Alley in Pisa, Italy

by Lars Kotthoff

submit your photo


Hall of Fame
View past winners from this year

Please participate in Meta
and help us grow.

Take the 2-minute tour ×
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

I’ve just gone through a round of testing with two telephoto zooms, on two different cameras, and both appear to have significant back focus issues at the 70mm end.

I just had the Nikon 70-200 f/2.8 factory serviced/calibrated (they went so far as replacing the flex board!) so I’m pretty confident with it being in spec. The other, Nikon 70-300 f/4.5-5.6 seems to be doing the same thing.

I even borrowed a Nikon D600 to compare against my D7000 and both seem to exhibit this back focusing.

Has anyone else noticed this behavior, or am I just imagining (reading too much into) this, or doing something wrong?

Test target images follow with one thing to note, during these test shots, I had the Nikon D7000 AF fine tune set to -5 which is why one of the shots at 200mm is actually slightly front focused.

Full-size 70-200 Shots: http://www.flickr.com/photos/90253723@N03/8614623879/

enter image description here

Full-size 70-300 Shots: http://www.flickr.com/photos/90253723@N03/8615730868/

enter image description here

share|improve this question
    
Are you fairly confident your test procedure is sound? Everything aligned perfectly? Did you do tests with and without live view? Centre focus point, or did you try various ones? I've read a fair bit about the D800 left focus point issues (alleged) and I've read that that issue tends to show up at wider angles, and there is a camp that believes that most people who think they have a problem just aren't testing correctly. I can't comment from personal experience, not having done significant testing of my own. –  MikeW Apr 3 '13 at 4:29
    
Earlier on, a few weeks ago, I did use live view for a couple of shots. I guess it is a good idea to re-run these tests using nothing but LV as a comparison. Along with using other and/or multiple focusing spots. It may take a few days, but I will post the results when I can. Thanks. –  ISOTropic Pixel Apr 3 '13 at 11:33
    
One thing you can use to check that the camera is actually trying to focus where you want it on the D600, is to set custom menu f2 Multi Selector Center Button > Playback > Zoom 100%. Then you can verify the focus point. –  MikeW Apr 3 '13 at 19:20
    
Live View, which uses contrast detection, should always be spot on and is not affected by the +/- settings in AF Fine Tune. LV will find the point of maximum contrast that the lens is capable of resolving. If LV is showing the same results, something else besides AF Fine Tune adjustment is going on here. –  Michael Clark Apr 3 '13 at 19:20
1  
@MichaelClark it will show the point that was active. If there is anything on the test chart within the area used by that focus point that can be also focused on, then they need a better chart designed. –  MikeW Apr 3 '13 at 20:26

1 Answer 1

up vote 8 down vote accepted

It is not necessarily true that all tele zooms back focus on the wide end. What is almost universal is that no zoom will be exactly calibrated at both the short and long end at the same adjustment setting.

Both Canon and Nikon recommend calibrating zoom lenses at the focal length you use the most. I tend to find the best setting on both ends of the lens and then split the difference. If my lens is +5 @ 70mm and +2 @ 200mm I'll set it at +3 or +4. There are some bodies, such as the Canon 1D X, that will allow you to calibrate the longest and shortest focal lengths of the same lens independently.

There are several issues that can affect the results of AF calibration tests:

The critical thing to remember when using tools such as the Lenscal or LensAlign is that the plane of the target and the focal plane of the camera must be parallel. Even a few millimeters variation can affect the results significantly. The LensAlign's designs assists in this better than the Lenscal. I'm more of a fan of flat target methods that allow you to optically align your camera with a mirror and then tape the target to the mirror. There is even FoCal software that will automate much of the calibration process.

Depth of field (DoF) is not symmetrical. That is, if the 0 on the ruler is the point of sharpest focus, the 2 above it should be sharper than the 2 below it. Looking at any DoF calculator will make it obvious. At shorter distances, there isn't enough difference to be noticeable, but at 20' with a 70mm lens @ f/2.8 on your Nikon D7000, the DoF includes 16.2" in front of and 18.72" behind the point of focus. At 100' this is really obvious with DoF stretching from 73.2' to 157.6' - 2/3 of the DoF is behind the point of focus! Zoomed to 200mm @ f/2.8, the difference at 100' is barely 4", and is much less noticeable than at 70mm.

Another thing that can sometime affect results of AF calibration is that phase detection AF is designed to favor speed at the expense of accuracy. Most AF systems are an open loop: the camera measures how far and in what direction the lens is out-of-focus, instructs the lens to move by that amount, and takes the picture without verifying how far the lens actually moved. Newer lenses have sensors that report back to the camera the position of the lens, but for any improvement in AF performance both the lens and the body need to incorporate the newer "closed loop" improvements. Roger Cicala, the CEO at LensRentals.com and technical guru, says the newer systems have about half the standard deviation in terms of focus variation from shot to shot compared to the "open loop" systems. But even then there is still deviation from one exposure to the next, just not as much. When using tools such as the Lenscal, a series of 3 or more measurements should be made at each setting.

Using the correct temperature light is also important. AF systems are centered around 5200°K. Using tungsten or especially fluorescent lights to illuminate your target can skew results. Of course, if you always shoot in a stadium under 4200°K lights, then maybe you should calibrate with the same type of lights.

One thing I've noticed when calibrating longer lenses is that the correct setting at 20 or 30 feet is not always the dead on setting when shooting at 100-200 feet. Of course the problem with calibrating at those distances is that a target such as the Spyder Lenscal becomes smaller than the coverage of your AF points and you can never be sure exactly what the camera is trying to focus on (Don't assume areas outside that little square are not covered by the actual focus point - they are).

A word about viewfinders and focus points: I'm not near as familiar with current Nikon cameras, but here's a map of the Canon 5DII's focus system. Other camera systems are similar. What is actually visible in the viewfinder are the eight small squares in the middle of each blue bar (but not the bars) and the slightly larger square at the center point, yet the actual coverage for each point are the areas covered by the blue rectangles and there is nothing in the viewfinder to indicate the position or coverage of the six red AF Assist Points that are active by default when the center point is selected and the camera is using AI Servo. So when you think only that little square in the very middle is all that is being looked at by the AF sensor, it is actually the entire area covered by the long vertical bar, the shorter horizontal blue bar and the red rectangles. The AF Assist Points can be disabled via the custom menu.

For further reading on AF systems, Roger Cicala's Autofocus Reality series is very insightful. Part 1, Part 2, Part 3A, Part 3B, and Part 4. And: How Auto Focus (Often) Works Also: Are zooms always sharper at one end than the other?

share|improve this answer
1  
Thanks for this. I admit I may have had the camera a few millimeters off level for a couple of the shots. But, this is not the first round of tests I’ve performed, just the most comprehensive. And they all have been leading to the same result. I’ve tried the FoCal software but had problems as far as functionality and consistency of results. I will try to re-run some of these tests and to get some real world shots of things, but looking at these shots almost the entire back of the scale looks in focus and does not seem like a DOF issue. –  ISOTropic Pixel Apr 3 '13 at 11:54
    
It does appear to be the case that the two lenses you tested are back focusing slightly at 70mm when calibrated at 200mm. I tried to write the answer to be a little more universal to anyone else who had similar concerns. Remember, even with the best cameras and lenses AF will vary slightly from one shot to the next. There's no such thing as a perfect zoom lens or AF system. At least not yet... –  Michael Clark Apr 3 '13 at 19:14
1  
Many thanks for adding the details for DOF. It helped me put this issue more in perspective. –  ISOTropic Pixel Apr 3 '13 at 20:20

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.