ISO Misconceptions

Started Oct 7, 2008 | Discussions thread
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Fred Briggs
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ISO Misconceptions
Oct 7, 2008

Just wanted to start a discussion on the topic of ISO settings on digital cameras, as I feel that there are a lot of misconceptions floating around, at least among a proportion of forum members. This has become increasingly irritating with the recent frenzy over the 5DII and 50D. Of course the following is just my opinion - no doubt if anything I’m about to say is factually inaccurate then I will be swiftly corrected!

Misconception 1: Increasing the ISO setting on digital cameras increases exposure.

Wrong – only larger aperture or longer exposure can increase exposure.

If the sensor is under-exposed with your required shutter and aperture settings at low ISO, it will still be just as under-exposed at a higher ISO setting. I know the camera’s metering will say it isn’t, but it’s lying! So, there is no such thing as a properly exposed high ISO shot.

Misconception 2: Increasing the ISO setting creates additional noise.

Not really – the noise you see at each ISO setting is the same noise that is there all along at base ISO, but just made progressively more visible.

Following on from the first point, you cannot increase real exposure with higher ISO settings, so what the camera is actually doing is taking readings from under-exposed pixels and multiplying them by a set factor for each ISO setting to simulate the reading which would be obtained from a proper exposure. However the higher the ISO setting used, the more under-exposed is the sensor, and therefore the lower the average pixel level signal to noise ratio.

In a properly exposed shot at base ISO the signal due to the light captured swamps that due to the inherent noise, at least in the pixels in most parts of the image, making the effects of the noise virtually invisible. However, that very same noise forms a greater and greater proportion of the readout as ISO settings go up, due to the progressively worse under-exposure at each ISO setting. This of course makes the noise more visible and/or triggers increased noise reduction, both with consequent reductions in image quality.

The key point I'm trying to make is that with each increase in ISO setting there is no increase in noise relative to the signal (or only an insignificant amount). There is just worse under-exposure, and consequently less signal, resulting in a poorer signal to noise ratio.

Misconception 3: Noise should not be visible in correctly exposed images taken at base ISO.

Not necessarily true – although whatever metering is chosen will hopefully be able to set a correct exposure for the important parts of the image, if the scene being captured is not uniformly illuminated, then some parts may well be significantly under-exposed. Readouts from the pixels in these areas will suffer from an inferior signal to noise ratio in just the same way as when the whole image is under exposed and a high ISO setting is used to compensate.

Fred

PS: I'm on UK time and I'm off to bed now, so won't be responding for a few hours ....

 Fred Briggs's gear list:Fred Briggs's gear list
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