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I am aware this is a very unusual idea but please bear with me. I am using an old analog SLR from the 80s. The inside focusing screen is removable. I was thinking whether it would be possible to attach a bigger focusing screen in front of the viewfinder after removing the small one inside and maybe with the aid of some more lenses and the right placement I could have this very big focusing screen on the back of the camera like on these really old ones. An alternative idea was cutting open the whole mirror array on the top and placing the bigger focusing screen on the top of the camera.

Anyone here with some deeper knowledge about optics who could come up with an idea or an explanation why this wouldn't be possible?

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    \$\begingroup\$ I don't know if this would do what you are looking for, but have you considered a camera with a waist-level viewfinder such as a Rolleiflex? It has a much larger focusing screen. \$\endgroup\$
    – Phil Anderson
    Jun 4, 2019 at 0:08
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    \$\begingroup\$ @PhilAnderson 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 and a Rolleiflex only has a larger focusing screen because it is a 6x6 medium format and not a 135 format camera. A 135 format camera with a waist-level viewfinder (a few of those do exist), will not have a larger focusing screen than any other 135 camera. \$\endgroup\$
    – jarnbjo
    Jun 4, 2019 at 9:59

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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.

    viewfinder magnifier

  • 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.

    waist-level finder

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  • \$\begingroup\$ Thanks! Great answer but not exactly what I'm looking for. I'd really love to be able to have a bigger screen I can physically access. Of course there is the option of getting another camera but I'd really love to use my existing one for its unique features. I am not afraid of damaging things while doing heavy modifications. And I know I would probably have to mess with the prism \$\endgroup\$
    – Jeffrey
    Jun 4, 2019 at 9:21
  • \$\begingroup\$ I don't see how it could possibly work @Jeffrey. Keep in mind lenses are designed for a specific format, so a lens for 135 film has an image circle slightly bigger than that format. You would barely see a lot more with a bigger screen. \$\endgroup\$
    – timvrhn
    Jun 4, 2019 at 10:45
  • \$\begingroup\$ @Jeffrey If magnifying the view of the current screen does not serve your needs, what is the reason you want a larger screen? \$\endgroup\$
    – xiota
    Jun 4, 2019 at 10:50
  • \$\begingroup\$ I am aware that the focusing screen is deliberately placed in the same distance as the film in order for it to accurately show you if the subject is in focus. I was contemplating if it was maybe possible to make up for the different distance with correction lenses. I'd like to have such a screen in order to then be able to place some foil in front of it and make markings as to where certain objects were on the frame in order to be able to compose another frame exactly the same \$\endgroup\$
    – Jeffrey
    Jun 4, 2019 at 14:08
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    \$\begingroup\$ "place some foil in front of it and make markings as to where certain objects were on the frame in order to be able to compose another frame exactly the same" – Sounds like an entirely different question/problem. \$\endgroup\$
    – xiota
    Jun 4, 2019 at 14:13

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