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"Real image" optical viewfinders

Doug Kerr

Well-known member
We often read of the direct-view optical viewfinders provided with some modern "compact" digital cameras described as "real image" viewfinders. I do not know whether in every case that description is apt. (The definitions of "real image viewfinder" quoted are often "fanciful".) But in any case, what does that term really mean?

In a real image viewfinder, the first ("objective") stage of the optical system generates a real image of the scene (from which the name is taken). That is an image that is actually formed by the convergence into a point (on the image plane) of the rays from each point of the scene.

In most cases this is an aerial real image - that is, it is not allowed to fall on, for example, a ground glass screen.

A real image can serve as the valid object of a subsequent optical system. Indeed, in a real image viewfinder, the real image formed by the first stage serves as the object for the second stage, through which the viewer looks.

The second ("eyepiece") stage generates, as its "output" for the viewer, a virtual image, which seems to the viewer to be perhaps one meter forward of the eyepiece.

Virtual image means that this image does not actually exist - there is no place which the rays from a point on the first image actually converge to a point on a second image. It is just that the rays entering the eye, out of the second stage, seem to have come from a point on something at this location.​

The common implementation of a real image viewfinder is essentially a Keplerian telescope with a wrinkle. The wrinkle comes from the fact that in a Keplerian telescope, the real image formed by the first stage is inverted, and the virtual image formed by the second stage is not inverted from that, so the image seen by the viewer remains inverted. This is accepted by users of astronomical telescopes, but would not be acceptable in a viewfinder.

So in actual real image camera viewfinders, there is some system (typically involving prisms) that "erects" the image. (We may exploit the prisms as well to "Z-fold" the optical path to best fit the available space in the cameras, especially with respect to the available "depth" for the viewfinder.)

The fact that, in my new Canon PowerShot G16, the input port of the direct view optical viewfinder is offset from the viewing port is consistent with the possibility that there may be a prism system (and Z-fold) involved.

What would a "not real image" viewfinder be?

Many simple direct-view viewfinders (especially when zoom tracking is not involved) are in effect reversed Galilean telescopes. The intermediate image in such a system is virtual (thus these are not said to be "real image" viewfinders). Both stages are non-inverting, so no erecting system is needed. There are various performance limitations of this simpler implementation (and I'm not sure that zoom tracking is even feasible).

Best regards,

Doug
 
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