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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'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 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. So how is it that both your eye and the sensor can see through the same lens? If I turn the camera sideways, you can see that my viewfinder here is actually not at the same level as the lens. Well, you probably guessed already that it's all done with mirrors.
The way this work is light comes in through the lens, enters the camera body, and bumps into a mirror that's parked right here at a 45-degree angle. So then it bounces up here into this area, where there's a weird five-sided prism called a pentaprism, and the light bounces around in there and then comes out here. So I am effectively looking through this lens. When I press the shutter button, the mirror flips up. Now, that cause my viewfinder to go blind. That's why the viewfinder blacks out when you take a picture, because with it up, light can't get up here; instead light passes into the camera body, past the mirror, through the open shutter, and onto the image sensor.
When the exposure is done, the mirror comes back down and now my viewfinder is restored. So, with this single lens--that's the SL part of single lens reflex--and the moving mirror-- that's the reflex part of SLR-- I can have an image sensor that can see through this lens and a viewfinder that can see through this lens. Now, you can actually see the mirror in your camera if you just take the lens off, which I'm going to do right now. I've also configured the camera so that I can get the mirror and the shutter open and they'll stay there, and put in bold mode and I have set it for mirror lockup.
So, this is the mirror right here. It's sitting inside the mirror chamber. And watch what happens if I put my hand behind the viewfinder. It's only behind the viewfinder, and you can see it actually reflecting in the mirror, so you can see that light's coming in the viewfinder and bouncing back off that mirror. So, I'm going to press the shutter button to flip the mirror up, and I'm going to shine flashlight in here so you can see this. That thing right there is the shutter of your camera. It's sitting in front of the mirror. And now I'm going to open the shutter, and there's my image sensor, and it's got this cool prismatic, holographic thing happening, because every individual pixel on the sensor--and there are millions and millions of them--every individual pixel has a tiny little lens on top of it to help focus the light.
So, all those millions of microscopic lenses are reflecting and refracting light in weird ways. So now I'm going to let go and the mirror comes back down and the shutter closes. Obviously, when I am taking a picture, all of that happens very, very quickly, and we have a slow motion view of that right here, showing the mirror and shutter of a different SLR. So what's the downside? Well, SLRs are larger than a typical rangefinder camera, which makes them little less convenient. They can't have the giant media sizes of the big view camera. They have got a lot of mechanical parts that break down.
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, 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-- 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 low-light performance, and the ability to shoot with shallower depths of field. With there 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're going to do in this course.
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