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05-24-2013, 03:51 AM   #1
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Does the 67 to 645 adaptor change the field of view

I haven't been able to find a discussion of this on line. I would guess that increasing the distance between the front of the lens to the film plane would change the field of view slightly. Does any one know of a chart showing what change if any the adaptor introduces? In other words, will a 100mm 67 lens act like a 95mm 645 lens when used on the 645 or vice versa, or is there no change at all? Thanks Norm

05-24-2013, 05:30 AM   #2
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There is a crop factor similar to analog 35 mm lenses on APS-C. See https://www.pentaxforums.com/forums/pentax-medium-format-645-6x7-645d/147228-...ion-chart.html

BTW - the distance to the focal plane is maintained, not altered, with the 67 to 645 adapter
05-24-2013, 05:45 AM   #3
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Thanks for the reference. Doing the math, if I am applying it correctly would mean that a 67 55-100mm lens would be the equivalent of a 33-60 mm lens when used with the adapter on the 645. Is this right? Thanks
05-24-2013, 06:02 AM   #4
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Hi Normo
The distance from the lens mount surface to the film plane has to be maintained for the lens (84.95 mm for the P67)
So the adaptor will add the difference between that and the 70.87 mm register of the P645.
The angle of view is related to the lens focal length and the film/sensor frame size.

For a P67 100 mm lens the AOV's will be:

The P67 frame is 55 * 70 so diagonal is 89 and the angle of view is 48.0 degrees
The P645 frame is 41.5 * 56 so diagonal is 69.7 and the angle of view is 38.4 degrees
The P645D frame is 33 * 44 so diagonal is 55 and the angle of view is 30.7 degrees

A standard lens typically has AOV around 47 degrees ( eg 50 mm on 35 mm film)
AOV = 2* atan(Diag/(2*FocalLength))

05-24-2013, 06:21 AM   #5
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QuoteOriginally posted by Normo Quote
Thanks for the reference. Doing the math, if I am applying it correctly would mean that a 67 55-100mm lens would be the equivalent of a 33-60 mm lens when used with the adapter on the 645. Is this right? Thanks
A 55-100mm lens is a fairly wide to normal zoom lens on 6x7. A normal lens on 6x7 is 90mm.
If you mount the 55-100mm lens on a 645 via the 67 to 645 adapter, you get a slightly wide to short tele lens on the 645. A normal lens on 645 is 75mm.

The focal length as such does not change - only the angle of view because of the smaller frame format.

It seems that math in the thread I linked to, isn't quite correct.

As the diagonal of the frame on 6x7 is about 90 mm, and the diagonal of the frame on 645 is approx. 75mm, the crop factor is about 1.2 (90 / 75 mm).
The 55-100mm lens when mounted on the 645 will the have approx. the same angle of view as a 66-120mm lens would have on 6x7.
05-24-2013, 06:28 AM   #6
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A 100mm 67 lens on a 645 body will have the same FOV as a 100mm 645 lens on a 645 body.
05-27-2013, 08:36 AM   #7
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The proper way to think of this is in terms of focal length, coverage, and sensor size. A lens throws a certain circle of light. If that circle is too small for the sensor, you will get dark corners or even an entirely circular image like the stereo fisheye-takumar folder some guy built (or the Samyang 8mm). If the circle covers, then the field of view will be equivalent for a given sensor no matter how large the circle is. You will get the same results shooting a 645 75mm as you would the P67 75mm or a large format 75mm lens.

There are now telecompressors on the market which do what you imagined - they take a 35mm image circle and squish it down for a crop sensor. You get the full frame field of view on the crop sensor, and because the circle of light is more concentrated and intense you also get a 1-stop speed gain, so for example a 50mm f2 would act like a 35mm f1.4 in field of view with the imaging characteristics (depth of field, etc) of a 50mm f2. There is some resolution loss due to the optics but this is more than compensated for by compressing the image - like resizing a JPG from 1024x768 to 640x480, you lose some resolution in the compression but the resizing compensates that. Sounds like magic but they have been commonly used in astrophotography for a long time and were used in many early DSLR systems to avoid huge crop factors on the small sensors (Minolta RD-175, Nikon E2, etc).

They require mirrorless bodies and SLR lenses in order to fit the optics in there, which is probably why they are only just coming out now. I've only seen them for the NEX system to date. Metabones Speed Booster is the prototypical example, but there are now Chinese knockoffs hitting eBay (search "focal reducer"). Hopefully we will see these for compressing MF to 35mm or crop as well, I'd love to use a P67 55/4 as a 28mm f/2 or something like that.

Last edited by Paul MaudDib; 05-27-2013 at 08:47 AM.
09-09-2013, 01:42 PM   #8
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QuoteOriginally posted by Paul MaudDib Quote
Hopefully we will see these for compressing MF to 35mm or crop as well, I'd love to use a P67 55/4 as a 28mm f/2 or something like that.
I really hope so...

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