We will limit our discussion to the SLR camera with the 50mm lens. The letters SLR stand for single lens reflex, which means that the viewfinder and image capturing lens are one in the same. The term "50mm" refers to the focal distance between the subject and the camera.
The SLR solves many of problems associated with the compact camera. Parallax error is corrected with a built in mirror and prism that allows you to view a subject through the same lens that takes the picture. Although this addition makes the camera heavier and more complex, as well as more expensive, allows the camera to do a lot more things. In close up photography for example, you are able too see exactly what you are photographing without any concern as to whether part of the image will be missing in the final photograph. This is because you are actually seeing through the lens of the SLR when you look in the viewfinder.
The lens works pretty much the same way the human eye does. As the light rays collected by the lens enters the camera body, they are reflected upward by an angled mirror into a prism housing on the top of the camera. Due to the characteristics of lenses the image is turned upside down and reversed on passing through the lens. The mirror turns the image right side up, and the prism turns it around and reflects it into the eyepiece. By the time it reaches your eye, the scene looks like the one you see without the camera, which will then appear in the final photograph.
Holding the camera
You should hold your camera in the same manner as the funny looking man in the image below. Don't worry, not all photographers have bad hair.
Focusing
Besides being able to see what the heck it is you're taking a picture of, the viewfinder exists to also help you accurately focus your image. To help you do this a focusing screen is placed just under the penta prism and above the mirror. It is actually the focusing screen you are looking at when you are focusing the camera. For the system to work the focusing screen is the same distance from the lens as film plane, so if the image appears sharp on the focusing screen it will be sharp on the film plane. In the center of the focusing screen there are two devices that appear as circles, one within the other, to help you accurately focus the image. (See the images of the images of the Asian woman below to see examples of focusing). The center one is made from two small triangles of glass, this is used when your subject has some sort of line in it, such as a nose or a window frame. If the camera is correctly focused, any line that passes through this area will appear unbroken, if on the other hand the image is not focused, the line will appear disjointed as shown in the diagram. The outer circle is called the micro prism and is formed from thousands of pyramids of raised glass, the micro prism is used where there are no lines in the picture. If the subject is correctly focused the micro prism will appear clear, if incorrectly focused the image will appear as a blur through the micro prism. Fascinating, no?
not focused.....focused
Shooting the subject
When you press the shutter button to take a picture, the angled mirror flips upward to permit light to pass to the back of the camera, where the film is. Miraculously, the shutter, a pair of curtains just in front of the film, opens to let the light hit the film. The shutter remains open for a precise amount of time, which is determined when you set the shutter speed. As soon as the shutter closes, the mirror drops back to it's normal viewing position. Because the mirror rises to allow the image to pass through to the shutter and then to the film, the viewfinder goes black momentarily when you take a shot. Then, by the grace of God, an image will expose itself on your film.
Metering
When you look through the viewfinder you should see, to the right of the image, a bunch of numbers. (See picture below.) These numbers refer to the correct shutter speed required to get the best possible exposure. The shutter is a mechanical means of controlling the amount of time light is allowed to pass through the lens to the film. The period of time is known as the shutter speed. Shutter speeds are calibrated in seconds and fractions of seconds i.e. ½ 1/4 1/15 1/30 1/125 1/250 1/500 Each shutter speed is doubling or halving the previous one and has a direct relationship with the aperture. Your camera has a built in electric eye, run by the power of your battery which judges the amount of light in the direction you are pointing your camera. There should be a little pointer-thingie next to one of the numbers in your viewfinder. Sometimes I feel like this pointer works a lot like a Ouija board in that you never really know where the information is coming from, nor can you count on its dependability. Most importantly, you do NOT have to follow its advice if you don't want to. In fact, you can achieve some pretty neat affects by making up your own mind with the advice of the light meter. Take a look at the images below. They are a couple of examples of what can be achieved with different shutter speeds.
high shutter speed...low shutter speed
Depth of Field
The distance behind and in front of the focused distance with in which the image appears acceptably sharp is called the "depth of field." Besides being greater the shorter the focal length of the lens and vice versa, this increases as the lens is stopped down and becomes greater the farther from the camera the lens is focused. Lens stops refer to the size of the aperture in the shutter when you open it to let in the light for your image. The larger the aperture, the less of your image that will be in focus. The smaller your aperture, the more of your image that will be in focus. See the images below as an example.
large aperture......small aperture
The near and far limits of acceptable sharpness can be determined from the depth of field scale on the lens barrel. With the lens focused at a given point the image will be satisfactory focus from the nearer value to the farther value on the distance scale indicated by the depth of field scale marks for the aperture to be used. This will give a reference in terms of meters for deciding just which f-stop you should use.
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