Q: “Why is the Pentacon Six called an “SLR” (a “Single Lens Reflex”) when it takes so many different lenses?”

A:
The name “Single Lens Reflex” is used to differentiate between this camera type and the type known as a “Twin Lens Reflex” (“TLR”).  I hope that the following diagram and photographs will make this clear.

The Twin Lens Reflex
Firstly, the basic principle of all “Reflex” cameras is that they have a mirror that reflects the image from a lens onto a glass screen that is used for focussing and composing. The outline sketch to the right illustrates the principle of the Twin Lens Reflex camera. Here, we are looking side-on at a box.  We can see that a mirror in the top half of the box reflects the image from a lens that we will call the “viewing lens” up onto a ground-glass screen at the top of the box, used for focussing and composing the image.  The photographer holds the camera at chest height, or higher, leans forward and looks down onto the focussing screen.  In a real camera, there would be flaps standing up round the focussing screen, to shield it from light coming from around it. The taking lens is in the bottom half of the box, and directs the image straight onto the film, which is held in place near the back of the box. Both lenses are mounted onto the same front component of the box, and this can be moved backwards and forwards within the box, to achieve focus.  Distances are so precisely calculated that when the image on the focussing screen is sharp, the image on the film will be, too. Compared with the Single Lens Reflex camera, the Twin Lens Reflex camera has some advantages and some disadvantages: Advantages Disadvantages The mirror does not move: without a mechanism to move it, the camera is quieter and may be lighter. The viewing screen does not go blank as the photograph is taken In theory, there is less delay between pressing the shutter button and the picture being taken (although in practice, this is rarely measurable or observable in the resulting pictures). The shutter is normally mounted within the taking lens and for many TLRs provides flash synchronisation at all shutter speeds. As the viewing lens is higher than the taking lens, there is a difference between what is shown on the focussing screen and what is recorded on the film.  (This is called “parallax”.)  This is particularly a problem with close-up photography.  Even with portrait photography, the top of the subject’s head may be cut off! Most twin lens cameras do not have interchangeable lenses: if you want a wider-angle lens or a longer lens, you will need a separate camera with the appropriate lens.  (This was the case with Rolleiflex cameras, but there were TLR cameras from Mamiya that did offer interchangeable lenses.) If you do use a TLR with interchangeable lenses, you obviously need to replace both the taking lens and the viewing lens.  They are always supplied together, but this must be reflected in the cost and the weight. The top shutter speed is likely to be 1/500 sec (or less on some TLRs).		[tlr_dia.jpg]
No doubt there are other advantages and disadvantages of Twin Lens Reflex cameras.  However, most of these cameras are fine, even superb, and they are often very satisfying instruments to work with.

The Single Lens Reflex
To illustrate the Single Lens Reflex principle, I am using photographs that I have taken of an immediate predecessor of the Pentacon Six, a Praktisix, cut in half.  Such cameras were occasionally supplied for use in camera shop showcases and windows, to explain the SLR principle while at the same time promoting the camera. We can see that this particular camera has the 80mm Biometar lens and a non-metering pentaprism on the top of the camera.  The five elements of the lens are clearly visible.  Underneath the pentaprism is a large rectangular lens that is within the prism housing.  This gathers the light rays from the focussing screen, which is immediately below it and is part of the camera. Below the focussing screen, we can see the mirror, which is at approximately 45°, just as in the TLR diagram above. Behind the mirror, just in from the camera back, we can see the pressure plate that holds the film flat.  As it has been cut in half, we can see its bright, metal surface as a narrow, vertical line.  We cannot see the shutter, which consists of two flat cloth curtains that would normally be just in front of the film, to prevent light from reaching it. Unlike the Twin Lens Reflex, the Single Lens Reflex has a single lens which must fulfill two functions: First, it must project the image onto the focussing screen, via the mirror, like the Viewing lens in the TLR. Then, when the shutter is fired, it must project the image onto the film, like the Taking lens in the TLR. In the next two pictures, we have used a laser pen to enable us to see the paths followed by the light rays as they pass through the lens.		[halfp6.jpg]

The laser pen was taped to a mini tripod, but tended to sag, so it was “steadied” by my free hand.  With my right hand firing the shutter on the camera that took the picture, my left hand was not that steady.  Because of the low ambient light level required to show up the laser, the exposure was 30 seconds.  In consequence of these two factors, the resulting laser line was somewhat diffuse.
In the first picture, we can see the laser light as a bright red point going through the five elements of the lens, slightly above the centre point.  It then strikes the mirror and is deflected up through 90° to the focussing screen.  It traverses the focussing screen and goes through the condenser lens at the base of the prism, then straight up to the top of the prism.  From there, it is reflected forward and down to the lower front face of the prism.  This part of its trajectory has not come out clearly in this image.  However, we can see how the laser light is reflected from that lower front face of the prism straight back to the rear surface of the prism, as a horizontal line in this image.  From the rear surface of the prism, it strikes the eyepiece lens on the back of the prism, through which the image is viewed by the photographer.  (But if you try to replicate this experiment, do not let the laser light shine into your eye, or anyone else’s!  It may cause permanent damage to your sight or to theirs!)		When we fire the shutter on an SLR, the mirror has to flip up to a fully horizontal position at the top of the chamber, so that the image being projected by the lens can strike the film (the focal plane shutter having opened).  In this sectional model of the Praktisix, the mirror mechanism unsurprisingly no longer works.  I could have “swung” the picture of the mirror up in imaging software.  It would then have been immediately below the focussing screen, preventing any light from entering the camera from above.  However, I have not done that, but by moving the laser pointer slightly towards the right-hand side of the camera (viewed from the front of the camera), I have managed to get the laser beam to brush past the edge of the half of the mirror (which should at this point not be there!) and onto the pressure plate, thus approximately simulating what happens when the film is exposed.  Note that while the image is being projected onto the film, it is no longer being reflected up to the focussing screen, which is why no image is seen in the viewfinder during the exposure.
[laserhalf_01.jpg]		[laserhalf_04.jpg]

• As regards composition, what you see is what you get.  No more tops of heads cut off in photos.
• For close-up work, especially, the advantage is enormous, since macro and micro photography are essentially impossible with a TLR.
• The immense majority of SLRs (but, surprisingly, not all of them) have interchangeable lenses, resulting in the one camera becoming suitable for virtually all types of photography (e.g., wide-angle panoramic shots, portraits taken with slightly longer lenses, wildlife and sports photography with extreme telephoto lenses, etc.).
• Each time you change a lens, you only change one lens, not two.  (And each time you buy a lens, you only have to buy one, not a pair.)
  

Since the late 1960s, SLRs have been the most popular cameras with most professional photographers (especially in photojournalism, sports and wildlife photography) and with most enthusiastic amateurs.  Most film-based SLRs were for 35mm film, but SLRs in larger formats (like the Pentacon Six!) are capable of producing images that are visibly superior to many equivalent images taken with 35mm SLRS.  The preference for SLRs (and digital SLRs) continues with many professionals towards the end of the second decade of the 21st century.

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© TRA First published: January 2018