D800 Autofocus issues II

Started Jan 25, 2014 | Discussions thread
olyflyer Forum Pro • Posts: 25,311
Re: I didn't know about the issue when I first saw it

RichRMA wrote:

Before I knew anything about a focus issue a friend, an early adopter, got a D800. He gave it to me to play with and I noticed right off that it was focusing with a prime lens about 1" off for every 48" distance. I figured, "It needs a fine adjustment for the lens on it." The friend ended up changing the camera three times, finally getting one that didn't have the focus issue. It would be nice to know if tolerance variation due to cost constraints in design are responsible for this, like in the old days when lenses were done by hand and only the most expensive could be guaranteed as consistent.

You have conducted an erroneous test. No lens is tested that way. Lenses are tested at around 40 or 50 x FL distance which means that for example a 50mm lens is tested at 2.5m (about 100"). If you want to test and tune the same lens at half that distance it is fine, but the accuracy in that case at longer distances may not be right.

Anyway, regardless how it is, of course if there is a 1" error in every 48" then that is a serious problem, it means that you can't focus on a flat surface from 48" because the total DOF is 1.9" at that distance so the flat surface would be too blurred. There is however another thing which many people miss, which is that the focus is regarded good if the DOF is 1/3rd in front of and 2/3rd behind the focus actual target. That's why when testing focus you must make sure that the focus target is flat and is not in an angle relative to the sensor, and that's why that widely used AF test method, with a focus chart (basically a ruler) in 45 degree angle is total rubbish. Yet, a huge number of people are using that and similar test methods. No matter what you are focusing on, even if your target is a perfect AF target even according to Leonard or page 100, you must keep it in parallel with the sensor, not in a slant angle.

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