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From my understanding of an (d)slr optical viewfinder the image is first projected onto a focusing screen forming a real image and then, using optics in the viewfinder, formed into a virtual image that the photographers eye focuses to be able to view it.

Does there exist viewfinders that instead of relying on the eye to focus project the image onto a viewing screen? I have seen pictures of twin lens relex cameras that looked like they did this but I have not found any confirmation of this.

If these exist, what are they called?

  • Question: Can you link some sources? My 5D III has a pentaprism, but I can very much see that the image I see is from the mirror (I can even see the small secondary mirror if I move around a bit). For a real image, you would ned a matte screen on the back that is illuminated by the mirror(s), correct? – flolilo May 6 '18 at 9:13
  • @flolilolio Sources for what? Yes I expect a matte screen would bee needed. I do not know if this have been done, hence the question. – lijat May 6 '18 at 9:20
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    I guess sources for your understanding/assumptions. – Saaru Lindestøkke May 6 '18 at 10:37
  • @lijat sounds very much impractical to me - it will darken/diffuse the picture and also, any light from the backside will reduce contrast. Also, focus distance of the eye will be a problem. Not saying that there isn't such a device, but it would be a quite incompetent design IMHO. – flolilo May 6 '18 at 10:47
  • @flolilolio why would focusing distance of the eye be a problem? I agree that the contrast probably is bad but if it exists I imagine the usecases to be rather specialised. – lijat May 6 '18 at 11:03
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Your understanding is not correct.

The image is indeed projected on a focusing screen, but what you see on the focusing screen is already more or less the final image.

You do not necessarily need any optical magic between the focusing screen and your eye. You could just as well look directly onto it and use the image you see to adjust focus and do the image composition. This is exactly how you use a waist-level finder, which however are usually only found on medium format cameras. A disadvantage is that the image is mirrored left to right.

On 135-film and digital SLR cameras, you usually find a pentaprism or pentamirror viewfinder. Here, you do find a prism or several mirrors between the focusing screen and the eyepiece, but they are only there to mirror the focusing screen image, so that you see it the correct way around. If you have a camera with a replaceable viewfinder, you could also on a (D)SLR take off the viewfinder and look directly onto the focusing screen and see the image.

  • You might also point out that the viewfinder projects a light field, not a virtual image. It's up to the user's eyeballs to focus that lightfield onto their retinas in another virtual image. – Michael C May 7 '18 at 8:28
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The Canon F-1 was a film SLR that had an option for mounting a direct view “Waist Level Finder”.

I believe any 35mm SLR with an interchangeable viewfinder will have a “Waist Level Finder” as an option.

Canon, Nikon, Pentax, Minolta, Miranda, and Topcon all had at least one SLR model that had an interchangeable viewfinder.

TTL metering was usually placed in the Pentaprism, and this eventually caused the manufacturers to stop offering a removable viewfinder option.

Photo source: Wikipedia

enter image description here

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Historically, some cameras, such as view cameras, or TLRs, did use a ground glass screen, rather than an eye-level viewfinder for the photographer to compose/focus. However, in the case of the view camera (the kind with a bellows, with no mirror in the body), the image on the ground glass would be inverted top-to-bottom and left-to-right. TLR waist-level viewfinders (where a single mirror is used), reverse the scene left-to-right. Correcting these reversals is the entire point of the pentaprism/pentamirror in a dSLR viewfinder.

Very few digital cameras would have anything like that these days, because of the LCD screen on the back of the camera taking their place. Flip-up or twist-out varieties of LCD screens also allow for waist-level shooting, but with better framing accuracy, and no L-to-R or top-to-bottom reversal of the image you see. And with digital data, there are a lot of additional advantages with features like magnification, exposure simulation, and focus peaking that you can never do with an optical viewfinder.

See also: Is waist level photography with a digital camera practical?

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The camera lens acts like a projector lens in that it projects an image of the outside world onto the surface of photographic film or digital image sensor. We can intercept this image using a semitransparent material like tissue paper or better, glass that has been roughed up by grinding its surface with an abrasive (ground-glass). The image formed is termed a “real” image. This is an image formed by converging light rays after negotiating a lens. In other words, light from a vista passes through a lens and its path is altered so that it forms a cone of light. A ground-glass is placed at near the apex of this cone of image forming rays. A key fact is the “real” image is formed at or nearly at the focal length of the lens. This “real” image is inverted but can be made right-reading by use of a roof prism (Penta Prism).

Inversely a “virtual” image is formed when light rays diverge. Such an image cannot be projected on a screen or ground-glass. The view of yourself as seen in a dressing mirror is a “virtual” image.

The twin lens reflex is a camera design that presents to the photographer a “real” image that is nearly the same as an image presented to photographic film. This is accomplished by mounting a nearly duplicate “viewing” lens close the taking lens. A mirror redirects the image rays of the “viewing” lens to a ground-glass viewing screen. The image is reversed right for left, but this design enhances composition and focusing when compared to cameras with less sophisticated optical viewfinders.

Ultimately the single lens reflex would prevail. This design utilizes a mirror and a roof prism (Penta-Prism) to correct the inverted “real” image.

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