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12-23-2015, 04:09 PM   #1
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How to describe a manual lens.

When my DSLR asks for the "focal length" of a manual lens just fitted, do I use the 35mm equivalent designation - eg 135 for my 135mmf/2.8 Takumar or the 35mm size multiplied by the crop factor, which for 135mm might be 135 x 1.5, or in the case of a model Q would be 135 x 4.7 ?

12-23-2015, 04:13 PM   #2
Imp
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You just input what the focal length of the lens is. The focal length is always the same, the only thing that changes is the crop factor, or the field of view. Your camera if a crop camera, so it knows how to account for the focal length. A 135 FF lens on aps-c is the same as an aps-c specific 135 lens, in regards to the camera stabilization system.

Last edited by Imp; 12-23-2015 at 04:54 PM.
12-23-2015, 04:21 PM   #3
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the focal lenght doesn't change, only the field of view in you image! See multiple threads about the crop factor in pentax forum. A 50 mm lens will always be a 50mm lens. This you have to tell your camera to keep shake reduction happy! See exif data of your modern lenses: 200mm is reported as 200mm aka 300mm in 35mm film equivalent!
12-23-2015, 10:50 PM   #4
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QuoteOriginally posted by danmdan Quote
When my DSLR asks for the "focal length" of a manual lens just fitted, do I use the 35mm equivalent designation - eg 135 for my 135mmf/2.8 Takumar or the 35mm size multiplied by the crop factor, which for 135mm might be 135 x 1.5, or in the case of a model Q would be 135 x 4.7 ?
Whatever it says on the lens


Adam
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12-24-2015, 05:43 PM   #5
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QuoteOriginally posted by Adam Quote
Whatever it says on the lens

What if is a zoom lens?


When I have put a manual zoom on my K7, there is nothing about focal length for zooms.
12-24-2015, 05:46 PM   #6
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QuoteOriginally posted by Adam Quote
Whatever it says on the lens
Thanks for all your replies. My K20D Pentax is my first ever DSLR, and has a lot of mysteries. For budgetary reasons I'm using "old glass", cheap to buy manual lenses from the distant past ! And never had a camera which asked me questions before.

A good additional question above about what to say for Zoom lenses ?
12-24-2015, 06:15 PM   #7
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QuoteOriginally posted by bigdavephoto Quote
What if is a zoom lens?


When I have put a manual zoom on my K7, there is nothing about focal length for zooms.
Personally, unless I know I'll be using it at one of the extremes (ie, 70mm or 200mm on a 70-200) I tend to turn SR off entirely and just compensate with faster shutter speeds. I found my IQ comes out better overall that way.
12-24-2015, 10:02 PM   #8
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QuoteOriginally posted by bigdavephoto Quote
What if is a zoom lens?
QuoteOriginally posted by danmdan Quote
A good additional question above about what to say for Zoom lenses ?
Input the focal length you're shooting at or close to it. Conventional practice is to err a little on the short side if you are shooting a range of focal lengths.


Steve

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