Back Focus is this test valid?

[anthony_bez]

I have not tested my lenses for back focus before, but wanted to try my new

ringlight so used this chart http://www.focustestchart.com/chart.html.

 

The camera was a D80 and the first lens a 50mm f1.4 AFD was bang on the center

line. The next lens 85mm f1.4 AFD was also perfect, so I smugly mounted my 12-

24mm f4. Well this was not as accurate but as it only goes to f4 the centre

line was still sharp at the edge of the back focused range. I wondered if it

was a problem with zoom lenses so tried my 18-70 AFS and this back focused

badly at f3.5 the target line was nowhere near in focus. With trepidation I

mounted my 80-200mm f2.8AFD and it also back focused but not as severe as the

18-70mm. As a reference I tried my 105mm 2.8 AFD it like the other primes was

perfect.

 

My conclusion is the D80 must be accurate because it works perfectly with the

fast primes. But are my zooms at fault or is the test showing a false reading,

as you shoot at the minimum distance range of the lens.

 

I am not that worried because real world photography is not showing problems,

but I use my 50 and 85 for wide aperture shots. I normally shoot at f8 with the

18-70 which may be masking the fault. But I would hate to lose faith in my 80-

200, do you consider it a fair test of the lens (at minimum focusing distance).

I was thinking of setting something up at a more normal shooting distance, any

ideas.

[tonybeach]

Minimum focusing distances are too close and that alone could cause your problem. If you were getting frequent (or even infrequent) back focus issues then this would be something to be concerned about, but it kinda sounds like your looking for a problem that you don't actually have.

[anthony_bez]

"kinda sounds like your looking for a problem that you don't actually have"

 

Anthony, Yes you are absolutely right! :-)

 

But I was just playing around with the ringlight, and now I am curious. I love my 80-200 and want to find out if it needs calibrating. I must admit when shooting tight head portraits at f2.8 I AF on the eye, then the nose to try to make sure of a keeper. Now I am wondering which would be the sharpest shot.

[mark_sirota1]

Here's the most well thought out autofocus test I've seen:

<p>

<a href="http://regex.info/blog/photo-tech/focus-chart/">Jeffrey Friedl's Autofocus Test Chart</a>

[conrad_hoffman]

I had trouble with the first chart listed, as it suggested my D200 was off by quite a lot at close distances. It isn't. Go to the Friedl site and follow the link to the Goodman site. That one worked well for me, and explained some of the issues with zoom lenses that can foul up test results. Try the Friedl target as well, but IMO all these tests are subject to some subtle errors.

[bjørn rørslett]

In my opinion, these tests are all subject to less-than-subtle errors. You are almost guaranteed to get "back-focus", unless you test with a very fast lens.

[rudymerz]

Bjorn,

 

and what would you use?

 

Rudy

[bjørn rørslett]

Same setup as the pro repair shops use. Staggered targets that are parallell to the film plane *and* fill a large area in the finder so full coverage of the AF-responsive area is ensured.

[leonard_evens]

I upgraded to a D80 after using a D70 for several years. I found what appeared to be consistent back focusing with two zoom lenses. so I got the same test chart you used and that confirmed the back focus. I don't have a fast non-zoom lens which might provide a more definitive test. My lenses had relatively large DOF even at the highest zoom factor and wide open. The focus point was still in focus, but far from centered in the DOF region as it should have been under the circumstances. I called Nikon and their Tech rep told me to exchange the camera, which I did. The new camera focused properly at normal focusing distances that I would use, which wasn't the case with the first camera. But much to my chagrin it still showed back focusing problems with the test chart and the same two lenses, but not as pronounced as with the first camera. My old D70 didn't seem to have any problems with the same lenses and the test chart.

 

I looked at Fiedl's webpage, and i don't think he says anything there which suggests the test chart made any difference, at least in my case.

 

I think from my experience and yours that some D80s may have some problems with zoom lenses when focusing very close to the minimum focus point and in relatively dim light. (I used flash, but remember that the focusing is done in ambient light.) I don't really know why this should be the case. But there are some things to consider.

 

First, as Friedl notes, the focusing area may not be coincidental with the bracketed region on the focusing screen. That could in some cirucmstances lead to an error in focusing. Apparently, the way the system works, as long as the black target line is within the focusing region, the camera will lock on it. It doesn't have to be centered in the region. Also, that suggests a possible scenario of how the system works in practice. Presumably the camera drives the servo motor in the lens back and forth until is it is statisfied it has maximized contrast between the target line and surrounding blank background. But, there may be a level at which it is satisfied beyond which it won't go. If the target line is in the DOF region, even if not centered, it may appear to satsify whatever criterion the system is using. If in addition, it can't move back and forth equal amounts about the real focus point, as might be the case when the lens is very close to its minimum focusing point, then it might not be able to do better than just focusing somewhere inside the DOF region and not necessarily at the best focus.

 

Any focusing system has to have an inherent error which is usually characterized as depth of focus. This is proportional to the f-number and depends on a fixed constant which measures the ability of the system to distinguish distinct points. The sensors in the D80 are just a bit under .01 mm apart. You would need at least two sensors to distinguish an edge. At f/4.5 that would yield a depth of focus of about .09 mm. If, as I suspect is the case, the focusing system needs to use more sensors than two, you would have to multiply that by some factor. That could produce a depth of focus of as much as .2 mm or more, and the lens position set by the automatic system could be off by that much in difficult focusing situations. That is approximately what I observed.

 

With your 50 mm lens at f/1.4, the depth of focus would be much smaller, perhaps as small as 0.03 mm. As the system examined the edges, things would go out of focus much faster given a small movement of the lens than would be the case with a slower lens. That might make it easier for the system to function. All, this, of course, assumes the center line was still in focus, but not centered in the DOF region. If it was actually out of focus, something else is happening.

 

By the way, I am not sending my new camera to Nikon to be adjusted since in actual shooting situations it seems to work well enough. But I'm glad I made the exchange because the first camera, in retrospect, turns out to have had some problems with exposure, particularly when using flash and was difficult to compensate properly.

[anthony_bez]

Leonard, Thank you for your in depth reply, I hesitate to blame the camera because it works flawlessly with my prime lenses. And it would seem strange that all three zooms are faulty. It would be interesting if you could test your camera with a prims.

 

Bjorn, If I set up three targets parallel to the film plane, would I put the left two inches in front of the middle and the right two inches back? Then fill the centre sensor area with the middle target.

[anthony_bez]

"You are almost guaranteed to get "back-focus", unless you test with a very fast lens"

 

Bjorn, I forgot to mention you exactly describe my testing.

[jeffrey_friedl1]

Bjorn, why do you believe one will get back focus when testing with slow lenses? I think I know what you intended to say, but perhaps not, so if you don't mind being a bit more verbose, I'd appreciate it.

 

Thanks,

Jeffrey

[anthony_bez]

"Bjorn, why do you believe one will get back focus when testing with slow lenses? if you don't mind being a bit more verbose, I'd appreciate it"

 

And so would I!