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I've got a Bronica ETR to EF lens adapter and also an EF to FE (Sonly full frame) adapter.
If I pair these together I have no idea what equivalent focal length this will give me for my Bronica lenses once on my full frame Sony.

How can I work this out and in general, is there an easy guide to determine this for future combinations of lenses/sensor sizes I may try?

My ETR Lenses are as follows

  • Bronica 50mm
  • Bronica 75mm
  • Bronica 150mm
  • Bronica 250mm

What I imagine is that the image when this lens is taken from a Bronica camera to the Sony camera via adapters will show the equivalent of a different lens, an image with some crop applied, i.e., an image not using all of the glass of the lens. One that looks as if it was shot on a longer telefocal lens. Is this the case?

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    \$\begingroup\$ Do you want to know how your Bronica lenses will look on a FF camera compared to lenses of the same focal lengths that were made for the FF camera? Or do you want to know how your Bronica lenses will look on a FF camera compared to how those same lenses look on a Bronica camera? \$\endgroup\$
    – Michael C
    Aug 30, 2021 at 16:28

3 Answers 3

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The crop factor is One relative to full frame because the sensor is full frame. The Bronica lenses will project a larger image circle than is required by the sensor, but 50mm is 50mm.

The lens adapters are required because the Bronica lenses have a greater back focus distance because the Bronica film plane is rearward of the large mirror (approximately 6 cm x 6 cm.).

[By request of the OP]

  • The relationship of a normal lens for the Sony versus a normal lens for the Bronica is a bit of a mess due to: the difference in formats (3:2 versus 4:3).

  • However, the diagonal of a full frame sensor is 43mm. The diagonal of a 645image (56mm x 45mm) is 73mm. So about 5:3 (or 3:5) for "normal" lens focal length.

  • A square out of a 35mm sensor is 24x24mm. 24 goes into 56, 2.33 times.

  • Cropping the Bronica image…well there are four ways to do it…

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  • \$\begingroup\$ This only part answers my question, so admittedly this is a failing of the question and should be updated. What I imagine is that the image when this lens is placed from a Bronica camera to the Sony camera via adapters will show the equivalent of a different lens, an image with some crop applied, ie an image not using all of the glass of the lens. One that looks as if it was shot on a longer telefocal lens. Is this not the case? How is this phrased ? \$\endgroup\$
    – AdamJones
    Aug 29, 2021 at 0:53
  • \$\begingroup\$ @AdamJones see revised answer...but the aspect ratio difference makes it a bit theoretical versus practically useful. \$\endgroup\$ Aug 29, 2021 at 1:15
  • \$\begingroup\$ The Bronica ETR is 4.5 x 6, isn't it? So that's a diagonal of 70 mm. \$\endgroup\$
    – Kahovius
    Aug 29, 2021 at 17:48
  • \$\begingroup\$ @AdamJones, I've updated the question with your comment (with a little editing). Feel free to roll back the change or further edit it if it's not what you meant. \$\endgroup\$
    – inkista
    Aug 31, 2021 at 2:32
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What I imagine is that the image when this lens is placed from a Bronica camera to the Sony camera via adapters will show the equivalent of a different lens, an image with some crop applied, ie an image not using all of the glass of the lens. One that looks as if it was shot on a longer telefocal lens. Is this not the case? How is this phrased ?

No, the Bronica 50mm lens's FoV will not be cropped by the Sony's sensor in comparison to any other 50mm lens on the Sony... and since "crop factor" is always related to the 35mm format, and that is what you are using, it will be 1x (not applicable). Another way to say it is that the crop factor for 6x6MF is .55x.

I think this excellent answer on system angle of view will probably help you understand better. What is "angle of view" in photography?

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Smaller sensors always make a lens frame as if it were longer, because using a smaller sensor is, in effect,cropping. See also this image demonstration from dpreview's original 5D review.

645 film has a crop factor, relative to 135/35mm format (so-called "full frame"), of 0.614x according to this sensor format table on Wikipedia. The crop factor is the ratio of the formats' diagonal measurements.

You can figure this out with the Pythagorean theorem (a2+b2=c2), if you know the sensor dimensions. 645 is 56mmx42mm, and has a diagonal of 70mm; 135 is 36mmx24mm, and has a diagonal of 43mm. 43÷70 = 0.614. The different aspect ratios of the two formats, however, isn't going to make this exact, despite the numerical precision. :D

So, while a 50mm lens remains a 50mm lens no matter the size of sensor that's behind it, on a Sony full frame (135 format) sensor, if you want an equivalent field of view to that a 50mm rendered on a Bronica ETR, you need a 50x0.614 => 30mm lens.

And the field of view the 50mm will yield on full frame will be equivalent to a 50/0.614 = 50x1.63 => 81mm lens on the Bronica.

So, if you're used to focal length/FoV translation on the ETR, then on the Sony,

  • the 50mm will frame like an 81mm would on an ETR camera
  • the 75mm will frame like a 122mm would on an ETR camera
  • the 150mm will frame like a 245mm would on an ETR camera
  • the 250mm will frame like a 407mm would on an ETR camera

Again, this won't be exact, since the two formats have different aspect ratios. But it's close enough for jazz. All other factors, DoF, perspective, etc. would look identical if both cameras were shooting the same subject with the same lens from the same distance. The reason larger formats appear to yield thinner DoF and different perspectives is mainly because with a larger format, you'll more typically either shoot closer to the subject or use a longer lens to get identical framing with the smaller format.

Footnote: Film 645 format, btw, by medium format shooters is also "full frame" while digital 645 is crop. :D 645D, as the sensor format is known, is only 44mmx33mm, and has a diagonal of 55mm, so its crop factor, relative to 645 film is 70mm/55mm = 1.27x. The 645D format is used by Pentax's 645 dSLRs, as well as Fuji's GFX cameras.

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