How much light does a DSLR focusing screen lose in the optical viewfinder?
Asked 6/6/2015
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In a DSLR, the matte focusing screen diffuses the image so you can see it through the pentaprism/pentamirror and eyepiece. Since that diffusion sends some light away from the optical path, how much light is typically lost at the focusing screen? Put another way, if you could somehow remove the focusing screen and still keep the same viewfinder image, how much brighter would the view be?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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!)
Originally by user41364. Source · Licensed CC BY-SA 4.0
user41364
11y ago
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Generated from our catalog & community — verify before relying on it.
A matte focusing screen does lose light, but there isn’t a single universal number for all screens. General figures for frosted/matte glass are often around 60–80% transmission, and DSLR focusing screens may do somewhat better depending on their design.
The bigger point is that a DSLR focusing screen is essential to forming a viewable image in the optical finder. The lens is focusing an image onto that screen; the pentaprism/pentamirror and eyepiece then let you view that projected image correctly oriented. If you removed the screen, you would not get the same brighter image—you would mostly lose the properly formed image and see an out-of-focus view instead.
So while some light is indeed lost at the focusing screen, it’s not meaningful to think of it as a simple “free brightness gain” if removed. The screen is part of how the DSLR viewfinder works at all. Also, this only affects the optical viewfinder path when the mirror is down, not the light reaching the sensor during exposure.
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