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03-04-2019, 08:56 AM - 2 Likes   #1
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Equivalent focal length - 67 lens on 645

Bonjour,

This has probably been asked before and I have searched on the Internet ... but, I am not certain of the calculation.

I have been recently putting my 67 lenses on the 645 (film) via a K&F Concept adapter (see images below). My question is what is the "equivalent focal length" in 35mm (135) when I do this ... I understand that a 67 lens on my 6x7 is halved and the ratio of about 0.625 applies to the 645 (film, not digital).

That's in advance for your input and "Salut", J

Pentax 645 with SMC Tak67 105/2.5






03-04-2019, 09:39 AM - 4 Likes   #2
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First, focal length is focal length. The view provided by a 6x7 format 105mm lens adapted to a 645 is identical to the view provided by a native 645 format 105mm lens on a 645. And a 105 mm lens from any large format system provides the same view on a 135 camera. The format of the lens does NOT affect the equivalent focal length unless you are trying to adapt a small format lens to a larger format camera and the image circle of the lens is too small to fill that larger format frame.

Thus, if you are asking about 135 equivalence on 645, the fact that the lens was originally designed for 6x7 is irrelevant.

But the true answer is tricky because of the aspect ratio issues. The frame sizes (and aspect ratios) for the three formats you mentioned are:

135: 36 mm x 24 mm (3:2)
645: 56 mm x 41.5 mm (4:3)
6x7: 70 mm x 55 mm (14:11) -- CORRECTED

And the crop factor depends on the aspect ratio of the final image.

If you are shooting wide objects in landscape orientation or tall objects in portrait orientation, then the framing is defined by the long dimension of the format. Thus, the crop factor for 645 is 36/56 = 0.64. That is, a 105 on 645 offers the same view as a 67 mm lens on 135.

If you are shooting compact/squarish objects in either landscape or portrait orientation, then the framing is defined by the shorter dimension of the format. Thus the crop factor for 645 is 24/41.5 = .58. That is, a 105 on 645 offers the same view as a 61 mm lens on 135.

And if you are comparing the view offered by a 105 on a 6x7 camera to the view created on a 135 format camera, then different crop factors are involved.

EDIT: desertscape found an error -- the Pentax 6x7 has a slightly wider and shorter frame size than the supposed standard for 6x7 described in Wikipedia. Overall, any crop factor calculation may be approximate because of the subtle discrepancies in actual frame size (e.g., the K-1 sensor is 35.9 x 24, not 36x24).

Last edited by photoptimist; 03-05-2019 at 03:26 PM.
03-04-2019, 09:44 AM - 1 Like   #3
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135, 645, and 67 all have different aspect ratios, so there's no exact equivalence. Anyway, focal length is focal length, so the 0.625 ratio you use does not change just because you are using an adapted lens made for a larger format.
03-04-2019, 10:14 AM - 1 Like   #4
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QuoteOriginally posted by photoptimist Quote
the crop factor depends on the aspect ratio of the final image.

a 105 on 645 offers the same view as a 67 mm lens on 135.
Kudos to @photoptimist for one of the best and complete answers on the question of equivalence. +1 for me.

As stated previously, a 105mm uncropped in 645 format would have the same field of view as a 67mm uncropped in 35mm.

03-04-2019, 10:21 AM   #5
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The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

The quick and dirty table in the article suggests any 105mm that covers the 645 image circle when used on a film 645 gives an image field of view of an approximately 65mm on 35mm film (but as mentioned the aspect ratio affects this a bit.)
03-04-2019, 03:13 PM - 2 Likes   #6
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Because of the different aspect ratios, I find it easier to equate focal lengths to the standard lens in the imaging format (note the 55mm on the 645D & Z is more like a FF 43mm equivalent than 50mm - which I prefer anyway). Thus a 105mm on a 645 (film) is roughly 1.3x a standard lens (75mm) = 65mm FF equivalent (50mm x1.3).
03-05-2019, 02:49 PM - 1 Like   #7
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QuoteOriginally posted by photoptimist Quote
6x7: 67 mm x 56 mm (5:4)
FYI-- The Pentax 6x7 is 55mm X 70mm.
03-10-2019, 12:40 PM   #8
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I asked the question of crop factors of the 645D and Z sensors to the 645 and 6x7 film lenses. the best answer I received on this site was,


645 film lens = 1.26 crop factor on the D or Z

6x7 film lens = 1.62 crop factor on the D or Z

I feel ok using this gage as a guide in buying and using lenses. I base my decision on the D sensor being smaller than the area of the 645 and 6x7 film camera. if it works when comparing aps-c to full frame, then it's gotta work with medium format.


