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07-11-2007, 10:34 AM   #1
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K10D and 6x7 lenses

Does anyone know what magnification factor a 6x7 (medium format) lens would have on the K10D?

I realize that 1.5x is the "effective magnification" on 35mm format lenses; but what about medium format lenses?

Thanks!

07-11-2007, 11:31 AM   #2
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If you pop a 200mm 6x7 lens onto a K10D, you will be getting the same FOV as with any other 200mm lens, whether it be DA, FA, 645, or from any other series.

Because the 6x7 has a larger image circle, though, a 66mm lens mounted on a K10D would produce the same image as the 200mm on a 6x7 camera. Technically, you could say that the "effective magnification" is 3x, but that in itself is very misleading...

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07-11-2007, 12:34 PM   #3
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I'm a bit confused how you arrived at that "conversion."

Maybe I didn't really word my question properly, let me try again...

A "normal" 50mm lens on a K10D (with the APS-sized CCD image sensor) produces an image similar to having a 75mm lens on a camera with a full frame sensor.

So, what would a Pentax 67 90mm (standard lens for a 6x7 format camera) be equivilant to? I would assume it would be, effectively, more like a telephoto lens on an APS-sized CCD image sensor.

Does that make sense?
07-11-2007, 12:58 PM   #4
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If you have Excel, you might be interested in downloading a workbook I made available some time ago. It has some a lens conversion worksheet as well as some other stuff on it.

Go to:

Tools for Photographers

A 90mm lens will provide a 23 AOV for a 35mm, a 41 AOV for a 6 x 7 and a 15 AOV for a Pentax DSLR.

A 31mm lens on a Pentax DSLR or a 48mm lens on a 35mm would be the equivalent to a 90mm on a 6 x 7 in terms of the image it would capture.

But a 90mm lens is a 90mm lens regardless of what camera it is on :-) So, it you put a 90mm lens on a Pentax DSLR it would be the same as a 135mm on a 35mm.

This should be enough to confuse almost anyone.

07-11-2007, 01:03 PM   #5
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mm are mm regardless of negative or sensor size. Your question is either not phrased correctly or represents a misunderstanding of mm = mm. The "conversion factor" has to do only with field of view (FOV) and not the actual focal length.

For example the diag FOV of a 50 mm lens on a 6x7, 35mm, and k10d is:

6x7...............83deg
35mm...........47deg
k10d............26.5deg

Your 90mm 6x7 lens would have a fov of ~18deg on a asp-c digital. A similar FOV to a 135mm on a 35mm or 270mm on a 6x7.

I feel I have done enough to confuse things.

see this FOV calculator if you have more questions.



An important thing to keep in mind is that the resolving power of a lens, lpi, does not increase just because you happen to be sampling a smaller portion of the image it provides. If a lens can only resolve 100 line per inch at 100 feet you will never see more than 100 lines per inch at 100 feet regrdless of the camera behind the lens.

Last edited by import silvia; 07-11-2007 at 01:08 PM. Reason: Added a point.
07-11-2007, 01:05 PM   #6
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The focal length is the focal length, regardless of whether it's intended for 35mm, medium format, APC sensor etc. Unless the spacing on the adapter is odd, the 90mm should perform just like any other 90mm on an APS-sized sensor. So yes, it would be a telephoto, like using a 135mm on 35mm film.

Sorry to repeat your points JMax and Silvia - you beat me to this

Oh and I have to say the whole "conversion factor" or "equivalent" is only really helpful for understanding the FOV of a given focal length on a sensor smaller than 35mm, if you are mostly familiar with 35x24mm format ... I see a lot of people now who have never used 35mm cameras posting on forums and getting massively confused because they think the "conversion factor" of their digital camera should mean something to them.

Last edited by ZaphodB; 07-11-2007 at 01:11 PM.
07-11-2007, 03:01 PM   #7
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QUOTE: "Your 90mm 6x7 lens would have a fov of ~18deg on a asp-c digital. A similar FOV to a 135mm on a 35mm or 270mm on a 6x7." -import silvia


I may not have asked my question very clearly, but, if I put my Pentax 6x7 90mm lens on the K10D, it would have the equivalent frame as a 270mm lens as compared to a 35mm "film camera."


So if I multiply the 6x7 lenses by three, I would get their approximate "equivilant" to 35mm format?

And, conversely, that would make a "normal" lens on the K10D about a 30mm lens?

Correct?

Thanks!
07-11-2007, 03:57 PM   #8
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QuoteOriginally posted by Col Flanders Quote

So if I multiply the 6x7 lenses by three, I would get their approximate "equivilant" to 35mm format?
No, a 90mm lens for the 6X7 will be the same as a 90mm lens for a 35mm camera. It's the same angle of view, except the lens is much larger.

07-11-2007, 05:04 PM   #9
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If you put a 90mm 6/7 lens on a K10D (APS-C) it should work the same as a 645 lens or 35mm lens because the K10D is only using the centre portion of the bigger lens. It should also give a good sharp result because the centre portion of lenses are the best areas for sharpness generally. So like the others have said 90mm would equate to 135mm in 35mm format.

I have a few good Pentax 645 lenses but I've been wondering if anyone has some pics taken using the K10D or K100D using a 645 lens and the available adapter.

