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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 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 and looking through the lens, how is there a 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 is just one lens, a single lens, and both you and the recording medium look through that same lens. To make that happen, there are a lot of mirrors involved. So my image sensor is back here, and my viewfinder is up here, and obviously here is my lens. Light comes in here.
It needs to get back to the image sensor back here when I finally take the picture, but it needs to get up here for me to be able to have any kind of viewfinder, and that's where all these mirrors come in to play. First of all, there is a mirror directly behind the lens. I'm going to take the lens off here and you can actually see it. There is a mirror right inhere. Now when I press the shutter button that mirror flips up, that's the reflex part of SLR, and with the mirror up, light can then get straight back to the image sensor that's behind the shutter.
In this video right here you can see in slow motion that shutter button being pressed and then the mirror popping up, the shutter opening and closing and then the mirror coming back down. So when the mirror is down like it is here, light is bouncing up here into this pentamirror that's up here and going back out to the viewfinder where I can see it. When I press the shutter button, the mirror comes out of the way, so that light can go straight back to the image sensor.
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