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Arriving at the best exposure for a photo is part science and part art. In Foundations of Photography: Exposure, Ben Long helps photographers expand their artistic options by giving them a deep understanding of shutter speed, aperture, ISO, and all other critical exposure practices. This course covers the basic exposure controls provided by all digital SLR cameras, as well as most advanced point-and-shoot models. Learn how to master a camera's metering modes, how to use exposure compensation and bracketing, and much more. By the end of the course, you'll know how to develop an "exposure strategy" that will allow you to effectively employ your exposure knowledge in any shooting situation.
As we discussed earlier, your camera has two mechanisms for controlling light: it has a shutter and an aperture. In this lesson, we're going to take a look at the shutter. A shutter is simply a mechanism that allows you to control how long the image sensor will be exposed to light, or film, if you're shooting with a film camera. Shutter speed is pretty intuitive. As the shutter is open longer, more light will strike the image sensor, and your image will get brighter and brighter. Let's go back to the pinhole camera that we looked at earlier. On the pinhole camera, this is the shutter, this little door here that I opened.
When I open it light is able to pass through the pinhole and expose the film that's in the back of the camera. So, to control shutter speed, I simply hold this door open for a longer or shorter time. Now because the sensor on your digital camera is so sensitive, and because you've got a lens to focus light onto it, your digital camera needs much shorter exposure times than this pinhole camera. So short, in fact, that a door like this one is impractical, because there is just no way to swing it all the way open and closed quickly enough to get the short exposure times that your digital camera needs.
So the shutter on your SLR, and on some point- and-shoot cameras, is composed of two curtains. When you press the shutter button, the first curtain begins to slide open, and then almost immediately the second curtain begins to slide closed. This creates a thin slip that passes in front of the image sensor, exposing it to light. I actually have a video of this happening, but before we watch it, there is something you need to understand about your SLR. SLR stands for Single Lens Reflex, meaning the camera has just one lens, and this is it, the big lens that's on the front of your camera.
Your image sensor is right back here. So, it's a pretty straight shot through the lens to the image sensor. That part is pretty easy to understand. The tricky thing about an SLR is that your viewfinder is up here, above the lens and above the image sensor. And yet you're still able to look through the same lens that is exposing the sensor. How does that work? It's all done with mirrors. If I take the lens off, you'll see inside that there is a mirror at an angle here. So light comes through the lens, bounces off that mirror, and goes straight up into this part of the camera.
This is called the pentaprism. It's a prism or in some cameras there is series of additional mirrors that then bounces the light straight back out through the viewfinder. So as long as this mirror is down, light coming through the lens goes up into the pentaprism and out the viewfinder, so I can effectively look through the lens. When I press the shutter button though, the mirror flips up, so that now light is going straight back, and the shutter happens. Let's take a look at that in this video. You can see the mirror flipping open, a shutter opening and closing.
Let's take a look that again. So you can watch that mechanism on your own camera. Just take the lens off the camera and press the shutter button. The shutter curtains will move far to fast for you to see though, but you'll be able to see them in mirror flip up and down. Now point-and-shoot cameras don't always have a physical shutter the way an SLR does. Sometimes instead of physical shutter, they just turn the sensor on and off for the length of the desired exposure. For different lighting situations you, or your camera, will choose to have the shutter open longer or shorter.
Now obviously in less light it will need to be open longer, while in bright situations you will want it open shorter. Shutter speed is measured in seconds, and because shutter speeds are usually very quick your shutter speed will almost always be a fraction of a second. Your camera provides a range of shutter speeds, and these are the standard speeds that you will find out on all cameras. Shutter speeds can also be very long. If you're shooting in extreme dark, you might have shutter speed that are seconds, minutes, or even hours long. So shutter speed is fairly intuitive, as is its effect on your image.
But your shutter is not the only mechanism for controlling how much light hits your sensor. As you saw earlier, like your eye, your camera also has an aperture.
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