the interpretation of the A2C2 is easy:
The A2C2 with an f/2.8 lens should be green, yellow and cyan (Nikon) or all greenish (Canon).
With a lens faster than f/2.8, the chart is allowed to look "worse": With a Nikon and f/1.4, some red or blue (deviations up to 20) is acceptable. With a slower lens, the chart should become increasingly green. Focal length, distance or sensor resolution have no or only little impact on the interpretation of the A2C2.
Below are two A2C2, one from the D800E (same as above, but a 2nd run for select AF fields) and one from the D3:
|A2C2 for a Nikon D800E with Nikkor 24-70/2.8G lens at 24mm f/2.8 at 1.2m distance.|
Second run for select AF fields.
|A2C2 for a Nikon D3 with Nikkor 24-70/2.8G lens at 24mm f/2.8 at 1.2m distance.|
Run for select AF fields.
It is obvious from the full measurement that the full left AF bank is affected by the issue and that the leftmost outer AF field isn't actually the worst one (at least not always). Other D800 or this D800E with other lenses have/has problems with the right bank too.
Please, feel invited to consult the full paper for further detail, to be found here:
If you go to Photokina 2012 in Cologne, maybe visit the Nikon booth and deliver your copy of the A2C2 chart to document that not only "a few" cameras have the problem and that you possibly expect an official word from Nikon.
I would appreciate too if visitors could leave a link to their copy of the A2C2 chart in the comments section of this article, with name of the lens used. Maybe before/after servicing comparisons too.
Thanks for stopping by,