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Forum: Sold Items
03-03-2008, 02:22 PM
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Forum: Pentax DSLR Discussion
02-28-2008, 11:33 AM
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ISO is another factor in calculating your exposure, along with aperture and shutter speed. In the simplest terms, doubling the ISO (eg. going from 400->800 or 800->1600) doubles the exposure. The values of aperture, shutter speed, and ISO are all directly related in this way, except that shutter speed is inversely related to exposure. So if you increase aperture by a full stop, you let twice as much light in. If you keep the shutter opened twice as long, you let twice as much light in. And if you shoot at double the previous ISO, you’re making the sensor twice as sensitive to light.
So, imagine a situation where you’re shooting in low-light. To get the shot properly exposed, you might choose to do a number of things. For example, if you have a tripod and a stationary subject, you can use a long exposure because the subject won’t blur by moving around and the tripod eliminates camera shake. Or you might opt to use a flash.
But what if you have a moving subject, or are in an area where you can’t use a tripod, and flash is out of the question? If you have a very fast lens – one with a very large aperture – to let in more light, you might be able to handhold the shot at a low ISO. Or you might not. Even an f/1.4 lens has its limits, and besides you might not want to shoot wide opened anyway since perhaps you want greater depth of field in your shot.
So what do you do? You can’t slow down your shutter, because you’re handholding the shot and you’re already at the slowest “safe” speed for your focal length / subject’s movement. You can’t use a flash. You can’t open up your aperture any further (either at all if you have a slow lens or because it won’t give you the shot you desire.)
What you do is bump ISO. Because of the direct relationship between aperture, shutter, ISO and exposure, if you double the ISO it’s as if you opened the aperture a full stop, or shot with shutter opened for twice as long. Going from ISO 200 to ISO 1600 for example, is three steps up in exposure value, so if you previously had to shoot at a minimum 1/60sec shutter speed, now you can shoot at 1/480sec (1/60 x2 x2 x2). Or you can move three full stops down to get better DOF. Or you can do a combination of the same.
The downside is that shooting at higher ISO introduces more noise to the photo, so it’s a trade off. For most people, the rule of thumb is to shoot at the lowest ISO the situation allows, to minimize noise. But there are certainly times when the higher noise is worth accepting, in favour of actually getting the shot!
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Forum: Sold Items
02-25-2008, 04:01 PM
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Forum: Sold Items
02-24-2008, 09:44 PM
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You've Got (Private) Mail re: the Sigma 55-200
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