MadSci Network: Engineering

Re: How does a diagphram work in a camera

Date: Fri Mar 24 16:37:50 2000
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Engineering
ID: 953676222.Eg

Dear Lorna, The diaphragm is another name for the aperture. The aperture is the hole through which the light enters the camera. It is positioned between the lense and the shutter. When you select the "f-stop" you are setting the size of the aperture, or diaphragm. The smaller the aperture, the less light that will come through, BUT you have a greater "depth of field". The depth of field is the range of distances from the camera that will be in focus. A small aperture allows a large range to be "in focus"; whereas a large aperture allows a small range to be "in focus". (There are usually markings on the focus ring that show you what the range of focus is for a select number of f-stops). If you made a pinhole camera, then you know that if the hole was bigger, things would have been less in focus (that's why we use a "pin" hole). This is also true for camera's with lenses. "So", you might say, "why wouldn't I just always use the smallest aperture possible?" The answer is that because there is less light allowed to enter the camera, you must leave the shutter open longer (a slower shutter speed) in order to get enough light to expose the film properly. This opens up the opportunity for movement in the image (or camera) that will cause other blurring problems. When you are trying to capture fast motion - like a runner, auto race, or dolphin show - you want a very fast shutter speed and thus a larger aperture. This makes focusing much more important when you use a fast shutter. Sometimes, a photographer only wants a small range of distances to be in focus - like when the he/she wants to emphasize the foreground. In this case, open up the aperture, focus the camera and set the shutter speed accordingly. When you are at the beach and it's real bright and you want your little brother AND the sharks in the water to be in focus, you can set the aperture small and get away with a reasonable shutter speed. The other parameter is the "film speed". This is the film's sensitivity to light. If you use a "fast" film (which is more sensitive to light), it allows you to use a smaller aperture and/or a faster shutter speed. The problem here is that faster films are "grainier", which is to say that they don't make as nice an image. Find a very dark photograph and look closely at it. Compare it to a normal one. You will see the "grains" of photographic material that cause this "graininess". Many cameras today automate the processes of detecting the film speed, setting aperture (f-stop), setting the shutter speed and even focusing the camera. In the past, photography was much more challenging because you had to balance these things yourself. It was a real art. For more info, I found a couple of web pages you might find helpful: This one talks about the "camera obscura" which is an interesting history of the pre-camera: http:// This one gives information on the parts of the camera. I would suggest checking out the other parts of this site, as it gives a pretty good tutorial on both cameras and photography: http:// Good luck, Todd Jamison Observera, Inc.

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