The Single-Lens Reflex Camera

• A reflex camera means WYSIWYG.

• Viewing mechanism has optics like a telescope’s.

• The lens acts like a single achromatic thin lens with flat focal plane, adjustable aperture and image distance.

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As you can see, the SLR camera appears to be a fairly complex device. The “Reflex” part means that the camera is equipped to let you see the same image that will be recorded on the film when the shutter opens -- the optical system that lets you do that makes the camera seem more optically complicated than it really is.

The lens in this case is removable, allowing other lenses of different focal lengths to be used. The one shown is a sophisticated compound achromatic lens, designed to produce a very flat focal plane at the film, but you can think of it as just a thin converging lens with a focal length of about two inches.

An adjustment lets you move the lens closer or further from the film -- basically to adjust the image distance to match the object distance for the subject you want to photograph. Adjusting the aperture changes both the intensity of the image at the film and the “depth of field.”