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 The lens on our camera is the most often neglected element for making great photographs. To understand the lens we first start with some technical infomation. Every lens is simply a piece of glass that can be used to focus light onto a point. We use a lens to allow the camera to have a LARGE opening to let in a lot of light and still have a sharp focus for the image.
Every lens has two technical specifications of interest to the photographer. First is the Focal length. This is the distance from the lens to the point of focus when the lens is focused on infinity. For 35 mm film when this distance is 50 mm the image we get will look normal as things look with our eye. The perspective or the appearance of the depth or space between objects will look normal with a normal lens. As the focal length gets shorter than 50 mm the angle of view the camera can see becomes greater. Depth of field becomes better with a wide angle lens and focus becomes less critical. Most point and shoot and most digital cameras use a wide angle lens for this reason. A wide angle lens will also tend to distort the perspective. Objects close to the camera will look much larger and closer than they really are and objects far from the camera will look much smaller and farther away then normal. Finally we have the telephoto lens. the telephoto lens shows only a small portion of our scene while making it much LARGER. The larger image tends to throw off the perspective by making objects seem to have little space between them. We call that compression. Things seem to be close in the background yet they are actually far away. The telephoto lens also has the least depth of field or depth of focus. The background is easily made out of focus with a telephoto while the background is easily fond in focus on a wide angle lens.
Finally the last technical piece for the lens we need to be aware of is the aperture. This is the size of the opening in the lens. It turns out that as the focal length gets larger this opening also needs to get larger in order to let in enough light to expose our image. This is because the amount of light gathered in by a telephoto lens is spread over a larger area thus making the brightness less. A telephoto lens is often hard to focus on a camera because there is too little light to make the image show up well on the cameras ground glass. To calibrate this light spread we use a calculation called f-stop. This number is equal to the focal length of the lens divided by the opening. This becomes f= FL / D It turns out that it is very difficult to make a lens that is as wide as its focal length so we will not easily find a lens with a maximum f-stop of 1. Most camera lenses have a maximum f-stop of 2 which means the opening is half as large as the focal length. The nice thing about f-stop is that the quantity of light hitting the film is the same, regardless of the focal length. Thus the amount of light from a 50mm lens at f2 is the same as a 200 mm lens at f2. The opening, however, for the 50mm camera is 25 mm and the opening for the 200 mm lens is 100mm.
 The aperture effects the depth of focus on our subject. The depth we see in our photograph depends on the size of the opening in the lens. The aperture is defined as f= FL/D resulting in a number we call an aperture or f-stop. We use a number like f2 but in reality this number is 1/2. An aperture of f16 is actually 1/16th and so on. Thus the larger the f-stop the smaller the actual opening on the lens. When the opening is very tiny the depth becomes greater and greater in our photograph. Ansel Adams found that an aperture of f 64 gave the best possible depth of field in a photograph. Most cameras only to to an aperture of f 16 of f22 though. The chart above illustrates how depth works. For a normal lens the depth is visibly less at f-2 than it is at f-16. But, when we focus the camera we are looking through the lens at an aperture of f-2, thus what we SEE is the lens at its WORST case depth. The final photo will be VERY, very different if we set the aperture to f16. To help us see this the camera has a depth of field scale on the lens that shows us the material in focus on the distance scale on our lens or some cameras have a depth of field preview button that allows us to preview the change in depth while making the lens very dark and difficult to focus. Depth of field gets less and less as the focal length gets greater. Thus the total distance in the photo above at f16 will be shorter with a 100 mm lens than with a 35 mm lens.
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