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| DotTune: New AF tune technique, no photos required |
Forgot to post this on the Nikon forum here. It's a new AF tune technique that is fast, accurate, and doesn't require you to take any photographs or even to use AF.
My original dpreview thread describing the method is here (I'm "horshack" on dpreview). That thread includes the background behind the technique.
1. Enter Live View and establish critical focus on a high-contrast subject. For cameras with poor LV clarity (D800/E), you may want to take a photograph and evaluate it on the computer to confirm critical focus. I recommend the focus chart here.
2. Leave the body and lens set to AF and configure your body for back-button focusing if not already configured so. This is necessary to disable AF engagement for shutter half-presses, which you'll be using in steps 3 and 4. To configure back-button focusing, set the "AF Activation" option to "AF-ON only". For the D4/D800 this is option a4, the D3/D3s/D700 it's option a5, and for D7000 it's option f5 (for D7000 AE-L/AF-L button will serve as the AF-ON button). Do not set the body or lens to MF as an alternative to back-button focusing; doing so will increase the confirmed focus range and make DotTunning inaccurate.
3. Look through the viewfinder to see if the camera thinks the subject is in focus. You'll need to half-press the shutter while doing this to keep the metering/rangefinder from going to sleep. If you see the green arrow pointing to the left, then the camera+lens is back-focusing, so you need to decrease the value of AF tune (negative adjustment). If you see the green arrow pointing to the right, then the camera+lens is front-focusing, so you need to increase the value of AF tune (positive adjustment).
4. Repeat step 3 until you get a green-dot from the rangefinder.
5. Depending on focal length and subject distance there will be a range of AF tune values which produce green-dot confirmation. For example on my D800 w/50G f/1.4 @ 5' subject distance, I get the green dot from -6 to -14. Conceptually the optimal AF tune value should be the value that's the middle of the range, which in the above case is -10. So you'll want to establish the range of green-dot values by progressively increasing the AF tune values until the camera stops showing the green-dot (it'll alternate between the green dot and an arrow when it's at the margin of the range)...and repeat that procedure in the opposite direction to find the other end of the range. Use the midpoint of that range as your final AF tune value.
When establishing the precise range of green-dot AF tune values you may need to examine the rangefinder for several seconds. I've found that for the extreme margins of the AF tune range the camera may take a few seconds to flicker the arrow/green-dot.
The canon version of this technique is here, which can also be used on Nikons.
Edit 2/10/13: Decided to call this technique "DotTune", to have a quick and searchable moniker when referring to it. It's a bit more identifiable than just "new AF tune technique"
Edit 2/17/13: I've created a simple midpoint calculator on my website. DotTune Midpoint Calculator
Edit 2/18/13: I've published a DotTune tutorial on YouTube. DotTune Youtube Video