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My understanding is, the focusing screen, being matte, will diffuse some amount of light in a direction that is not captured by the optical viewfinder. I am aware of some technical developments towards decreasing this loss but haven't found any specific numbers (comparing one focusing screen to another focusing screen does not count). So, how much light is really lost?

Or, in other words: if we remove the focusing screen (and perhaps adjust the viewfinder so it still provides the equivalent view through the lens, like in a binocular) how much brighter the picture would be?

  • Without a focusing screen, you don't get a picture in the viewfinder. That's kind of the point of a focusing screen. – inkista Jun 7 '15 at 18:24
  • You will have a picture without focusing screen, but it will only cover the central part of the field of view. – Edgar Bonet Jun 8 '15 at 10:59
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    @inkista No, the purpose of the focusing screen is to see exactly what your camera sensor (or film) will see - and without the focusing screen (but still with eyepiece/viewfinder) we would get an optical system similar to a binocular, where the focus would also depend on your eyepiece and the depth of field would also depend on your eye's iris, not just the camera aperture. – szulat Jun 8 '15 at 11:03
  • I think this will depend on the density of the focusing screen, won't it? The material (plastic or glass) and it's thickness should be considered. – SailorCire Jun 13 '15 at 14:58
  • since the matte screen in my understanding isn't "eating" any light, I guess not brighter at all. apart from that, the human eye wouldn't notice any difference due to adaption. – thengineer Jun 13 '15 at 19:39
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You can get some general figures in the 60–80% range by looking up transmission rates of materials like frosted glass. DSLRs often use laser-etched glass that may have higher transmission.

But a camera is not like a set of binoculars. A DSLR lens focuses light onto the focusing screen (when the reflex mirror is down), and the viewfinder flips the image orientation so it appears upright. So you're looking at a projection of the image through the viewfinder, not the object itself. Binoculars focus light directly onto your retina.

Without the focusing screen, you would just see everything out of focus. Also, none of this matters when the reflex mirror is up, since the light converges on the image sensor instead of the viewfinder.

(To make things even more complicated, the reflex mirror isn't completely reflective. Some of the light passes through the main mirror, and is reflected by a secondary mirror to the autofocus/autoexposure sensor at the bottom of the chamber. The focusing screen doesn't even start with 100% of the light passed through the lens!)

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