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Old Jun 23, 2006, 5:15 PM   #1
chrisdsa's Avatar
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In one of the many preview articles on the Sony A100, there was one that mentioned that the autofocus was built into the Sony body and the lenses did not have as much electronics. This apparently made the AF in the Sony/KM lenses "slower" than those like for example on the USM canon lenses.

Can someone explain the significance of this... assuming that it is true?

Also is that why the Canon USM lenses are so much more expensive ?
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Old Jun 23, 2006, 5:43 PM   #2
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You really need to take each lens on a case by case basis when comparing focus speed, whether or not the focus motor is in the body or the lens.

Nikon and Minolta opted to go with an in body focus motor with their cameras.

But, both manufacturers also have some lenses with focus motors built in. Nikon's AF-S lenses have a built in focus motor. Ditto for Minolta's SSM lenses. These are similar to Canon's USM lenses.

But, Minolta only has a couple of lenses with SSM now. So, most are reliant on the in body focus motor. However, looking at focus speed comparisons, I sometimes see reports (in controlled conditions) that a model like the Konica Minolta Maxxum 5D can focus faster and in lower light compared to entry level DSLR models from Canon, Olympus, Pentax and Nikon.

For example, the Konica Minolta can focus twice as fast as a Rebel XT in EV 0 lighting, and the Canon can't focus at all in EV -0 lighting (where the Konica Minolta can still focus), using a 50mm f/1.4 on both models. ;-) The Konica Minolta's closest competitor with entry level DSLR models is the Nikon D50 as far as focus speed goes. The others are left behind, despite focus motors in the lenses. I suspect the Sony A100 will test even faster (more refined algorithms). We'll have to wait on controlled tests to find out. But, I think the AF sensors and focus motor are likely to be the same as in the 5D.

Sometimes the noise/vibration from in body focus motors makes it seem like AF is taking longer than it is compared to lenses with quieter focus motors built in. But, if you measure AF times, most Minolta lenses of equivalent quality are pretty fast on KM DSLR bodies compared to the competition.

How a lens is geared plays a big role, too. If more revolutions of the focus motor are required, then focus takes longer. An example of a slower focusing lens would be a macro lens (where it's geared for much finer focus precision).

The brightness of a lens also plays a big role. The brighter a lens, the more light reaches AF sensors to help them "see" better for focus purposes. So, don't use a dim lens in low light if you want fast Autofocus, regardless of camera brand.

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