Digital SLR glossary

What makes an SLR?

The term SLR (single-lens reflex) is just a fancy way of saying that a camera‘s viewfinder looks through the same lens that exposes the image sensor (or film). SLR viewfinders show more accurately than a point-and-shoot camera’s viewfinder what the image sensor will capture, including the effects of any lens attachments.

As in any camera, the lens focuses light through a shutter and an aperture. But in a digital SLR, a mirror placed in front of the focal plane (where the image sensor sits) bounces the light that comes from the lens upward into a prism and then out the eyepiece. When you press the shutter button, the mirror flips up out of the way so the light can pass to the focal plane (see illustration, below).

Because light doesn’t reach the image sensor until you press the shutter, you can’t use the camera’s LCD as a viewfinder. Fortunately, most SLRs tend to have nice optical viewfinders that deliver good coverage, usually showing 95 to 98 percent of the final image.

Other terms:

35mm equivalency
What a digital lens would be equivalent to in terms of a 35mm film SLR camera. Used as a standard for discussing the field of view and magnification power of a lens. In 35mm equivalency, lenses over 50mm are telephoto, whereas lenses below 50mm are wide-angle or fisheye lens.

In a digital SLR, a mirror placed in front of the focal plane (where the image sensor sits) bounces the light that comes from the lens upward into a prism and then out the eyepiece. When you press the shutter button, the mirror flips up out of the way so the light can pass to the focal plane.

67/645
Older lens mounts made by Pentax.

Buffer
In a digital camera, the RAM that is used to temporarily hold images while they are being written to the storage card.

Burst mode
A special mode for shooting a sequence of images in rapid succession. Also known as continuous .

Depth of field
A measure of the area of an image that is in focus, measured as the depth from the focal point of the image.

Focal length
The distance, usually measured in millimeters, between the lens and the focal plane in a camera. As the focal length increases, the field of view decreases.

Interlocking controls
To avoid accidentally changing a camera parameter, some cameras require you to use multiple controls simultaneously to change a setting. This prevents accidental setting changes while carrying the camera.

ISO
A measure of the light sensitivity of a piece of film or a digital sensor. The higher the ISO, the greater the sensitivity. ISO is one of three exposure controls on a digital camera, in addition to aperture and shutter speed selection. Each exposure control allows you to affect different image characteristics.

Pentaprism
The prism that sits on the top of most SLRs. The pentaprism transmits the light reflected upward by the camera’s mirror into the camera’s eyepiece.

Rangefinder
A type of viewfinder that uses one lens for focusing light onto the focal plane, and another for framing your image. Optically, a rangefinder is much less complicated, and therefore less expensive, than an SLR configuration.

APS-sized film sensors
Short for Advanced Photo System, APS film comes in a cassette that automatically advances to the first frame when you close the back of the camera. At the end of the roll, the camera rewinds the film back into the cassette and closes the film cover. Most digital SLRs use image sensors that are the size of a frame of APS film, rather than 35mm film. Higher-end SLRs use full 35mm-sized sensors.

Cropping factor
Many digital SLRs use an image sensor that is smaller than 35mm film. If you attach a lens to one of these cameras, its 35mm equivalency will be multiplied by the cropping factor—a manufacturer’s specification. For example, if your digital SLR has a cropping factor of 1.6x, then a 50mm lens mounted on your camera will have a 35mm equivalency of 80mm. That is, the 50mm lens will yield the same field of view as an 80mm lens on a 35mm camera.

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