Does the adapter have a glass element? If it does then that would probably subtract a bit from image quality like a teleconvertor. Also depending on how the adapter mounts on the DSLR will determine whether you get infinity focus or not. M42 lenses are good because the adapter mounts flush meaning you get infinity focus and they are even better because there is no glass element between the lens and camera.

So if anyone has some results using 645 lenses and the adapter on a DSLR that would be helpful to me.

Tim H
07-11-2007, 05:26 PM   #10
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QuoteOriginally posted by Timbuctoo Quote
If you put a 90mm 6/7 lens on a K10D (APS-C) it should work the same as a 645 lens or 35mm lens because the K10D is only using the centre portion of the bigger lens. It should also give a good sharp result because the centre portion of lenses are the best areas for sharpness generally. So like the others have said 90mm would equate to 135mm in 35mm format.

I have a few good Pentax 645 lenses but I've been wondering if anyone has some pics taken using the K10D or K100D using a 645 lens and the available adapter.

Does the adapter have a glass element? If it does then that would probably subtract a bit from image quality like a teleconvertor. Also depending on how the adapter mounts on the DSLR will determine whether you get infinity focus or not. M42 lenses are good because the adapter mounts flush meaning you get infinity focus and they are even better because there is no glass element between the lens and camera.

So if anyone has some results using 645 lenses and the adapter on a DSLR that would be helpful to me.

Tim H
Some people have used the Pentax 645 lenses on their DSLRs, unfortunately the adapter is rare and expensive. The pictures I've seen have been taken with the much cheaper and widely available Canon EOS adapters. The register distance for the 645 is *much* larger than the K mount, so there is no need for an optical element.

All the pictures I've seen have been smashing.


To the OP - a 50mm lens is a 50mm lens, period. End of story. It will look on your camera like every other 50mm lens in existence. It will look like the short end of the SMC Pentax-DA 50-200mm lens. In other words, no matter if the lens was designed for digital, 35mm, 645, or 6x7 - it will look like every other 50mm lens on your camera.

This, of course, looks like a 75mm lens on a 35mm film camera.

But the origins of the lens don't matter - the focal length is what dictates the view.
07-11-2007, 06:13 PM   #11
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Density Help

Can I ask the question one more way.

A lens made for 35mm has a 1.5 magnification factor on a C size sensor based on the fact that you are sampling less of the image circle than the lens was designed for.

A 6 X 7 lens has a larger image circle and should have a larger mangification factor.

The lens magnification factor of a 6 X 7 lens on a C size sensor is?
07-11-2007, 06:30 PM   #12
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QuoteOriginally posted by joelovotti Quote
Can I ask the question one more way.

A lens made for 35mm has a 1.5 magnification factor on a C size sensor based on the fact that you are sampling less of the image circle than the lens was designed for.

A 6 X 7 lens has a larger image circle and should have a larger mangification factor.

The lens magnification factor of a 6 X 7 lens on a C size sensor is?
I know you don't mean it this way, but 'magnification' is constant. A macro lens on a 6x7 that focuses down to 1:1 magnification will focus to the same point (and the same 1:1 magnification) on 35mm film and your digital SLR.

As for the cropping factors, use one of those calculators posted above.

My point is - who cares? It is what it is. If you buy a 90mm lens originally designed for 6x7 film, it will look exactly the same as a 90mm lens designed for DSLRs.
07-12-2007, 06:16 AM   #13
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QuoteOriginally posted by joelovotti Quote
Can I ask the question one more way.

A lens made for 35mm has a 1.5 magnification factor on a C size sensor based on the fact that you are sampling less of the image circle than the lens was designed for.

A 6 X 7 lens has a larger image circle and should have a larger mangification factor.

The lens magnification factor of a 6 X 7 lens on a C size sensor is?
2.875

(based on the horizontal dimension of the actual image area--69mm x 56mm)

Incidently, the 1.5 magnification factor, an unfortunate term, is based on the horizonal dimension also. The Pentax sensor is 23.7mm and the 35mm film is actually 36mm or 1.52 times the Pentax sensor. Some think a diagonal measure should be used instead of the largest dimension.

A good illustration of angle of view and film/sensor formats can be seen at:

Angle of View
07-12-2007, 03:32 PM   #14
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QuoteOriginally posted by carpents Quote
If you buy a 90mm lens originally designed for 6x7 film, it will look exactly the same as a 90mm lens designed for DSLRs.
THAT was NOT my question!

I realize that a 90mm lens designed for a 6x7 camera "appears" the same as a 90mm lens designed for a DSLR.. I am asking about mounting a 90mm lens designed for my Pentax 6x7 mounted on a DSLR with an APS-sized CCD image sensor (such as the K10D.)

I truely appreciate everyone's help and patience...

Let me try asking this way...

If I put a 50mm lens designed for a 35mm camera on a DSLR with an APS-sized CCD image sensor, I no longer have as wide a field of vision. effectively, the 50mm lens has become a 75mm lens.

Correct?

So, what happens when I mount a 6x7 90mm lens on a K10D using the S-mount adapter, of course.

Last edited by Col Flanders; 07-12-2007 at 03:41 PM. Reason: clearified question
07-12-2007, 03:45 PM   #15
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QuoteOriginally posted by jmax Quote
2.875
So, 90mm (6x7 lens) x 2.875 = 258.75mm

Right?

Thanks to everyone again.
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