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Old Apr 25, 2010, 7:52 AM   #1
Join Date: Mar 2010
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Default Why do lens reviews complain about inaccurate focusing?

Maybe I'm misunderstanding how SLR focusing works, but I'm confused as to why comments about focusing are in lens reviews instead of body reviews.

"Fast" focusing, I can understand... some lenses certainly have faster focusing mechanisms than others. But why complain that the focusing "missed" or "hunts" in regards to the lens? Isn't that the cameras job?

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Old Apr 25, 2010, 8:09 AM   #2
Join Date: Jun 2003
Location: Savannah, GA (USA)
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When looking at those types of reviews, I'd take into consideration the body/lens combination being used and how well they work together for Autofocus Speed/Accuracy.

In addition to lens gearing and AF motor speed, many things can influence how well a given lens focuses on a given camera model. How bright the lens is can be very important (because a camera's Autofocus Sensors can "see" better to focus when a brighter image gets through).

How sharp the lens is (and how contrasty a lens) is at wide open apertures can also be important to AF speed and accuracy (because a modern camera always focuses and meters with the aperture wide open, only closing down the aperture to the selected f/stop when the shot is taken). So, if you have a sharper image with more contrast being seen by the AF sensors, you're more likely to see accurate focus.

How compatible a lens is with a given camera model also enters the equation, as a camera's AF system may try to move the focus (either via screw drive or via electrical signals) by a certain amount and the lens may not move the focus by the amount the camera thought it would move it. It's not uncommon to see compatibility issues with third party lenses requiring "rechipping" of them to work better with some cameras, because they're either misinterpreting the commands and moving their focus too little or too much, or not reporting things like AF distance and Focal length correctly to the camera body (since many Phase Detect type Autofocus Systems are designed to take Depth of Field into consideration when focusing quickly).

Sometimes those types of issues with how well a lens works with focus changes can vary at different focal lengths, too (where you may start seeing back focus or front focus issues at one end of the zoom range or the other, versus consistently back focusing or front focusing with a given camera model). You can also have issues with the way an optical system is designed, where focal length may shift some with focus changes.
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