D7000 Autofocus Fine Tune: Is the EXIF FocusDistance Parameter Useful?

ggbutcher

So, I've taken a few pictures with the new camera, and it may be me or it may be the camera, but some shots didn't seem to focus well, So, I've been all over the forums, reading about folks' AF experiences with the D7000. I've reviewed about 5 general categories of AF evaluation, downloaded all sorts of pdf target images, and done some ad-hoc testing of my camera that would indicate my D7000/18-200mm has a slight backfocus difference in the lower focal lengths.

I'm going to do a slight adjustment based on that to do Christmas shooting, but I'm going to do a more rigorous test after the holiday. In doing so, I'm wondering if the difference between the distances reported in the EXIF FocusDistance parameter for a LiveView-focused image vs. a AF-focused image would be useful for quantifying the correction needed? Here's one example from my ad-hoc testing, at 18mm, data extracted with exiftool from the NEF:

LiveView: 0.67 m
AF: 0.79 m

Diff: about +4.7 inches.

I shoot the LiveView image first, then turn off LV and shoot with AF; in doing so, I see the focus ring shift when I half-press the shutter release to take the AF picture.

I've searched for anyone doing something similar, but all I find is about how FocusDistance is an inaccurate measurement of actual distance. For this purpose, however, I'd think that the difference between the two measurements (LV, AF) would be appropriate to use for AF fine-tuning, given that the two focus mechanisms are acting on the same device. I'm also thinking of developing a table of difference measurements for each fine-tuning step; I can't find anyone who's done that, which probably means it's a dumb idea for some reason (non-linearity, differences in camera-lens magnitudes, etc)...

I know that AF fine tune and the "D7000 problem" have been beaten to death, but I can't find any prior discussion along these lines. Comments and criticism are welcome here...

Ironheart

It's not a problem, it's a fact of life for interchangeable lens systems, and not unique to the D7000. Remember that AF fine tune is global, meaning if you set it for the shorter focal length, it may throw your longer length focus off.

When Nikon adjusts focus for a zoom lens, it has the ability to adjust the focus at several (perhaps as many as a dozen) focal lengths with their proprietary firmware and via hardware at the service center. The super zooms are the most tricky when it comes to this. Is the lens older? Did you buy it with the camera? In any event your best bet is to take the camera and lenses to a sevice center and have them calibrate the lot. AF fine tune will possibly help, but may not get you all the way there.

ggbutcher

@Ironheart: Yeah, the 'global' part is challenging, as there is no shift in the focus ring at the longer focal lengths. The lens is about 2 years old, bought for the D50. WRT the service center, that's good to know. For the forseeable future, I plan to use only the 18-200, so getting it calibrated to the camera by a technician would probably be worth it.

I'm going to keep the question open, 'cause I'm interested in what folks know or think about using EXIF FocusDistance for AF fine-tuning...

Nuke

My experience with that number in the EXIF has not been great. Not bad for short distances but as you get more and more distant, it becomes more and more inaccurate.

The number is also based on what lens is being used and comes from the lens. Something like an UW thinks that anything beyond 12 feet is at infinity.

funtagraph

It's not only the focal length but also the distance which can cause AF problems and increase the difficulties to solve them with only one AFA setting. In general, the super zooms are less critical due to comparatively slow apertures and therefore a little bit more DoF. Nevertheless there's only one correct setting of the AF. Sigma adressed this with 4 different distances to set up and if it's a zoom, those 4 distances at 4 different FLs = 16 different settings (and tons of test pictures). It's very time consuming and there is still the slight possibility of an unreliable camera based AF anyway. So, with critical shots, it's only LiveView to be trustworthy. Or a more decent Af module than the D7000 has, although I don't want to spoil your pleasure. With super zooms, there' s no free lunch.

ggbutcher

I did some messing around with setting AF fine tune increments and inspecting the FocusDistance; it really didn't line up to the difference seen when I would do LiveView on, manual focus to infinity, shutterbutton 1/2 to focus on the target, LiveView off, then shutterbutton 1/2 to AF focus, and observe the lens distance ring movement. Doing this iteratively through AF fine tune settings got me to -4, which produced no/minimal adjustment at 200, 70, 35 and particularly 18mm zoom. Oh well. But, I'm ready for Christmas...

Golf007sd

I highly recommend doing what Ironheart has suggested. Hence, send it to Nikon. Moreover, I would send in all your lenses that you would consider using on your D7000. Just make sure you have it pack properly so that you don't have to deal with the BS of Nikon saying it was damaged due to "impacted."

Should you still consider doing the AF Fine tuning yourself, I will email you a file that might help.

haroldp

Canon allows AF fine tune for long and short FLs for zooms, hopefully Nikon may follow suit, probably a firmware change if they choose to.

.... H

The_Other_Steve

Yes, I've been down this road with the D7000. Remember your short focal length will have more DOF.

Anyway, the solution I adopted was the moire method I mentioned previously, then to simply be aware of the issue and to make a shooting judgement. For artless documentary stuff, you can give up ISO or choose more flash to get a better DOF and safety margin.

It's something that Canon has over Nikon, a short and long AF adj.

Msmoto

One issue for me when using AF is the necessity to get the focus system working....i.e., to either trigger AF by pressing the release halfway, or the AF button, and be certain this is done a half second or so prior to shutter release. Sometimes i will just hammer the release button and when this is done I get some images not accurately in focus. On the other hand, when i use the "pre-focus" by pressing halfway, and keep the focus active by consistently pressing half way on and off when viewing my subject, I find the accuracy of the AF system is much higher.

Have a great Holiday.