d800 focus testing with different targets

Started May 15, 2013 | Discussions thread
Per Baekgaard
Contributing MemberPosts: 994Gear list
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Re: d800 focus testing with different targets
In reply to RajahX, May 15, 2013

Thanks for doing this test -- shows how difficult it actually is to establish a good repeatable and reliable set of methodologies, including targets. However much I disagree with the statistics and releavance of most of "the p.100 commenter's" posts here, it is surely correct that establishing a good and consistent methodology/target is not straighforward! Also, lighting might impact this -- you could get slighly different results in daylight.

I've used the dot tune method on my lenses, as well as other methods that has included calculating the MTF of selected points and finding the "best" focus zone. This has worked well for me in practice; I've improved the amount of "critically sharp" shots in practical use -- which was the reason for me doing this in the first place.

The advantage of the dot tune method is that it takes out one element of variability that results from repeated movement of the optical elements and the hysteresis in the helicoil. You just need to establish the "best" focus once and the the rest of the procedure does not involve any mechanical adjustments.

However, it relies on 1) the assumption that there is indeed ONE best focus position and not several (e.g. due to mechanical tolerances variable decentering), 2) that you can find that, and 3) that the PDAF zone is symmetrical around the focus point.

Both 1) and 3) are reasonable assumption to some degree, but finding the right position can be difficult to achieve without help. We're talking about small adjustments here, and I think it can be somewhat challenging to see the difference between e.g. an MTF50 of, say 1800 and 2200 just by looking at the rear LCD. You might want to use some other method of achieving critical focus.

RajahX wrote:

First, i conducted the test at a distance of 0.5m from the target. Here are results:

  1. Zebra, dot tune range: [-9, +3]
  2. ISO Center, dot tune range: [-16, +7]
  3. Horizontal Lines, dot tune range: [-19, +5]
  4. Checkerboard, dot tune range: [-20, 0]

0.5 m from the target is probably too close for a "general" adjustment, unless you use this setup mainly for macro shots. The "usual" distance for "general" adjustment is often mentioned as >25x or better 50x the FL, so for a 35mm lens, it would be 1.75m.

But the results above would suggest a tune value of -3, -4.5, -7 or -10.

Did you maintain the exact same focus position between the shots or did you re-focus when changing the target? How did you focus?

My experience is that +/- 2 units is about the precision you can achieve. If you discard the outliers, you setup might be around -5 for most targets at this distance.

Next, i conducted tests at a distance of 1m from the target. Here are results for that:

  1. Zebra, dot tune range: [-15, +4]
  2. ISO center, dot tune range: [-19, +5]
  3. Horizontal lines, dot tune range: [-19, -4]
  4. Checkerboard, dot tune range: [-10, -3]

... which would indicate values between -5.5, -7, -11.5 and -6.5 at this distance.

I think the actual tune value depends on the distance to the targert, and the relative posisions between your targets 1, 2 and 3 are roughly the same as above (in the 0.5m test you end up with 67:100:156:222 whereas in the 1m test you have 79:100:157:86 -- so 4 is clearly an outlier).

This MIGHT indicate that 1) the checkerboard is not the best target and if you disregard that, then 2) the other targets are reasonably consistent between them and 3) you have as expected some variability with distance. Targets 2 and 3 are actually very consistent between them.

I dont think that different targets leading to slightly different values is surprising.

Apart from a general bias in the negative direction for all targets, the targets give wildly differing ranges.

... but not totally inconsistent

And these ranges change as we increase the distance to the target.

Exactly.

Given the high degree of AF variability between targets and distances to targets, i don't see how one computes a good AF tune setting.

What I did was to use the method as a guidance and then examine a lot of real-world shots with that particular setting and check for any possible skew of the front-focus/back-focus ratio and re-adjust as needed. There will always be shots that are slightly oof, but the key is to avoid consistent front or back focus.

The only target that appears to produce somewhat expected result is the ISO target. --- the range for this target shifts slightly as the distance is increased. The rest of the targets do something fairly random as the distance is increased ?!?

Maybe not completely random?

I think one reason for the targets working differently at different distances is due to how they are "seen" by the PDAF. I'm not aware of the resolution of the PDAF sensor(s), but at some distances the repeating patterns in e.g. the checkerboard will "fool" the sensor. Also, if you re-focused between shots, maybe the way you see the best focus changes and/or is impacted by optical defects in the system?

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