I

---------- Post added 03-10-19 at 03:57 PM ----------

QuoteOriginally posted by photoptimist Quote
First, focal length is focal length. The view provided by a 6x7 format 105mm lens adapted to a 645 is identical to the view provided by a native 645 format 105mm lens on a 645. And a 105 mm lens from any large format system provides the same view on a 135 camera. The format of the lens does NOT affect the equivalent focal length unless you are trying to adapt a small format lens to a larger format camera and the image circle of the lens is too small to fill that larger format frame.

Thus, if you are asking about 135 equivalence on 645, the fact that the lens was originally designed for 6x7 is irrelevant.

But the true answer is tricky because of the aspect ratio issues. The frame sizes (and aspect ratios) for the three formats you mentioned are:

135: 36 mm x 24 mm (3:2)
645: 56 mm x 41.5 mm (4:3)
6x7: 70 mm x 55 mm (14:11) -- CORRECTED

And the crop factor depends on the aspect ratio of the final image.

If you are shooting wide objects in landscape orientation or tall objects in portrait orientation, then the framing is defined by the long dimension of the format. Thus, the crop factor for 645 is 36/56 = 0.64. That is, a 105 on 645 offers the same view as a 67 mm lens on 135.

If you are shooting compact/squarish objects in either landscape or portrait orientation, then the framing is defined by the shorter dimension of the format. Thus the crop factor for 645 is 24/41.5 = .58. That is, a 105 on 645 offers the same view as a 61 mm lens on 135.

And if you are comparing the view offered by a 105 on a 6x7 camera to the view created on a 135 format camera, then different crop factors are involved.

EDIT: desertscape found an error -- the Pentax 6x7 has a slightly wider and shorter frame size than the supposed standard for 6x7 described in Wikipedia. Overall, any crop factor calculation may be approximate because of the subtle discrepancies in actual frame size (e.g., the K-1 sensor is 35.9 x 24, not 36x24).
why would you be comparing it 135mm when you have th 645 and 6x7 film areas to compare it to?


Last edited by bull drinkwater; 03-10-2019 at 12:58 PM.
03-10-2019, 05:42 PM   #9
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QuoteOriginally posted by bull drinkwater Quote

why would you be comparing it 135mm when you have th 645 and 6x7 film areas to compare it to?
Good point.

You can certainly express equivalence of lenses from any format to any other format. It just depends on which format you use most or which has the most natural personal associations between focal length and angle-of-view. Is a 25 mm lens a UWA, normal lens, or a telephoto to you?

In this case the OP asked for 135 format equivalence.

Last edited by photoptimist; 03-10-2019 at 05:53 PM.
03-11-2019, 06:45 AM - 1 Like   #10
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QuoteOriginally posted by bull drinkwater Quote
645 film lens = 1.26 crop factor on the D or Z

6x7 film lens = 1.62 crop factor on the D or Z
I'm not sure this makes any sense to me.

With crop factor, the lens format is not relevant, nor is the size of the image circle. The only relevance is the focal length and the reference image sizes. Obviously the minimum image circle must still be there.

So a 100mm 6x7 lens and a 100mm 645 lens (both film era) and a 100 mm DFA 645 lens all give the same field of view when mounted on the 645D or 645Z. And furthermore those same lenses on an APSC sensor work like a 100mm APSC lens. The field of view however will be narrower than the original format if the new sensor is smaller. So someone used to 135 film might say any 100mm on APSC is "longer" 1.5x than on 135, but the reality is even an 8x10 lens that is 100mm will give the same rough picture in terms of angle of view on that same APSC sensor as an apsc only 100mm would.

The lens itself may be capable of a larger image cirvle in the case of the 6x7 lens but the sensor size limits it. The crop factor is only between formats of sensors not lens formats.
03-18-2019, 04:35 AM   #11
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35 mm equivalent for 645

Bonjour !

The 645N has a film size of 41 X 56 mm and the lens equivalence factor is 0,6 (a 100 mm lens on a 645N gives the angle of view of a 60 mm lens on 35 mm FF).

The 645Z has a 33 X 44 mm sensor and the lens equivalence factor is 0,8 (a 100 mm lens on a 645Z gives the angle of view of an 80 mm lens on 35 mm FF).

A 100 mm 6X7 Macro lens with an adaptor is still a 100 mm lens when mounted on a 645 body. It's only the ratio between film or sensor sizes that matters.

Richard L.


Facteurs d'équivalence de 0,6 pour le 645N (film) et de 0,8 pour le 645Z (numérique).

Last edited by RICHARD L.; 03-18-2019 at 06:53 AM.
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