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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 that 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 have 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 a particularly speedy shooting. In a twin lens reflex camera, you look through one lens, and a second lens exposes the film. However, if I am 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. To make that happen, there is a mirror inside your camera. Now, here is how this works. Normally, light comes through the lens, it enters the body, and it bumps into a mirror that's sitting right here at 45 degrees.
The mirror bounces light up into this thing, which is called a pentaprism. Inside this part of your camera, there is a five-sided prism that knocks the light around until it comes out the viewfinder here. So when you look through the viewfinder, thanks to this series of prisms and mirrors, you're looking through this lens right here; this single lens on the camera. When you press the shutter button, the mirror that sits right here flips up. When that happens, light can pass into the camera body, and instead of bouncing up here, it just keeps going straight.
The shutter opens, it passes through the shutter, and hits the image sensor. When the shutter -- when the exposure is done, the shutter closes, and the mirror comes down. When that happens, light goes back to being bounced up here. This is why, when you press the shutter button, your viewfinder goes dark for a moment, because when this mirror pops up, the viewfinder basically goes blind. So with this single lens, I can, thanks to the mirror, get light out the viewfinder, and get light back to the sensor. That's the SL part of SLR.
The reflex part is referring to the fact that the mirror moves; that it goes up and down. You can actually see the mirror in your camera. I am going to turn the camera around to the front, so that we can look inside the mirror chamber itself. Now, if I take the lens off the camera --I'm just going to take it off, just like I normally would here. This is the mirror chamber. So you can see the mirror right there. It is, in fact, sitting at a 45° angle.
Your lens sticks into this chamber as far back as these metal contacts here. Sitting behind the mirror is the shutter, which is closed right now, and then behind that is the image sensor, obscured by the shutter. I have got the camera in a mode where if I press and hold the shutter button down, the mirror will stay up, and the shutter will stay open for as long as I hold the button. So I am going to do that right now. I am going to pop the shutter open, and that mysterious glowing thing back there is your image sensor. Now, your image sensor doesn't actually glow, but you can think of it that way.
What's going on here is every pixel on the surface of the sensor has a little lens over it. It's part of how light gets focused properly onto the sensor, and those mirrors are tiny microscopic things, and they're all reflecting and refracting light in this weird way that's giving us this cool rainbow effect back there. I am going to let go of the button now, and the shutter and mirror are going to come down. Here is a slow-motion movie of this whole thing happening on a different camera. You can see the mirror flip up, the shutter open and close, and the mirror come back down.
So what's the downside? Well, SLRs are larger than a typical rangefinder camera, which makes them a little less convenient. They can't have the giant media sizes of a big view camera. They have got a lot of mechanical parts that break down, and they can be noisy, but overall, today's SLRs, is particularly digital SLRs, offer the best all-around camera design, allowing for incredible flexibility of lens choice, shooting options, portability, and ease of use, while all giving you a nice big bright viewfinder. While there are a lot of great point-and-shoots on the market -- and a point-and-shoot is often the best camera choice, depending on the shooting situation -- 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 quality, better lowlight performance, and the ability to shoot with shallower depths 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, and that's what you are going to do in this course.
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