# DotTune: Video Tutorial for AF tuning without photographs

Started Feb 18, 2013 | Discussions thread
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 Re: Kaj's question is a good one IMO In reply to TOF guy, Feb 21, 2013

TOF guy wrote:

The 1/3 -2/3 rule is far from general of course. But Kaj's objection certainly would apply IMO in any situation where the DOF does not extend the same in front and back of a target.

Example:

• Testing a 200 mm lens at 10 feet (about 3 meters). Distance of focus is centered (give or take a very small difference) in the range of distances within the DOF (rule of 1/2 1/2)
• Testing a 28 mm lens at 10 feet. Distance of focus is 1/3 away from the nearest distance within DOF and 2/3 away from the furthest (rule of 1/3 2/3)
• Of course there are many other ratios possible, up to "infinity" (far distance extends to "infinity")

For the typical distances used to test a lens one would most likely be within the range of a 1/2 1/2 rule (you're more likely to test the 28 mm with a target closer to the camera), but there may be a correction to apply in this test as Kaj suggests i.e. consider how far the DOF extends behind and in front of the focus plane and apply the necessary correction. See for instance this calculator(which I did use just prior to posting just to make sure ).

Very valid point The O/P should add a comment in his video IMO.

The center of the focus-dot confirmation range is a proxy for the where the camera itself would focus, so the balance of front/rear DOF should be the same vs. a body+lens combo requiring no fine tuning. In the 'additional notes' section of the video I discuss the option of using a 3-D focus target like a LensAlign for those who want to DotTune to a precise point within the DOF if they want control over the balance of front/rear DOF.

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