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| DotTune: New AF tune technique - looking for testers |
It's free, it's fast, and it's much more accurate than any other method I've seen so far. What makes it faster and more accurate is that it doesn't rely on actual phase-detect focusing to derive the AF tune value. Because PDAF has shot-to-shot variability, all existing AF tune techniques require multiple images taken at each tune value to eliminate that variability from the equation. And it's the need for multiple shots that make those techniques more prone to AF tune measurement error. The concept is based on a recent discovery about the true nature of AF tuning - you can read about that here (I'm "horshack" on dpreview).
I developed and tested this method first on a Nikon D800, and it has been confirmed by several other owners so far (Nikon thread here). I've also tried it on my 5DM3 with a Sigma 35mm and it appears to work. I'm looking for feedback from other Canon owners to confirm it on their bodies+lenses.
Canon version of technique
1. With the camera mounted on a tripod, enter Live View and establish critical focus on a high-contrast subject over the center point. Exit Live View.
2. Set the lens to MF (Manual Focusing). Be careful not to jostle the focus ring when you move the switch - you don't want to alter the focus you established in step #1.
3. Set AF tune to 0.
4. Look through the viewfinder to see if the camera thinks the subject is in focus by checking if the green focus confirmation dot is lit. You'll need to half-press the shutter (or AF-ON) while doing this to keep the metering/rangefinder from going to sleep. Assuming you get confirmation, increase the AF tune value and recheck the viewfinder, leaving the lens set to the original focus you established in step #1 - do not refocus the lens. You want to keep increasing the AF tune value until you no longer get a green confirmation dot. As you reach the edge of the range the green dot will either fail to light up or it may flicker...you may need to cycle the half-press of the shutter several times to confirm that the confirmation you get is consistent. You're done when you find the highest positive AF tune value that gives you a consistent green-dot confirmation - that value represents the positive end of the AF tune range.
5. Return AF tune to 0 and repeat the procedure in step 4, but this time decreasing the value of AF tune (going negative). When done you've established the negative end of the AF tune range
6. Calculate your final AF tune value, which is the midpoint of the range you established in steps 4 and 5. For example if your range was -2 to +8 then the midpoint is +3. If your range was 0 to +12 then the midpoint is +6. Set your AF tune value to your calculated midpoint. You're done.
* If you don't get a green confirmation dot at an AF tune of 0 then increase or decrease the AF tune value until you do. Your goal is to find the range of AF tune values that show focus confirmation - that range may be all positive or all negative, instead of the -/+ range described in the steps above.
* If either end of the AF tune range (-20 or +20) still gives you focus confirmation, this means the range of AF tune values is wider than the camera's supported +/-20 AF tune range. Marianne Oelund came up with a clever workaround for this here.
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"