Richard

There are a couple of reasons for testing. Tests have shown the AF Fine Tuning adjustment is different at different focal lengths - even being positive at one focal length and negative at another for the same lens.

My guess is you will want to test at various focal lengths in order to understand the optimal adjustment. For example, you might find with a 24-70 lens that there are different values at 30mm, 50mm and 70mm. Not only is the value of the adjustment important, but it is important to understand what nearby values might produce. You are better off testing at multiple focal lengths and then evaluating the results of a single AF adjustment applied to all the focal lengths.

The other issue is the distance to the target. We've seen some variation in results based on distance to the target. Here is where you most common distance to the subject might make sense. LensRentals had a thread about the dramatically different results they got testing a macro lens at non-macro distances. Likewise a telephoto does not need to be tested at 25 feet when you use it for photographing large mammals at 150 feet.

Eric Bowles
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