Why do dslrs use a pentaprism instead of another mirror?

[dlwjiang]

Image quality? If so why not two pentaprisms? Size?

[prabbit237]

Google and wikipedia are yer friends

Single-lens reflex camera - Wikipedia, the free encyclopedia

Basically, the lens flips the image around backwards and the multiple reflections in the pentaprism flip it back around again. A simple 2nd mirror wouldn't show the image correctly (binoculars work using 2 single prisms to do the same thing. See Binoculars - Wikipedia, the free encyclopedia.)

Astronomical telescopes flip the image upside down and left-to-right but it doesn't matter in astronomy. The image on the film or sensor is also flipped and reversed (but again, it doesn't matter.) But we humans find it much easier to see terrestrial things if they're right side up so.....

Actually, the old large-format cameras would have a piece of frosted glass in the back and you'd actually look at the image through it and focus it and then close the back, close the shutter, slide the film in from the holder and take the picture. Then slide the film back out and back into the holder. With those, either they had the lens focusing differently so that the image came through the lens, got focused and reversed and then through another lens and reversed back again and then re-focused on the film or (I'm thinking) the photographer just had to get used to seeing the image upside down and backwards when composing it.

[daft_biker]

Originally Posted by dlwjiang

Image quality? If so why not two pentaprisms? Size?

Cheaper DSLRs do use a 2nd mirror. See pentamirror - Pentamirror - Wikipedia, the free encyclopedia

"This is cheaper and lighter, but generally produces a viewfinder image of lower quality and brightness"

[Aegea]

Remember that the setup is not like a periscope, where you are looking straight through at the object. In a DSLR viewfinder you are looking at an image on a ground glass screen. a simple mirror would give you a picture that was upside down or laterally reversed or something (even after a cup of coffee I can't think that through )

[OsmosisStudios]

Originally Posted by prabbit237

(I'm thinking) the photographer just had to get used to seeing the image upside down and backwards when composing it.

That's it. WHen you look at the ground-glass screen, the image is reversed top-to-bottom and left-to-right. It's a royal pain when youre not used to it.

[inkista]

Originally Posted by OsmosisStudios

That's it. WHen you look at the ground-glass screen, the image is reversed top-to-bottom and left-to-right. It's a royal pain when youre not used to it.

But it could also explain why so many view/technical camera shooters were masters of composition. Viewing an image upside down tends to fast-track you into right-brain mode and you perceive shapes not objects (why so many of us keep recommending Betty Edwards's Drawing on the Right Side of the Brain). I remember seeing a short documentary film on Ansel Adams, and being relatively unsurprised that when he used his enlarger to project the negative against the photopaper on a wall, he did so with the image upside down, as he would have seen it on the ground glass of the camera.

As for why not a pentaprism/pentamirror instead of a mirror: well, kind of hard to get a big chunk of prism out of the way of the lightpath from the lens to the sensor, vs. just flipping up a mirror.

[dlwjiang]

"Some optical instruments, such as periscopes and binoculars use prisms instead of mirrors to reflect light around corners. This is because mirrors do not reflect light as totally as prisms do (mirrors only reflect about 95% of that reflected by prisms under TIR conditions). Also refraction distortion can result in using a glass fronted mirror. Therefore the image is crisper and brighter. In prismatic binoculars, total internal reflection in prisms is used to extend the path length between objective and eyepiece, effectively `folding' the optical path. This makes them compact and easy to carry"

Total Internal Reflection (TIR)

I'm not sure, but I think two mirrors flips the image correctly. You need an even number of reflections. Pentaprism + mirror = 6.

[prabbit237]

Originally Posted by dlwjiang

"Some optical instruments, such as periscopes and binoculars use prisms instead of mirrors to reflect light around corners. This is because mirrors do not reflect light as totally as prisms do (mirrors only reflect about 95% of that reflected by prisms under TIR conditions). Also refraction distortion can result in using a glass fronted mirror. Therefore the image is crisper and brighter. In prismatic binoculars, total internal reflection in prisms is used to extend the path length between objective and eyepiece, effectively `folding' the optical path. This makes them compact and easy to carry"

The issue with amount of light being reflected can be overcome by using front-silvered mirrors (telescopes use such) but those tend to tarnish & scratch easier so that may be the main reason for using prisms as opposed to several mirrors.

Originally Posted by dlwjiang

I'm not sure, but I think two mirrors flips the image correctly. You need an even number of reflections. Pentaprism + mirror = 6.

Not quite so simple. That'd be true as long as the mirrors are not between the lens and the focal plane. A lens actually inverts the image in BOTH directions (sideways and vertically) before it hits the focal plane so a single mirror would invert that back again in one direction but not in both (think of how a bathroom mirror swaps sides but doesn't flip ya upside down.) Two mirrors in a corner will produce an image that puts you back correct sideways. But the lens has also flipped ya. So you have to use more tricks, such as the length of the light path, or mirrors set at several different angles, etc. to totally reverse all flips. The pentaprism does have several different angles to do just that.