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Any advice on determining if something is focusing is bad with lens, body, or myself?

edited October 2014 in Nikon
Hello everyone,

I have a Nikon 24-70mm f2.8. I've used it w/ my Nikon d600 for almost 2 years now. At 24mm, the focus seemed to be very off, but it didn't bother me much because what I was shooting at 24mm. at 70mm, the focus was much better, but not necessarily perfect. I thought something was not right with the body. I recently got the d750, but focusing is still off.  I never bothered doing any sort of AF fine tuning until I started shooting with the d750. The focus seems to be more off w/ this body.

I just got my d750 a few days ago and starting using it w/ my 24-70. It focuses really quickly, but my focus is still off. I looked up dot-tuning method and decided to try it  out really quickly. I put up something w/ a lot of contrast (black text on white paper) on my wall and put my camera on tripod (about 6ft from the wall).

I was able to adjust the fine tuning such that the lens focused properly at f2.8. However, bcz it is a zoom lens, the fine tuning needs to be changed as you change the focal length. This was a quick test done about 6 ft from the wall at f2.8. I used the #3 in the middle as my focusing target (see attached image).

Here are my results for d750:
@ 70mm: 10
@ 24mm: -9

Here are my results for d600:
@ 70mm: 7
@ 24mm: -15

I didn't try a focal length in the middle. I realized it after I put everything away.

I have two other lenses: 70-200 f2.8 and 50mm 1.8. I haven't considered AF Fine Tuning these because the pictures  have always looked sharp to my eye.

Also, after I did the af-fine tune, I checked the results by taking shots at each focal length w/ the tuning on and off. There was a clear difference. The tuning made the focusing accurate. The problem is because the tuning is so different between 24mm and 70mm. It's not really practical to change it before every shot. And choosing a value in between causes both focal lengths to suffer. It seems like I have to sacrifice one range for the other. I bought the 24-70 to cover that focal length. If I have to choose one focal length over the other,  it would be better to just use primes.

A couple of questions:
-How do you AF fine tune for zoom lens? Any particular focal length? At what distance?
-Is the result normal? or is something wrong with the lens?
-Since, this was a very quick test, I am planning to repeat the dot-tuning in a few days. What distances should I test from? I felt that 6ft is a reasonable distance for both 24mm and 70mm.

Thank you very much for any advice and responses! 

Comments

  • AF fine tuning a zoom lens is difficult on the Nikon because, as you've discovered, you can only set one value regardless of focal length or focus distance. An approach could be to evaluate the lens at different focal lengths (as you have done, but go a little further with it such as 24, 50, and 70mm) as well as some different distances. See what values you get and whether you can make a compromise. For example, if you had ranges from +5 to +9 on these, then you could settle for +7. If you get wildly different results (which it appears like you have), then you might need to make a choice of what focal length and distance you use most often. Or choose to not AF fine tune it at all.

    I have not tried DotTune before, but I've read about it on some forums. Conceptually, the main issue I have with it is that you are trying to make a precise measurement (an AF fine tune value) with a very imprecise device (the focus confirmation "dot"). We all know the dot has a ton of slop in it, giving a wide range of what it considers to be in focus. In other words, the dot is much less precise than we want it to be. So a major assumption is being made that the exact center of this imprecise range is the correct value. Is this a valid assumption? Maybe it is, maybe it's not. It would be like trying to measure a voltage to 3 decimal places with a meter that only has 1 decimal place of precision.

    That being said, it doesn't hurt to try it.

    On your 24-70, there could be something wrong with it. I'm occasionally disappointed with my 24-70 particularly when I use the outer focus points. But it's generally pretty good. If as you say the focus is "very off" at 24mm, that doesn't sound right.

    Good luck!
  • Thank you very much for your reply. I really appreciate it. I'll definitely try out a few more tests and then I'll call Nikon for assistance. The camera is still under warranty.

    The focusing dot is used to indicate to the user from the camera letting him/her know what the camera thinks of the focus.

    If the camera thinks test-subject is focused or not, it will indicate appropriately with the dot. AF tuning helps with accurate autofocusing.

    Dot tuning approaches focus tuning from a different mindset compared to the traditional way we do focus tuning.

    Traditional: Adjust tuning and focus test-subject until camera focuses correctly.

    Dot tuning: Start with accurate focus and adjust tuning until camera also indicates test-subject is focused.

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