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08-08-2014, 01:09 PM   #1
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Focus limiter

Hi everyone ,

Is there a way to limit the focusing "search" on a lens.

Main reason is that when I try to capture birds in flight, I often end up searchin with autofocus searching from one end to the other....

Would be nice if I could limit the autofocus to, let's say 40 feet to infinity

Thanks

08-08-2014, 01:13 PM   #2
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Depends on the lens. Some lenses do have limiters, 100mm macro comes to mind but it would be a lens function, nothing you can do with the camera if the lens is not already capable of it.
08-08-2014, 01:39 PM   #3
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How does the autofocus worc on screw drive ? Does it just hunt till the screw CANNOT turn anymore then just screw the other way ?

Would ibe harmful if I were to put some kind of blocking gadget on that..... somehow ?
08-08-2014, 01:41 PM   #4
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That's pretty much how it works. You couldn't put any kind of blocking gadget on it without modifying the lens.

08-08-2014, 02:23 PM   #5
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QuoteOriginally posted by devouges Quote
How does the autofocus worc on screw drive ? Does it just hunt till the screw CANNOT turn anymore then just screw the other way ? Would ibe harmful if I were to put some kind of blocking gadget on that..... somehow ?
Not exactly, it seems to work back and forth before it locks on, and if the initial back and forth finds nothing in focus then it racks the lens all the way out and back. Which is fine for most things but BIF causes problems.

Blocking the lens screwdrive from turning all the way would be considered a "bad thing". As in either stripping gears in the lens or ruining the motor in the camera. Bad idea.
08-16-2014, 06:25 AM   #6
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One way around it is to use the Pentax SMC-F 1.7x autofocus adapter (AFA) in combination with a fast telefocus lens. When you set the telefocus lens to infinity, the AFA will do the focusing, but with a limited range. It works like a charm, although often the sweet spot for a bird in flight is a tad closer than the minimum focusing distance. This means you have to adjust the focus on the telephoto lens to something closer than infinity, but the closer you get, the more narrow the focusing range of the AFA gets, which means a lot of times you'll have the bird flying just outside the range. With a lot of practise you'll learn to anticipate this, but it can still be a bit cumbersome.
08-16-2014, 10:31 AM   #7
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Thanks..... I will look into that
08-16-2014, 11:11 AM   #8
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I reset focus to infinity or at least beyond my expected target after each series of shots. Pentax cameras almost always start racking in when looking for focus.

When your camera misses the target, rather than letting the lens rack all the way and return, use the Quick-Shift feature (for lenses that allow Q-S).

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