>>So you did not actually test the method I proposed >yourself? > >No - not yours. Many others though, including the commercial >units I've reviewed. > >>The whole idea of leaving *only* one dot visible when >focusing >>is to eliminate the typical errors coming from targets >placed >>at 45 degrees. Indeed, the rest of the target is here >pure >>white and cannot contribute to the phase detection of the >>focusing system. This is easily confirmed by the total >>inability of the camera to focus if the collimator is not >on >>the dot. > >In my experience, cameras can sometimes assume focus, for a >variety of reasons, when no normally obvious focus target is >present. For example, in inconsistent lighting, the camera can >sometimes read a faint shadow edge sufficiently well to >attempt focus and allow the shutter to fire even when no >obvious focus target is present on an ostensibly clean sheet >of paper. >
Sure it can happen. But only very occasionally does my camera fire when I aim it at a blank sheet of paper in focus priority mode. If it did more systematically, I would call this a bug in the focusing system.
>I'm not sure what you mean by the reference to a collimator. >The hole in the top sheet which exposes the target is not a >collimator - it's a mask. A collimator is particle >modification device which is used to realign (collimate) a >beam of particles (e.g., photons). A camera lens is a kind of >collimator. Are you referring to the lens being used? If so, >I'm missing your meaning.
Sorry for that; I used (AF) collimator as a synonym for AF point.
> >An edge of the mask can also be a focus target as often as the >target print itself. A flush or unflush far mask edge being >grabbed hard (instead of the target because it's too small in >the frame for the selected AF sensor) by the AF system, >coupled with a lens that tends to focus slightly ahead, >coupled with a camera AF system which focuses slightly ahead, >may give a test result which looks spot on.
This is why you need to cleanly cut the hole as indicated. The focusing system analyses all the information available. A badly cut edge would provide some at a different place than intended. On the other hand, the hole should be small compared to the expected depth of field to analyse the resulting pattern when the dotted paper is exposed. Typically, it should appear of similar size as the AF point in the viewfinder. If the camera focuses on the front of the dot or its back as a result of the cut out, it does not make a difference.
>There are all >sorts of combinations of these three factors alone, which when >coupled with 45 degree testing in particular, give >photographers absolute fits of AF fine tune angst. To get a >sense of what the camera is actually doing - not matter what >testing metholdology is being used - the tester has to take a >hundred photos or more just to chart the variability inherent >in the particular camera and lens combination being tested. > >
On this one you will have to trust me if you don't find the need to try out: I took hundreds of photo to play with all my lenses, and the variability is negligible compared to the size of the sharp zone of dots you have to observe on the target. I therefore strongly believe that the camera (D800) is very systematic at picking the dot consistently when there is nothing else around it except white paper. I think most if not all of the variability usually observed with other methods comes from the fact that the target has several parts at different distances (especially a ruler aimed at 45 degrees) that are picked a bit randomly if they are too close.
>I'm not trying to shoot down your efforts. I'm only injecting >the important variables that I think should be fully accounted >for in any AF focus testing system. The simplicity of your >approach is genuinely attractive, but I don't see any way for >it to be used, as-is, to produce accurate test results on a >consistent basis. Then again, a 100-200 shot test series might >produce enough variability data to make a judgement about AF >fine tune. I just don't have the time to give your proposed >test method a serious try, but I hope that someone else can. > >What I'm suggesting though is that any testing at a 45 degree >angle is difficult, at best, to do accurately. The angle in >and of itself is a compromise solely for simplicity of test >set up, but it materially challenges any camera's AF system in >an inappropriate way. It's only ever been promoted by the >producers of hobbyist AF testing kits because it's simple and >small enough to fit into a manageable retail package. Angled >test kits don't exist for any other reason. Parallel/plane >target focus testing at various distances is generally a much >more effectively accurate AF testing method, but it still >requires more room and greater measurement accuracy to >establish a parallel/plane relationship between camera sensor >and target. It also generally produces more reliable results. >Using your single target point by itself, in a parallel/plane >test set up, would determine how effectively a cross-type >sensor can accurately lock on to it. Maybe you've designed a >target that is better than many of the others I've seen >(noting too that some others still, have proven to be very >good test targets no doubt). That last bit would be a real >coup for you - seriously. > >Once again, I'm not in any way criticising your goal of >accurate AF testing and calibration, just any methodology >which needlessly incorporates a 45 degree angle setup. To my >way of thinking, the compromises and the test results are >unacceptable. > I totally agree with you that one has to get rid of the problems generated by aiming at a target at 45 degrees. What I am claiming is that by exposing only one dark dot on the target when focusing, you are not aiming at 45 degrees or any other angle. With only a small dot to aim at, there is no reference to any direction. It's just like looking at a point floating in the air.
I welcome constructive critism about the methodology proposed. In fact, I share absolutely all the objections you raise about using targets aimed at an angle (there is a lot of randomness in the sample pictures taken). It is precisely because of them that I started to think about an alternative that still offered a view of where the acquired focus point lies in terms of front or back focusing. What I am saying is that if you only give the focusing system one dot to analyse, variability is negligible in practice (at least with my camera and objectives). And honestly, if my camera was not able to pick consistently such an easy target, I would not felicitate Nikon .