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04-08-2008, 06:36 AM   #1
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Dialing in focal length on K20

When you mount old manual lenses on K20 a menu with focal lengths appears on the monitor.
Should I dial in exact focal length or focal length multiplied by 1.5?
I wasn't able to locate that information in the manual but I think there shouldn't be any need for compensation on operators part since all manual lenses are old, hence FF, and camera can do the compensation. (Jeez, I'm glad my english teacher from highschool is not a Pentaxforums user).

04-08-2008, 06:42 AM   #2
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Focal length is always focal length.
04-08-2008, 08:40 AM   #3
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I actually had kind of the same question. I just acquired the superb Voigtlander 125mm Macro and 125 is not offered among the possibilities of focal length in the entry menu of the K20D. Should I enter 120mm or 135mm, which are the two closest options offered?
04-08-2008, 08:41 AM   #4
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QuoteOriginally posted by Pnuts Quote
I actually had kind of the same question. I just acquired the superb Voigtlander 125mm Macro and 125 is not offered among the possibilities of focal length in the entry menu of the K20D. Should I enter 120mm or 135mm, which are the two closest options offered?
I've found picking the closest, 120mm in your case, is the best.

04-08-2008, 08:42 AM   #5
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Thanks a lot for this piece of info, tybeck
04-08-2008, 09:06 AM   #6
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As egordon said: Focal length is always focal length. It is the same on crop or FF sensor. The difference is only in FOV, which is smaller (narrower) on a crop size sensor.
I had hard time with this 1.5 factor myself, until I mounted a lens on my 35mm film camera and looked through the viewfinder. Then it was all clear.
04-08-2008, 09:11 AM   #7
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I'm guessing this popped up because you had SR turned on, as it uses that information to try and give you a sharper picture. So how do you answer the question for a zoom lens? I rarely use SR, because I mostly shoot on a tripod, but when I use my TC it asks that and I never know what to put in, because I have a zoom, plus a TC, what number does it really want? If I put my 50-135 on a 1.7X TC, for example, does it want me to say 135, or 230, or something in between?
04-08-2008, 09:39 AM   #8
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QuoteOriginally posted by travis_cooper Quote
I'm guessing this popped up because you had SR turned on, as it uses that information to try and give you a sharper picture. So how do you answer the question for a zoom lens? I rarely use SR, because I mostly shoot on a tripod, but when I use my TC it asks that and I never know what to put in, because I have a zoom, plus a TC, what number does it really want? If I put my 50-135 on a 1.7X TC, for example, does it want me to say 135, or 230, or something in between?
There is no simple answer for zooms.
For maximum effect you need to change it every time you zoom in or out.
If you don't want to do that then the safest option is to use the lowest lenght you will use (weaker SR is certainly better than no SR, but too strong SR will just make things worse)
My guess is that multiplication with 1.7 would also be needed.

04-08-2008, 10:03 AM   #9
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If you are going to use SR, put in the lowest you are going to use. That would be the logic, at least, and I have experimented a little to confirm it, at least the SR goes total overdrive when set to ~200 on a 50mm Just handheld random tests, but the 50 performed OK enough with SR set to 77mm, although at a short distance.

If I had a 50-135 with a 1,7x TC, I would input ~90-100.

Edit: Very, very slow, started writing at the same time doing some other stuff.
04-10-2008, 02:41 PM   #10
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Thanks, I'll keep that in mind. I haven't used the SR often, because I am usually on a tripod, but if I do need it that will be helpful to know.
04-10-2008, 03:29 PM   #11
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QuoteOriginally posted by KjetilH Quote
If you are going to use SR, put in the lowest you are going to use.
SR is a counter movement of the sensor.

Let the amplitude of this counter movement be A. With a perfect SR system this means that the image without SR would shake with the same amplitude B=A.

The resulting shake has amplitude C=A-B (=0)

Now assume that real focal length f_b and dialed in focal length f_a differ, then

C = A-B = a f_a - b f_b = a (f_a - f_b).

This is minimal if you choose f_a in the middle of the focal range you are going to use.


However, the efficiency of SR is easily reduced from 3 to only 1 step then. But still better than off for less than 4x zooms.
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