What is camera noise and ISO?
So let me try and explain about ISO and noise; larger sensors, such as those found on many DSLRs today are less prone to 'noise'. This is because the larger photosites found on such sensors collect light more effectively and therefore don't need as much amplification (high ISO) to capture a poorly lit scene as a small sensor; Photography using a smaller or cheaper camera or camera phone could mean that you end up with speckled or 'grainy' images.
Have you noticed that this happens on your phone when you’re taking that selfie or inside shot with your friends? Likewise, in digital photography, increasing the ISO setting amplifies the electronic signal from the sensor, it’s like all these electronic little fireflies helping to brighten the image become part of the photograph, amplifying the background, electronic 'noise' the camera's circuitry makes this in interference – also you could think of it as 'visible hissing'. This distorts the image signal and creates a speckled or 'noisy' image, which can ruin a great photo; it can add to some images especially if it’s for wedding photography at a wedding, for the Grooms men images but most of the time it gives poor looking photos.
It's sensible to use the lowest ISO setting that will still allow a fast enough shutter speed to avoid camera shake resulting blurred images . Alternatively, use a tripod, as you'll be able to use a lower ISO setting and slower shutter speed than is possible when holding the camera. I tend to use a tripod at weddings when its interior photography, If you're using slow shutter speeds you could try using the camera's 'Noise Reduction' facility if it has one however I think this makes the image look too smoothed out, like people have plastic skin, not a good look.
Because these little fireflies (photosites) are set farther apart on larger sensors, increasing the ISO setting will cause less interference, and so create less noise, this lets you obtain pretty good results from a DSLR when the ISO is set high, even ISO 6400 looks pretty good with today’s technology, I will quite comfortably use ISO 6400 if I am providing the photography for a client in a church, there is one particular Church in Somerset that even in the summer months, it’s so dark, there really isn’t any other option as flash is not allowed, I prefer not to use flash during a wedding ceremony anyhow and naturally ISO will get better and better.
Most cameras will have an automatic ISO option; when selected, the camera will monitor the shutter speed and if the speed drops below the point at which camera shake could occur (Try and keep shutter speed as double the focal length of the lens) it will automatically increase the ISO setting. This can help you avoid taking blurred images, but it can also introduce noise. On some cameras the automatic ISO range is limited, so the camera avoids using very high ISO settings. Remember; manually set the lowest possible ISO; if it’s possible try using a tripod as there's no danger of camera shake and you’ll get better noise-free images.
So how do you choose your ISO, remember first think about the image you want, look at the ambient light, decide on what F-Stop you could choose or get away with a lower ISO, if you want more in focus by selecting let’s say F5.6 and the light is poor, you can reduce the shutter speed but remember that camera shake, then adjust your ISO to get that photo you want, this is generally the way I work when providing my wedding photography service in and around somerset.