Can an SLR be modified to use a larger focusing screen or waist-level screen?
Asked 6/3/2019
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I use an older 35mm SLR with a removable focusing screen and wondered if I could replace it with a much larger screen, either at the back of the camera or by opening the top and mounting a larger screen like on older waist-level cameras. Is there any optical reason this would not work, and could extra lenses make it possible?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Changing the size of the focusing screen is not reasonably possible. jarnbjo explains why the focusing screen is the same size as the film format:
The focusing screen is always the same distance from the lens as the film you are intending to expose, otherwise you could not use it to focus. It is therefore also the same size as the film format...
The size of the imaging circle, mirror, prism, etc are all fixed. Just installing a larger screen wouldn't be enough. You'd be effectively building an entirely different camera. The presence of a moving mirror prevents placement of additional optics in front of the screen. You'd also have to account for the different focusing distances caused by moving things around.
Instead of messing with the screen, it would be easier to change the viewfinder optics. Move it back, make it larger, increase magnification. The screen would be the same size, but it would look larger.
There are viewfinder magnifiers you can try. I haven't used one, but based on what I've read, they might affect viewing the edges of the frame. The trade off might be worthwhile if you need the magnification to assist focus.
Phil Anderson suggests using a camera with a waist-level finder. Some cameras let you swap out the entire prism assembly with a waist-level finder. The screen itself is still the same size, but the change of optics behind it increases the viewing size.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
7y ago
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In practice, no—not without redesigning most of the camera.
An SLR focusing screen must sit at the same distance from the lens as the film plane, or what looks in focus on the screen will not match the film. For a 35mm SLR, that also means the screen is effectively tied to the film format size.
Beyond that, the rest of the viewing system is built around that screen size and position: the lens image circle, reflex mirror, pentaprism/pentamirror, and viewfinder optics are all matched to it. Simply attaching a larger screen elsewhere would not give a correct or usable image. The moving mirror also leaves little room to add optics in front of the screen.
So while it is theoretically possible if you build a new optical path, it would amount to creating a different camera rather than a simple modification. A more realistic approach would be changing the finder system itself, such as using a camera designed for a waist-level finder or a larger viewing screen.
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