SLR cameras without mirror?

The term "Single Lens Reflex" (SLR) indicates that the camera has only one lens, used for both composing and shooting the image. Earlier there were also cameras with two lenses, one for viewing and one for taking the picture, but nowadays SLR cameras rules the market for system cameras. It's a cheaper and more presise construction because only one lens is needed and the viewed image is identical to the photographed image.

The principle for an SLR camera is that light passes through the lens and hits an angled mirror which reflects the light upwards to a focusing screen. The projected image from the lens is displayed on the screen's matte surface. Ergonomically, the most practical position for the viewfinder eyepiece is to place it on the back of the camera instead of looking down on the focusing screen. This requires that the image must be angled backwards and rotated to show it upright in the viewfinder. For this purpose, a five-surfaced prism (hence the term "pentaprism") is used. To save weight and costs, the prism is sometimes replaced by five mirrors (pentamirror).

Why mirror reflex?
The mirror reflex principle has been unchanged since the first SLR cameras appeared on the market in the early 1950's. The advantage is that you need only one lens and the viewfinder displays a near identical image as that being registrered on the film or image sensor whether the lens is a zoom lens or has fixed focal length. The disadvantage is that the mirror must swing upwards to allow the light to pass straight through to the film or image sensor. As a result of this, the viewfinder image disappears during exposure, but reappears immediately afterwards as the quick-return mirror swings back.

Traditionally, the only alternative to a mirror reflex solution was an optical viewfinder (the correct term is "rangefinder"). It works fine on cameras with a fixed, non-interchangeable lens, even with a modest zoom lens. However, as it is positioned beside the lens, the image has a slightly different perspective and a less precise angle of view. Perspective compensation mechanism or guide lines are sometimes implemented to overcome this problem. When a zoom lens is fitted to the camera, the optical viewfinder has a zoom function that corresponds more or less with the focal lengths of the lens. However, physical limitations restrict the zoom factor of the lens when an optical viewfinder is used.

Electronic viewfinders (EVF)
In order to permit lenses with larger zoom range in a digital camera, manufacturers installed a new device called electronic viewfinder. This is basically a tiny TV screen, mostly LCD, showing exactly the same image as on the large LCD screen on the back of the camera. By using an electronic rather than an optical solution, the information displayed in the viewfinder may be selected by the user, exactly as on the rear LCD.

Since the electronic viewfinder has the same kind of eyepiece as an SLR camera, showing exactly the same image as that being recorded, one could say that the electronic solution combines the advantages of the focusing screen in an SLR camera with the advantages of the LCD screen.

Until now noone has managed to produce an electronic viewfinder with a resolution and sharpness that can match a mirror reflex viewfinder. Their refresh rate is also too slow, creating a blurred viewfinder image when panning the camera. However, this is "only" a technological problem and from experience we know that such obstacles will always be solved. All it takes is some research and priority over other developments.

Other disadvantages of conventional D-SLR cameras are the lack of real-time image display on the rear LCD screen. Olympus solved this problem with an NMOS Live View image sensor from Panasonic, first introduced in the Olympus E-330, followed by the Panasonic Lumix DMC-L1 and the two new Olympus E-410 and E-510, soon becoming available.

System camera with electronic viewfinder?
Theoretically, it is possible to implement an EVF in a single-lens reflex camera and thereby avoiding the mirror. To make such a solution useful, the image quality of the EVF must be improved considerably. Based on recent developments and improvements in display technologies, I'm convinced that we will soon have EVF's with outstanding image quality. When this happens, I think we may kiss the mirror reflex solution goodbye as electronic solutions tends to be cheaper than mechanical ones.

The advantages of an EVF over a mirror reflex viewfinder are many. If we assume that there will be no major difference in the finder image quality,  an EVF may provide 100% coverage even in entry-level cameras, reviewing the image in the viewfinder will be possible, information display may be user selected, no viewfinder black-out during exposure, no noise from the mirror mechanism and it will be possible to make a tiltable eyepiece.

I suppose that since the mechanical restrictions of the mirror, focusing screen, prism and eyepiece will be avoided, it should be possible to place the EVF screen in a position that would give a super-large viewfinder image. With a Live View image sensor it should also be possible to make video recordings. Imagine HD quality video in a photo camera with interchangeable lenses!

Apart from the nostagic feeling of a "real" viewfinder image on a focusing screen, I can see no disadvantages of using an EVF rather than a mirror reflex viewfinder.

So far, noone has announced such a design, but I'm convinced that it will come as soon as the industry has succeeded in making a suitable EVF.