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10-25-2010, 09:00 AM   #1
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645 lens to k mount questions

I've been searching back through the treads on this and haven't quite found the right answer. So here goes:

I've recently purchased the original Pentax 645 camera and some A lenses. I've also picked up a 645 to K mount adapter. What I've been unable to determine is does the adapter play a role in the field of view/equivalents of lenses?

Ole's post here is a help and I get how this works. I'm assuming that it's just that simple but if the adapter plays a part in this I can't figure that out.

Any help here? Or does anyone have a quick and dirty simple rule to convert 645 focal lengths to APS-C equivalents?

Oh, and one more question. What value should be entered in my K10d for the equivalent focal length? I'm guessing it should be the actual length of the lens but I'm not sure either.

Last edited by blackcloudbrew; 10-25-2010 at 09:12 AM.
10-25-2010, 09:27 AM   #2

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Putting a 645 or 6x7 lens on your APS-C will be the same crop factor as putting the same focal length in a FF 35mm lens on your camera. After all, they are the same focal length. You enter the focal length noted on the lens when prompted by the camera. But of course your choices are based on common 35mm focal lengths and not the ones you may encounter in medium format. In that case, I guess, you select the nearest option.
10-25-2010, 09:43 AM   #3
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About a .6:1 ratio (62/100)

I built a M645 - PK adapter using lens and body caps to use my Mamiya's on PK bodies. Fun but no IQ advantage.

Here's a coupl'a common examples (from the Mamiya M645 lens catalog):

645 to 135

35 = 22
45 = 28
55 = 34
80 = 50
100 = 62
110 = 67
120 = 73
150 = 93
200 = 124
210 = 130

I've found it helpful to think of this issue "backwards" and consider how much of the fixed angular FOV (for lack of the correct term) of the lens is 'intercepted' by a given sensor/format.

EG., no matter what the application, the angular FOV of a 50mm lens is always the same, but the space the sensor occupies within the FOV depends on the format ratio and distance from the base of the lens which effectively "crops" the FOV.

Not a perfect example, but it works for me as "useful fiction" anyway. The same thing applies with extension rings or bellows extension.

Likewise, if the angular FOV is fixed, the amount it "wiggles" is also fixed so the SR setting is dependent on the actual FL of the lens.


Last edited by pacerr; 10-25-2010 at 10:01 AM.
10-25-2010, 09:55 AM   #4
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Original Poster
Thanks for the responses. I think I was over analyzing the problem. I just got my adapter this morning and was trying to rationalize the field of view on the Pentax A-75mm lens when put on my k10. This helps.

10-25-2010, 12:26 PM   #5
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The field of view on a K10D is near identical for the following lenses.
- 6x7 75mm 4.5
- 645A 75mm 2.8
- FA 77mm Limited
- DA 70mm Limited
- DA* 50-135 @75mm

The crop is not the lens, it is the sensor-size.

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