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03-08-2009, 02:05 PM   #1
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Why do some DA lenses have Autofocus switch?

I have been wondering why some DA lenses like the 50-135 and 16-50 have the A-M switch at all. It's not like any of the pentax bodies works like the Nikon D40 that doesn't have in-body motor (and therefore can only turn on AF on the lens) Am I missing something here? Are the switch just for convenience?

I am guessing that the Autofocus switch is actually an SDM switch... please enlighten me =)

03-08-2009, 02:12 PM   #2
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Some people like to manually focus, and it can be more accurate in some situations.
03-08-2009, 02:20 PM   #3
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The FA* lenses also had an AF/MF switch on the lens - on those lenses you push/pull the focusing ring in order to change mode.

I find the FA* system more convenient than having to look for the tiny switch on the camera body.

So perhaps the switch is just for convenience or perhaps the future holds a camera without built in focusing motor? On such a camera the DA* (and FA*) lenses would still offer manual focus.
03-08-2009, 02:53 PM   #4
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One advantage is that it allows "catch in focus" use with these lenses -
  • activate catch in focus (or whatever it's called) in the custom menu
  • put lens switch in Manual
  • leave body in AF
- hold down shutter release while focusing, camera will fire when it detects focus has been achieved.

03-08-2009, 04:50 PM   #5
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QuoteOriginally posted by SpecialK Quote
Some people like to manually focus, and it can be more accurate in some situations.
Yes, but there is already a switch on the camera body is what I think the OP was referring to. Why have both?
03-08-2009, 05:41 PM   #6
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usually it's on tele lenses, i assume that the longer the lens, a duplicate control can be handy
03-08-2009, 09:11 PM   #7
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Yeah it seems to make sense now since the Auto switch is closer to the top than the in-body one. My K100D's auto-manual switch isn't that easy to flip, probably because they dont want us to complain about it. Thanks for the answers everyone.
03-09-2009, 02:33 AM   #8
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Try using a lens like the DA* 300mm. If you want to quickly change from AF to MF without losing track of your subject in the viewfinder, the lens switch is extremely convenient.

You cannot do it using the body switch unless the lens ison a tripod.

03-09-2009, 04:24 PM   #9
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So you can leave the body switch on AF, and the lens switch overrides it?
03-09-2009, 06:04 PM   #10
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I don't know anything about the Pentax switches but the additional AF/MF switch on my Sigma 28/1.8 is nice in that it allows to decouple the focus ring from the focus gear in AF mode.

This means, you can have a nice grip on the lens because no focus ring turns while AF does its work.
03-09-2009, 06:10 PM   #11
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QuoteOriginally posted by Arpe Quote
So you can leave the body switch on AF, and the lens switch overrides it?
Yes, that's exactly how it works.
03-09-2009, 07:01 PM   #12
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QuoteOriginally posted by Class A Quote
I don't know anything about the Pentax switches but the additional AF/MF switch on my Sigma 28/1.8 is nice in that it allows to decouple the focus ring from the focus gear in AF mode.

This means, you can have a nice grip on the lens because no focus ring turns while AF does its work.
Now that's interesting =) why dont more lenses have this feature? Perhaps it would cost more to make...
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