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Archive 2013 · AF fine tune..? confused to say the least :)
Dustin Gent
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p.1 #1 · AF fine tune..? confused to say the least :)

Ok, so I have been following the thread about the Sigma 35mm 1.4 that is amazing and now I want one . Then I read another thread about the said Sigma and some fine tuning.

Forgive me for my lack of knowledge on this subject. I have not had an AF lens for several years and just picked up an 85mm 1.8G last month to use with my D700.

My question is how would I know if it needs to be fine tuned? I used the 85mm a few times so far - and wouldn't be surprised if it needed some sort of tuning - but I am too dumb to know for sure

Thanks for any help!!

Jan 22, 2013 at 08:58 PM
binary visions
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p.1 #2 · AF fine tune..? confused to say the least :)

If you're getting consistently sharp focus in the areas you're focusing on, it probably doesn't need tuning.

The only way to know for sure is to carefully test it using a static subject and a tripod. You can use one of the test charts that you angle at 45 degrees and then focus on the midpoint to see how far off it is. Alternately, you could just shoot a series of test shots: shoot with the AF fine tune at zero, then shoot at +5, then at -5, +10, -10, etc. and see which one is the sharpest.

There are more sophisticated programs like FoCal which will automate this process for you.

If you aren't having problems... I tend to not worry about that kind of thing.

Jan 22, 2013 at 09:35 PM
binary visions
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p.1 #3 · AF fine tune..? confused to say the least :)

If you'd like to just do a quick gut-check on it, you could set up a test chart or a piece of newspaper or something, put your camera on the tripod, and use a zoomed in live view with manual focus to get a perfect focus. Take a picture.

Then, spin your focus ring to defocus it, and allow the camera to autofocus. Take a shot. Compare the two: are they the same sharpness?

Just make sure you're doing all of this on a sturdy tripod with the test subject in a fixed location so you don't have other variables creeping in.

Jan 22, 2013 at 09:38 PM
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p.1 #4 · AF fine tune..? confused to say the least :)

The current manufacturing tolerances of bodies and lenses are not tight enough for critical focus with very fast lenses. You can end up with a top tolerance body and top tolerance lens and you will get back focused pictures for example.

Jan 22, 2013 at 09:55 PM

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p.1 #5 · AF fine tune..? confused to say the least :)

Just use it. If you're getting sharp pictures, it doesn't need tuning, if you muck around with it you're just as likely to make it worse

The 45 degree ruler test can be misleading because the actual AF point can be in a slightly different location to the mark in the viewfinder. Also, the AF can have trouble focusing on a 45 degree object.
- for testing, you are better off focusing on a flat subject but also include a 45 degree object in the frame so you can see where the sharpest focus ends up (this is how the LensAlign test target is set up)

Another way to do it is to set up several standing targets (such as AA batteries) at staggered distances from the camera. AF on the middle one and see if a different one ends up in focus

Make sure your target is at about your 'normal' focusing distance for the lens (ie don't tune the focus for ultra closeups when you normally use the lens at 10 feet from your subject)

Jan 22, 2013 at 10:30 PM
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p.1 #6 · AF fine tune..? confused to say the least :)

I always recommend staying away from the close-range 45 ruler test, unless you're using the lens for almost exclusively near-macro or close-up shooting. The focus distance is to short.

Do a controlled test from tripod at the lens's normal usage distance, in normal usage light. That's where you see the real-world implications of focus errors for you.

Jan 22, 2013 at 11:26 PM
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p.1 #7 · AF fine tune..? confused to say the least :)

And always shoot lens wide open for fine tuning / focus testing so you get the shallowest depth of field possible to see where the focus plane is.

Others seem to have good luck out of the box, but it's very rare for me personally to find a lens / body combo that does not benefit from fine tuning. Very worthwhile and quite easy to do, no special tools or software needed at all.

Check out the instructions here most of the way down the first page - I'd use 7 feet minimum distance for your 85mm lens.


Jan 23, 2013 at 12:09 AM

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