Yet Another Noob Question

Discussion in 'Beyond the Basics' started by jophassa, May 24, 2006.

  1. jophassa

    jophassa TPF Noob!

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    OK, I am fairly new to the whole photographer world and have only ever used cameras which are totally automatic so these questions are sort of justified.

    What exactly is Aperture and what exactly is happening when Aperture priority is used?

    There are just a few more silly questions to come. So don't worry too much.
     
  2. duncanp

    duncanp TPF Noob!

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    Apature is the size of the diaphram, an apature of 2.8 is a wide aperture using a very big hole, as the hole is big the light rays become dispersed quicker and you have smaller depth of field (how much of the picture is in focus). also the larger hole lets more light through and you need a shorter shutter speed. f22 is a small aperture, it would give you a large depth of field, but you would need a longer shutter speed.


    hope this helps...
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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  4. JamesD

    JamesD Between darkrooms

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    The "Aperture" is the opening in the lens which permits light to enter the camera. You can vary it by telling the camera to open or close an iris-like diaphragm inside the lens. If you look at your own eye, the pupil is the aperture, and the iris is the diaphragm. If you shine more light on your eye, the pupil contracts to control the amount of light entering your eye. In dimmer situations, it dialates to let more light in. Same thing with Camera.

    Aperture is measured by the F-ratio. This is the ratio of the diameter of the aperture to the focal length of the lens. For instance, f/2.8 means that the focal length is 2.8 times the diameter of the aperture. If the lens focal length is 50mm, then at f/2.8, the aperture will be about 17.8mm in diameter. That's why as the ratio gets smaller, the aperture itself gets bigger... at f/1.0, the aperture of a 50mm lens will be 50mm wide, letting in a lot of light. Close it down to f/2.0, and only a quarter that amount of light will be let into the camera because the diameter of the aperture is now only 25mm instead of 50. (Keep in mind, when you make an opening half as wide, it only lets a quarter the light in, because you have to consider height of the opening as well, ie, the total area of the aperture opening).

    Aperture Priority is an autoexposure mode used to let the camera automatically set the exposure so you don't have to. The only two settings that affect exposure are the aperture size and the shutter speed. (ISO refers to the film speed, and has nothing to do with exposure; just make sure you get the exposure right for the film speed you're using).

    In fully automatic mode, your camera automatically sets both the aperture and the shutter speed. In fully manual mode, the camera doesn't set anything, and you have to manually select both aperture and shutter speed.

    In Aperture priority mode, you select the aperture setting (it's given priority; shutter speed is considered "unimportant"). The camera then selects a shutter speed which will give correct exposure at the aperture setting you've selected. It's sort of a semi-automatic mode.

    You would use Aperture Priority (or AV, for aperture value) any time you want automatic shooting with control over the aperture size. For instance, if you want to control how much of the foreground and background are in focus (larger apertures (smaller f/numbers) make less of the foreground and background in focus... this is called "shallow depth of field")

    You would also use AV when you are using flash. The flash from a camera is extremely short (1/10,000 second?), so if your fastest shutter speed is 1/4000 second, it's still going to get the whole flash, and a lot of time when the flash isn't on. (However, your camera has to be able to synchronize the shutter at that speed--but that's a different issue). So, since the flash is of a specific brightness and infinitesimally short duration, shutter speed will not affect it, meaning that only aperture can be used to control the exposure for flash. If your subject is close and will reflect intense light, you use a small aperture; if the subject is far away and will not get much light from the flash, you use a large aperture to gather as much of it as possible. Note that the important distance is not camera-to-subject, but flash-to-subject.

    There are other uses for AV mode, of course, but these two are the ones I most commonly encounter myself.

    Another, similar mode is Shutter Priority ( Time Value, or TV mode). It's the same as AV, except that the camera controls the aperture, while you set the shutter speed. I virtually never use this mode, so I can't tell you much about it's uses; I expect that it'd be used for stop-action photography, like sports, or indoor lighting under fluorescent lighting (for which shutter speed needs to be slower than 1/60 second in the US, 1/50 second in many other countries, because fluorescent intensity varies with the AC powerline cycle). Or for taking pictures of a television screen... if you take a picture of it at about 1/120 second, you're only going to get a quarter of the image because it paints the television picture about 30 times a second.

    Heh... I've waxed long-winded, as usual. Hopefully I've been clear, rather than confusing. If I've been the latter, someone will be along presently to sort me out.
     
  5. JamesD

    JamesD Between darkrooms

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    It's interesting to note that our eyes are basically cameras in aperture priority mode, with adjustable film speed. The persistence of vision limit is about 30 cycles per second, meaning that if you flash a light 30 or more times per second, it will appear to be on continuously, rather than flashing. That's roughly equivalent to shutter speed. Our brains (or some other part of the nervous system, I dunno) control the aperture (pupil size) based on the light available.

    And then, as we all know, when you go from a brightly lit room to a dark room (or turn out the lights), it takes a while to get your eyes to adjust, to get that extra sensitivity, similar to upping the film speed. Or, someone turning on the lights in the middle of the night...
     

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