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Old Jul 12, 2010, 1:51 PM   #1
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Default Question about using manual lens with a Pentax K-x

Ok, so I just bought my K-x this weekend and already had a couple of older macro lenses that are k-mount. I'm getting good results but unclear on one area - The "Input Focal length" prompt that comes up when I turn the camera on. I'm probably overthinking it, but humor me. Do I use the low end, the high end, or the 35mm equivalent here? I understand this mostly has to do with the anti-shake feature but read the manual and still don't have a clear picture.

If I have a 50-200mm macro lens, when it prompts I should select 200 for example, right? Also, if I understand correctly, with the equivalent multiplier, I'm technically getting 300mm out of the lens?

Thanks for any input/clarification.
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Old Jul 12, 2010, 3:00 PM   #2
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depends what focal length you are using it at - if your using the zoom at 200 mostly put in 200 - if your using it at 50 put in 50 - same for anything in between
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Old Jul 12, 2010, 3:22 PM   #3
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Interesting... So I should change that setting each time I move and re-zoom, ideally? I tried it both ways on Saturday and didn't notice much difference, but I was outside in bright light... Mostly it just affects the anti-shake, correct?
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Old Jul 12, 2010, 6:09 PM   #4
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Yes, it affects that anti-shake, the only reason you need to put it in. It is nice when looking at the exif information, if you have more than one manual lens.

If your shutter speed is pretty high I don't think it matters all that much. For macro I'll sometimes "cheat" and put in a longer focal length (i.e., my macro lens is 105 and I usually put in 100. But sometimes I'll put in 125 or something along those lines if I'm doing 1:1 macro or using an extension tube or something). I don't think it would make that much difference, and if I remember from my instruction book from the K100, it said it was OK to put in the longest focal length of the lens. It would make a difference if you were shooting at 25mm and you had entered 300 or the other way around, I think.
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Old Jul 13, 2010, 5:36 PM   #5
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Here is a link to the best guide for using Pentax manual lenses I have seen www.robertsdonovan.com/?p=1181. It got me started with a Vivitar 24mm prime. A tip I use all the time with manual lenses is catch-in-focus. It works great on the Kx.
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Old Jul 28, 2010, 10:16 PM   #6
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This question is asked a lot at PentaxForums.Com and the usual answers are:

* Set the INPUT FOCAL LENGTH to the maximum FL of the zoom; or
* Set it midway between the minimum and maximum of the zoom; or
* Set it near the focal length you intend to use most that time; or
* Don't set it; use a fast shutter or a tripod with any manual zoom.

Macro is even trickier. Nominally, you would enter the lens's focal length PLUS any extension distance. A 50/4 macro at 1:1 magnification has 50mm of extension, so the INPUT FOCAL LENGTH would be 100mm. But the manual say that SR doesn't work as expected with close subject.

So the definitive answer is: try it yourself and see what works.
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