What is an SLR?

All cameras have at least one thing in common: they have a lens that sits in front of a focal plane. On that focal plane is a recording medium, either a piece of light-sensitive film or paper or a digital image sensor. The focal plane needs to sit directly behind the lens because the lens is used to focus light onto your recording medium. Another way to think of it is that the recording medium looks through the lens. What's tricky about camera design is that if the recording medium is sitting there looking through the lens, how is there room for you to look through the lens to frame your shot? Camera designers have wrestled with this problem since the beginning of photography, and they've come up with lots of solutions.

For example, with a view camera, you actually take the recording medium off so that you can look through your lens to line up the shot, and then you put the recording medium back on. Needless to say, this doesn't make for particularly speedy shooting. In a twin-lens reflex camera, you look through one lens and a second lens exposes the film. However, if I'm shooting up close, my framing might be off due to the parallax shift between the two lenses. Similarly, in a rangefinder camera, I look through this viewfinder while the camera looks through this lens.

I still might have parallax issues, but with a camera like this I can actually change lenses and still have a viewfinder that works. The SLR, or Single Lens Reflex, solves all of the issues with these other designs. With an SLR there is just one lens, a single lens, and both you and the recording medium look through that same lens. So how is it possible that that same lens can expose the image sensor and give me a viewfinder? You know where the lens is on your camera and of course this is the viewfinder.

My image sensor sits right back here, directly behind the lens, so light can come straight through the lens directly to the image sensor, but how is it that it can also get up here so that I can see it out the viewfinder? On this camera it's all done with mirrors. Light comes into the lens and hits a mirror that's sitting right here like this. That bounces the light up here into another system of mirrors called a pentamirror. That pentamirror in turn sends the light back out through the viewfinder where you can see it. When the mirror is up, light is no longer being bounced up into the viewfinder.

That's why the viewfinder goes black when you press the shutter button. Now you can actually see a lot of this stuff if you just take the lens off of your camera. You can look here into the mirror chamber and see that there is in fact a mirror right there. In fact, you can even look up there and see other mirrors. Now watch what happens when you press the shutter button. This is some high-speed video of another camera. Notice the mirror flips up, the shutter opens and closes and then the mirror comes back down, and sitting directly behind that shutter was the camera's image sensor.

So what's the downside? SLRs are larger than a typical rangefinder camera, which makes them a little convenient. They can't have the giant media sizes of a big viewfinder. They have got lots of mechanical parts that can break. They can be noisy. But overall today's SLRs, particularly digital SLRs, offer the best all-around camera design, allowing for incredible flexibility of lens choice, shooting options, they give you portability, and a lot of ease of use. While there are a lot of great digital point-and-shoots on the market--and a point-and-shoot camera is often the best camera choice depending on the shooting situation-- in spite of that, SLRs score over their smaller point-and-shoot counterparts, both in terms of image quality and shooting flexibility.

With their larger sensor size, they provide better quality better, better low-light performance, and the ability to shoot with shallower depth of field. With their interchangeable lenses, fast burst rates, and advanced features you can shoot just about any subject with an SLR. Now you just have to learn how to use it, but you will learn all about that in this course.