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01-11-2010, 08:49 PM   #1
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What's this doohickey on my A*400mm?

See the photo: I'm referring to the ring with the knurled knob with the red "O" in the center. Can someone explain what it's for and how to make use of it?

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01-11-2010, 08:59 PM   #2
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Is it a focus limiter ?

I think I had a similar screw on my Tamron Adaptall 300/2.8 that I could use as a focus limiter.
01-11-2010, 09:06 PM   #3
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Or perhaps even a focus lock screw.
Only one way to find out - screw it tight and try to move the focus ring...
01-12-2010, 05:03 AM   #4
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Its a focus stop.
It's so that you can return to a specific focus distance quickly and easily.

01-12-2010, 06:39 AM   #5
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QuoteOriginally posted by Wheatfield Quote
Its a focus stop.
It's so that you can return to a specific focus distance quickly and easily.
Ah, yes, I see. It's a soft 'stop': you can adjust in either direction but when you get to the selected focus distance, it gently clicks into place. I can imagine situations where that might be handy. Thanks!
01-12-2010, 07:25 AM   #6
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Beautiful lens!
01-12-2010, 10:12 AM   #7
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QuoteOriginally posted by Douglas_of_Sweden Quote
Beautiful lens!
It is, Douglas! As you can tell I'm just getting acquainted with it. I'm unworthy of it in terms of skill (actually, that's true of a number of my lenses ;~) but I have aspirations.
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