The mystery revealed
So what is an SLR? SLR is an anagram for Single Lens Reflex. What this means is that the camera uses one lens that reflects the image to a viewfinder by means of a mirror.

You may think, "Well, duh, every camera has a single lens." Yes, these days, every camera you see has only one lens, but there was a time when TLRs, or Twin Lens Reflex cameras were very popular. That's right, two lenses. One lens was for viewing and composing the image, the other for actually taking the image. The most popular model was the Rolleiflex shown in Figure A. But now though, for the most part, cameras just have a single lens for both viewing and taking the picture.

FIGURE A


The Rolleiflex TLR was the first reflex camera. Roll over picture for a larger image.

How it all works
How does all this reflecting work, and why is it better? Well, it all works out like this...

Light enters the front of the lens, passes through and hits a mirror. The image is then reflected up to a ground-glass focusing screen for viewing, and sometimes to a five-sided pentaprism and to a viewfinder. When you release the shutter, the mirror flips up out of the way, and the film or image sensor is exposed. Figure B illustrates how an SLR works.

FIGURE B


My crude drawing shows how an SLR camera works. Roll over picture for a larger image.

The biggest advantage to the Single Lens Reflex camera is that you see exactly what the lens sees. What you see is what will be captured on the film or image sensor. When I got my first SLR, my dad explained it to me by saying, "What you see, is what you get."

I personally hate waist-level finders, where you view the image directly on the focusing screen. The problem with these is that the image you see is reversed left-to-right, because it's a mirror image. This makes composing kind of difficult because you have to remember to move the camera right to adjust the image to the left. I much prefer a prism finder, so that the image you see is corrected left-to-right and top-to-bottom.

This is better why?
In addition to being able to see exactly what the camera sees, another advantage of the reflex camera is a depth-of-field preview. Most SLR cameras have interchangeable lenses with full aperture control. These lenses generally have manual and automatic aperture settings. By putting it on manual, you can adjust the aperture setting and see how the depth-of-field will be affected. On automatic, you see the image as if the aperture were wide open. But when the shutter is released, it will automatically close down to whatever aperture you have set.