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03-08-2014, 03:21 PM   #1
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645 lenses on K-mount focal lenght

Hello!

I have the 120mm Macro 645 lens on a K-30 at the moment.
When I urn on the K-30 it asks for the focal lenght.
Now I wonder should I choose 120mm here or do I have to calculate 120*1,5 = 180mm or is the focal lenght, due to the thikness of the 645 to K adapter, the same. The manual of the adapter states that the focal lenght will be the same. So in that case I have to choose 120mm even on the K-30?

Thank you in advance
Szenario

03-08-2014, 03:26 PM   #2
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Use 120mm. Focal length = focal length = focal length. What changes with the various sized sensors is the angle of view, but the focal length remains the same.

Hope that helps....

03-08-2014, 04:02 PM   #3
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Original Poster
Thanks! I will do so.
But still I wonder... Every lens one uses on a cropped sensor will have another efective focal lenght then written on the lens.
Does that not apply on 645 lenses? Will the effective focal lenght not be 180mm?
03-08-2014, 04:57 PM   #4
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Whatever it says on the lens, it is. Even the Ricoh GR and the Sigma DP series, which are marketed by their 35mm equivalent fields of view, have the actual FLs on the camera bodies.

03-08-2014, 05:05 PM - 1 Like   #5
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QuoteQuote:
Every lens one uses on a cropped sensor will have another efective focal lenght then written on the lens.
No it will not. The image produced will have another field of view than the lens would produce on an uncropped sensor, but the focal length is a physical property of the lens and remains unchanged. "Effective focal length" is a very unfortunate misnormer; it makes sense to think in 35 mm equivalent terms because everyone knows what field of view to expect of a given focal length in this format, but any 120 mm lens will remain 120 mm on any camera. Whether the lens will cover the image circle partially (as with an APS-C lens on 645), fully (as with an APS-C lens on APS-C), or more than fully (as with a 645 lens on APS-C), is an entirely different matter...
03-08-2014, 06:01 PM   #6
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The lens is labeled with a focal length and is mounted on a camera. The lens projects an image on to the sensor. Depending on the size of sensor determines the actual image that you capture. Take this image below...



A smaller sensor will capture a small portion of the overall image projected by the lens. When you compare this to what a full frame sensor would see, the smaller sensor has what is referred to as a cropped view. The view is a small part of what is available to the larger sensor. The focal length of the lens did not change - only what the smaller sensor captured.

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