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by garik

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Many of the lens reviews tend to comment on their autofocus performance, but they seem to be mostly subjective in nature (fast, slow, hunts around etc). I understand that the autofocus performance is also a function of the camera body being used (the number and type of AF points vary greatly), along with the lighting conditions. AF servo performance is another function that would be useful to quantify, particularly for action photography.

However, if we were to keep these factors (same camera body, standardized lighting) a constant, would it be possible to measure and quantify the autofocus performance of a lens?

Also, on this note, which influences the AF ability of a camera more - the lens or the body (say the Canon EF-S 18-55mm kit lens on a 7D vs the EF-S 17-55mm f/2.8 on a 600D)?

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@mattdm The question you have linked to seems to be complementary - camera related vs lens related. Does it make sense to merge the two, as there doesn't seem to be any standard measurement available for either. –  ab.aditya Dec 15 '11 at 10:13

1 Answer 1

Speed

The best value to compare the autofocus speed is to know how long it takes (attached to your camera body) to move the focus lens from the minimum position to the maximum position.

Detection

The most difficult part of comparison is to measure the autofocus-detection-speed. This strongly depends on the camera body you use, so you need to try it or find a review about it.

Before buying

Before buying (or even comparing) an autofocus lens, you should think about it in greater detail. For most photography (according to my experience) you do not really need a very fast autofocus, so the question of speed defeats its purpose. Even if you want to do sports or wildlife photography where you need really fast autofocus, you should be satisfied with nearly every lens that has a silent-wave motor.

Also, on this note, which influences the AF ability of a camera more - the lens or the body

This strongly depends on the lens you want to use, some are slowed down by the camera body and some are slower than the body could be.

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