What field of view do Bronica ETR lenses give on a Sony full-frame camera?
Asked 8/27/2021
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I have Bronica ETR lenses adapted to Sony full frame via ETR-to-EF and EF-to-FE adapters. I’m trying to understand whether adapting medium-format Bronica lenses changes their effective focal length or crop factor on a 35mm full-frame Sony body.
My lenses are:
- 50mm
- 75mm
- 150mm
- 250mm
Will these lenses behave like longer lenses because the Sony sensor is smaller than the Bronica 645 film frame, or does a 50mm lens still act like a 50mm lens on full frame? More generally, what’s the right way to compare angle of view between medium format and full frame when adapting lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
5
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…
Originally by user95237. Source · Licensed CC BY-SA 4.0
user95237
4y ago
0
Generated from our catalog & community — verify before relying on it.
On a Sony full-frame body, the crop factor is 1× because the camera is full frame. Adapting Bronica ETR lenses does not change their focal length: a 50mm lens is still a 50mm lens, a 75mm is still a 75mm, and so on.
What does change is the image circle the lens was designed to cover. Bronica ETR lenses were made for 645 format, which is larger than 35mm full frame, so on Sony the sensor uses only the central portion of that larger image circle. That does not create extra magnification; it just means the lens covers more than the Sony sensor needs.
If you want to compare angle of view to how the lens looked on Bronica 645, 645 has a crop factor of about 0.61× relative to full frame. So the same lens mounted on full frame will look narrower than it did on the Bronica body.
Rule of thumb: crop factor depends on sensor/film size, not the lens or adapter. Compare formats by their diagonals to estimate equivalent angle of view.
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