Quote Originally Posted by canon-ball


I saw that some cameras have an iso that is expandable and go as far as 104,200!


When and how would you use an iso that high?
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Ignoring the reality that those ISO settings aren't suitable for use, it's very simple: it's all about the exposure triangle:


Big apertures mean thin DoF but lots of light, while small apertures mean thick DoF but little light.


Long shutter speeds mean things happen during the shot but lots of light, while short shutter speeds mean stop-action but little light.


High ISOs mean lots of noise but lots of (apparent) light, while low ISOs mean little noise but little (apparent) light.


If you find a shot that you want and let the camera figure aperture/shutter/ISO in full/partial auto, then want to change the artistic outcome of the shot, you'll have to adjust some of the parameters above. If you want to move one or two to the right, you'll have to move one or two to the left to maintain a similar brightness of the resulting shot.


So, if you want thick DoF and motion blur during a night scene, you might need high ISO.