EF 100-400mm f/4.5-5.6L IS USM or equivalent off brand

Started May 18, 2012 | Discussions thread
R2D2
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Look at it this way...
In reply to Mike Engles, Jun 3, 2012

Mike Engles wrote:

I cannot see how a lens with no built in sensor can give up focussing in tricky situations. It is the AF sensor in the camera that gives up, generally because there in no detectable contrast in the scene, for it to focus on.

You are right of course that it is indeed the Camera that calculates focus, and either locks on or gives up depending on the amount of contrast present.

However, some lenses pass better image information (contrast) to the AF sensors than others. This is a well-established fact, and can be a determining factor in how well a camera/lens combo will AF.

I've seen this first hand when using different lenses in the same shooting situations. Some will simply AF faster, more accurately, and more decisively than others. Same camera body.

If you still don't believe that a lens can affect the body's AF, then try this: take a lens that is a fine autofocuser, and put a terrible UV filter on the end (or rubber-band a piece of plastic wrap over the end of the lens), then see how well the combo will autofocus in those tricky AF situations.

In real life, all lenses have their strengths and weaknesses. One of those is AF ability. And whether or not this is an issue for a photographer depends entirely on their own specific shooting requirements. And mebbe the OP will prefer giving up AF for Reach in some instances. I do that myself on occasion.

R2

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