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Camera Anatomy - Single Lens Reflex (SLR) Cameras
Ensign Speed Reflex c1928

As already discussed, focus is fundamental to photography, both in terms of what is - and what is not in focus. The rangefinder camera determines what should be in focus without actually demonstrating the degree. The TLR goes one step further, by using a second viewing lens but it is the Single Lens Reflex (SLR) that solves the problem fully.

In this type of camera a mirror intercepts the light that passes through the lens and projects it onto a ground glass screen where it forms an erect (upright) but mirrored image. Now the photographer is truly viewing through the lens and able to accurately determine precisely both the focus and depth of field. When the photograph is ready to be taken the mirror is retracted allowing the light to pass directly to the film, when the shutter is opened. In the earliest models the mirror was retracted manually, but by the time the Speed Reflex (at left) was around in the mid 1920s pushing the shutter release first retracted the mirror before firing the shutter. The early SLRs did not have the familiar prism of today, the image was inspected by viewing down a leather tunnel to the ground glass screen. Another common feature of SLRs necessitated by their construction was the need for the light to pass through the lens to the reflex mirror unhindered, this lead to the shutter mechanism being placed just in front of the film - the focal plane shutter. As an aside the Speed Reflex shown here is also a bellows camera - for hidden within is a small focussing rack with the lens mounted in front of a short bellows.

Praktica LTL3 The familiar, until the 1990 that is, 35mm SLR with the distinctive prism housing on top that first appeared on a Contax camera in 1949. The prism serves to reflect and flip the mirrored image from the ground glass screen to the viewfinder, resulting in an erect and true image which is bright and often magnified by the viewfinder optics. The use of 35mm film allows these cameras to be relatively compact which removed one of the SLRs drawbacks. With the shutter positioned just ahead of the film within the camera's body it is possible to change lenses without exposing the film making the design very flexible. The principle shortcoming is that the focal plane shutter uses a variable gap to vary the shutter speed and that only a longish exposure time will synchronize with flash. Most SLRs kept the lens aperture open fully, so that the user had the brightest possible image in the viewfinder whilst composing the shot, the aperture closing to the value set as the shutter release was pressed. Some, but not all, had a preview button, allowing the aperture to be closed to the value set whilst composing the image, so that the depth of field could be reviewed.
Kodak Retina Reflex III

The Kodak Retina Reflex III at right is one of a small number of SLRs that attempt to solve the flash synchronization problem by using a leaf (diaphragm) shutter ahead of the mirror. In order for this to work the shutter has to have the main shutter open for focussing, whilst a second trap door protects the film at the focal plane. Whilst a number of manufacturers persevered with this design, the complexity caused both reliability and cost issues and the concept didn't survive beyond the 70s.

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