DotTune: Video Tutorial for AF tuning without photographs

Started Feb 18, 2013 | Discussions thread
Daniel Lauring
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Re: This guidance is not fully reliable
In reply to Leonard Shepherd, Feb 20, 2013

Leonard Shepherd wrote:

I find it amazing the number who come up with a suggested calibrating system and ignore Nikon's guidance on what can cause poor auto focus accuracy.

The suggested target is a regular geometric pattern. Page 100 of the D800 instruction book, and also the instructions for every Nikon camera I have owned in the last 13 years, confirm geometric patterns are not always consistent with accurate focus.

Although I have seen nothing official from Nikon, on the Thom Hogan site there is a target which does not have equal geometric spacing which some suggest is used by Nikon. Whether this second target comes from Nikon or not is a secondary issue, the important detail is it avoids possible poor focus accuracy which sometimes occurs when a regular geometric pattern.

With a good autofocus target the video link suggestions are a reasonable starting point. Whether relying on the autofocus confirmation light for calibrating to the resolution potential of a D800 with an f1.4 lens used wide open at minimum focus is another topic.

What should not be another topic is Nikon's guidance that the suggested autofocus target may not be reliable enough for accurate fine tuning.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.

Leonard,

You made a point.....once.  Now your continuous spouting of the same thing is getting tired, without anything to back up why a particular focus target is not good.

Referring to some general statement in the manual is not backing it up.

Proving a target used is not as accurate is.  Be very specific about what is wrong with the target.

If you want to be helpful, suggest a good auto focus target...one that people can download from the web.  Or make one yourself.

In my experience, all you need to have a reliable focus target is a single horizontal thick line with good contrast (for Nikon anyway.)  This works with all Nikon's PDAF focus sensors.  A thick checker pattern works pretty well too...but I prefer having something that gives me two points of contrast running through the focus box.

I have seen a poor target used, like the one that had all the lines moving outward, originating from the center of a circle.  If one were to put their auto focus sensor over the center it would be looking at very fine lines going outward in all directions.  This fine pattern isn't a very good focus target.

On the other hand, I've always had quick and repeatable focus with a single thick horizontal line.  The advantage of having lines of multiple thicknesses, is you end up with lines that are the right thickness for different distances (such that you can get both sides of the line within the focus box.)  There may also be less chance of aliasing issues vs. a repeatable pattern, but if the lines are thick enough that shouldn't be an issue anyway.

Specifically, the target used in this video, appears to be pretty reliable and the results people are getting is repeatable.

What problem do you have with that?

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