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Old Dec 28, 2004, 12:28 PM   #1
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Can someone please explainthe consequences of setting ISO to 'Auto' (i.e.,does theactual ISO settingthen depend on the lighting conditions, or what)? :?
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Old Dec 28, 2004, 2:46 PM   #2
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In a film camera, the ISo is fixed by whatever film you load. That only leaves two variables to consider for exposure settings...aperture and shutter speed. An AE system will use the exposure program to decide on a default exposure based on the lighting conditions.

But, a digital camera is capable of altering the sensitivity if the sensor (changing the relative ISO) and that enables the camera to have a thrird variable in the exposure equation...aperture, shutter speed AND ISO. If the light levels are low, the camera will try to get the right exposure with the aperture/shutter speed combination but if the exposure exceeds the specs of the camera (for example if the aperture can't open up any more or if the shutter speed required is more than the camera can deliver) then the camera can change the relative senisitivity of the sensor to bring the exposure back to a range that the camera can handle.

In other words, if the camera can't achieve correct exposure at ISO100 then it will increase sensitivity until it can...up to the limit of that parameter.

Setting the camera ISO to auto lets thishappen but it also allows the camera to make a decision that might leave you with a picture that has more noise than you want. If light levels are low, you might end up with an ISO800 picture which will have lots of noise but might be better than not getting the shot at all!
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Old Dec 28, 2004, 3:40 PM   #3
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Meryl Arbing wrote:
Quote:
In a film camera, the ISo is fixed by whatever film you load. That only leaves two variables to consider for exposure settings...aperture and shutter speed. An AE system will use the exposure program to decide on a default exposure based on the lighting conditions.

But, a digital camera is capable of altering the sensitivity if the sensor (changing the relative ISO) and that enables the camera to have a thrird variable in the exposure equation...aperture, shutter speed AND ISO. If the light levels are low, the camera will try to get the right exposure with the aperture/shutter speed combination but if the exposure exceeds the specs of the camera (for example if the aperture can't open up any more or if the shutter speed required is more than the camera can deliver) then the camera can change the relative senisitivity of the sensor to bring the exposure back to a range that the camera can handle.

In other words, if the camera can't achieve correct exposure at ISO100 then it will increase sensitivity until it can...up to the limit of that parameter.

Setting the camera ISO to auto lets thishappen but it also allows the camera to make a decision that might leave you with a picture that has more noise than you want. If light levels are low, you might end up with an ISO800 picture which will have lots of noise but might be better than not getting the shot at all!
Thanks Meryl... I sort of guessed that was the case, but I do appreciate the confirmation. Based on your explanation,I can't seeANY downside to using the 'Auto' ISO setting. Sure I may get a noisy pix, but if I don't/can't use flash, the alternative is (as you said) no pix at all.

My own comment (above)promptsa relatedquestion... How does the Auto ISO setting know if I will be using flash? After all, using flash would have a huge impact as to the need for a higher ISO!
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