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Archive 2013 · AF Fine Tuning... Torn on zoom. Help?
  
 
JeepTX
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p.1 #1 · p.1 #1 · AF Fine Tuning... Torn on zoom. Help?


Here's the story. I'd been having some issues with sharpness at 1.4 on my 50mm 1.4G Nikkor, so I grabbed a copy of FoCal (side note - pretty cool and easy to use sw). I ran through tests and got a good setting for the AF fine tune on my D300. +17. Wow, I think to myself, that's a lot; but who cares - focus is improved dramatically across all apertures. Awesome. Mission accomplished.

Then I decide to start in on my other lenses, figuring to do my 17-55 2.8 Nikkor next since it's my 'general purpose' lens. Off to the races, I run the tests at 17mm first and get a recommended setting of +1. Then I run the tests at 55mm and get a recommended setting of +14. Hmm, pretty wide range I think to myself, so I run the test in the middle at 35mm and get +14 again.

So now I wonder whether to favor 17mm or 35mm and beyond. I think I use 17mm more, but to make sure I jump over to Lightroom to check. Lo and behold, it's actually about 50/50 between 17mm and 35-55mm when I use the lens. Ugh! Now I'm torn on what direction to go.

Anyone else encountered a similar situation and have thoughts or advice on which path to take?

Thanks!!

Ed



Feb 19, 2013 at 04:26 PM
binary visions
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p.1 #2 · p.1 #2 · AF Fine Tuning... Torn on zoom. Help?


Consider your subjects.

What you're finding is that your autofocus is off. That's different from what in your image is sharp.

For my uses, at 17mm, you have a greater depth of field and are more likely (typically) to be shooting a wide angle image, possibly at a smaller aperture or focusing further away. I haven't done a statistical analysis but I'm guessing that at focal lengths smaller than 20mm, my aperture is usually f/5.6 or f/8 and focused closer to infinity a lot more than at focal lengths longer than 35mm. Thus, for my purposes, it'd make more sense to adjust for that longer focal length.

Additionally, with the greater DOF at shorter focal lengths, your room for error is a lot bigger. This is where it's important to understand what you're adjusting: the error of the autofocus, NOT the sharpness of your image (per se). Using a DOF calculator, a subject 5 ft. away, at 17mm f/2.8, will have a DOF of around 3 ft. That same subject, at 35mm f/2.8, will have a DOF of 0.66 ft. So if your AF is a little off at 17mm, it's unlikely you'll notice. If the AF is off at 35mm, suddenly it might be a problem.

The upshot... I'd set my AF adjust to give me the best results at the longer focal lengths unless I had an extremely strong preference for the shorter focal length in my shooting.



Feb 19, 2013 at 04:55 PM
Jan Brittenson
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p.1 #3 · p.1 #3 · AF Fine Tuning... Torn on zoom. Help?


JeepTX wrote:
Anyone else encountered a similar situation and have thoughts or advice on which path to take?

Is it under warranty? If so, send it to Nikon for service. There should be no variation across the range, the fact that there is indicates decentering. If it's not under warranty and you don't want to pay to get it fixed, tune it for the long end. Focus is more critical at 55mm than 17.



Feb 19, 2013 at 05:34 PM
Baywing
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p.1 #4 · p.1 #4 · AF Fine Tuning... Torn on zoom. Help?


There will be variation in af fine tune for zooms. I usually set mine to the long end and check the short end. If there is a lot of difference, I might bias the long setting a little to compromise. In the end, it's better than no adjustment at all. I'm using the FocusTune device and software and in the instructions it says that zooms will vary and they suggest tuning for the long end. As said above, you aren't likely to notice as much if the short end is a little off. You can add in another factor, temperature. I've noticed that my calibrations change slightly with temperature changes. If you calibrate inside at 70F and go out into 20F it will probably be off a little.


Feb 19, 2013 at 07:37 PM
 

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Jan Brittenson
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p.1 #5 · p.1 #5 · AF Fine Tuning... Torn on zoom. Help?


My 14-24, 24-70 and previously 16-85 DX had a ton of variation across the range. Sent them to Nikon and they came back spot on. Zero tune needed anywhere. The 14-24 and 24-70 both got a complete rebuild. I forget what they did to the 16-85, but it involved parts replacement.



Feb 20, 2013 at 01:10 AM
JeepTX
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p.1 #6 · p.1 #6 · AF Fine Tuning... Torn on zoom. Help?


Thanks for the great response! The comments on DOF considerations is great, as I hadn't yet considered that. Makes a great case for going with the longer focal length fine tuning versus the wide angle. I also just checked my lightroom library, and indeed many of the 17mm shots were at f4 or smaller, but a lot of the 55mm shots were more portrait-type at f2.8.

I'll give it a whirl with the +14 tuning for the longer end and see how it goes for a while!

Ed

binary visions wrote:
Consider your subjects.

What you're finding is that your autofocus is off. That's different from what in your image is sharp.

For my uses, at 17mm, you have a greater depth of field and are more likely (typically) to be shooting a wide angle image, possibly at a smaller aperture or focusing further away. I haven't done a statistical analysis but I'm guessing that at focal lengths smaller than 20mm, my aperture is usually f/5.6 or f/8 and focused closer to infinity a lot more than at focal lengths longer than 35mm. Thus, for my purposes, it'd make more sense to
...Show more



Feb 20, 2013 at 03:06 AM
JeepTX
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p.1 #7 · p.1 #7 · AF Fine Tuning... Torn on zoom. Help?


Jan Brittenson wrote:
Is it under warranty? If so, send it to Nikon for service. There should be no variation across the range, the fact that there is indicates decentering. If it's not under warranty and you don't want to pay to get it fixed, tune it for the long end. Focus is more critical at 55mm than 17.


No, not under warranty any longer - that lens and my D300 have been with me for quite some time. I had never tested it, because honestly I have never had any qualms with the performance and I still don't honestly.

The comment has me concerned, though. I'll go test my 70-200 2.8 and see if there's variation there. I've been 100% blown away by the quality of the 70-200 across all focal lengths, so will be interesting to see.

thanks!

Ed



Feb 20, 2013 at 03:10 AM





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