High ISO Doesn’t Necessarily Mean Noisy Images
It's very easy to overlook one of the biggest advantages of digital cameras over film cameras – namely the ability to change ISO on the fly. ISO is simply a measure of light sensitivity, and digital ISO adjustment is a revolutionary breakthrough.
If you wanted a faster ISO or 'film speed' on a film camera you literally had to put in another roll of film. With digital cameras you can choose from the whole range, from low ISOs (say 100) right through to very high settings in the 'expanded' modes which more expensive cameras now offer.
Higher ISOs mean your camera is more sensitive to light, and make it so much easier to work at dusk or at night without resorting to flash – check out Tony Worobiec's tutorials. There is no such thing as a free lunch, however, and the downside of pushing up the ISO is that you start to get more noise.
This is simply image interference that is caused by electronic signal modification. A good analogy is your hi fi. Depending on how good your set-up is, you can crank the volume right up, but you will start to hear distortion and rattling as the amp and speakers are pushed to their limit – substitute speckled, muddy colours for audio distortion, and you get the picture.
To see what noise looks like, take a picture in the same light at ISO 100 and then ISO 3200 and zoom into primary colours. At higher ISOs the colours will be less clear, smooth and consistent in texture.
Factors affecting noise
It's not as simple as just avoiding higher ISOs to keep noise down, however, as there are many factors at play here. Imagine you are taking a portrait inside a darkened church, exactly the kind of place you would want to use a higher ISO.
If artificial light or candle light is illuminating most of the subject's face, noise won't be too much of a problem, even at ISOs above 1600.
If the person is poorly lit or mostly in shadow, and you end up having to fix the exposure in software, you will certainly notice more noise on the final image.
This brings up another important point. Noise will be generated when you need to fix underexposed shots, even if you used a relatively low ISO. This is another reason to err slightly on the side of overexposure if you shoot raw.
Using Auto ISO
Your camera automatically works out the right ISO setting for the scene, and modern auto ISO functions are are lot more sophisticated than they used to be.
If you are worried about noise, you can restrict the maximum level so it never goes above, say, ISO 1600. While you may get noticeable noise at this level, it's relatively easy to control.
Note also that noise tends to be less of an issue with larger, full-frame sensors than with smaller APS-C chips.
The shutter speed factor
Another good reason to be more adventurous with higher ISO is that it enables faster shutter speeds. The rule of thumb is that your shutter speed should equal or surpass the focal length you are shooting at, and when the light falls this can be hard to achieve.
Just bumping the ISO from, say, 100 to 400 can give you these faster shutter speeds and the amount of extra noise generated at this level will be negligible.
Higher ISOs should not be used as a quick fix for using 'slow' lenses, though, so be prepared to invest in lenses with constant wide apertures (e.g. f/2.8).
Wider apertures let in more light, so they are godsend when used with higher ISOs in tricky lighting conditions where you don't want to use flash.
Good noise and bad noise?
Don't get obsessed about having super 'clean' shots, as they can look very digital and synthetic. If you look back at the work of classic black and white film shooters, they often valued film grain, and while digital grain rarely looks as nice, it can add drama and texture.
I own an Olympus OM-D, for instance, and many owners believe that the noise at higher ISOs, if controlled, can also be pleasantly 'grainy' in character.
Removing noise in software
Finally, don't assume that Lightroom or Photoshop are the only choices when it comes to removing excessive noise in software. Often the raw editing software supplied by the camera maker can deal with noise more effectively than the 'one size fits all' approach of Lightroom.
When it comes to removing noise, be very careful with the sliders. You can end up fixing one problem but creating another, particularly with portraits – people's faces can take on a plastic, waxwork look!