Understanding how these three elements effect your photographs will give you not only more understanding of how your camera works but also put you in a lot more technical and creative control. Back in the days when cameras were all fully manual, getting any decent photograph required some understanding of how the speed of the film [ISO or ASA], the size of the hole that let the light in [aperture] and the amount of time that hole was open for [shutter-speed]. Nowadays pretty much every camera – and certainly every consumer camera – has a fully automatic ‘Program’ [P] mode, leaving the photographer in control of just one thing; pushing the button to take a photo.
But to get beyond just ‘pointing and shooting’ is going to involve, at some point, getting away from ‘P’ and beginning to use the other modes available on the camera; Aperture Priority [A or Av], Shutter Priority [S or Tv] and the Holy Grail of Manual Mode [M].
Getting out of Program Mode needn’t be daunting but to do it means understanding more about ISO, aperture and shutter-speed, all of which enable you to let more light into the camera but in a different way from one another and with different strings attached.
What is ISO: Remember film? Well, if you do you may remember that it came in different flavours; 100, 200, 400 were probably the ones you would have seen most of. The number corresponded to the ‘speed’ of the film which, in short, meant its degree of sensitivity to light: 100ISO [or ASA, as it was also known] was ‘slower’ than 400ISO film. You would use 100ISO film for all your daylight, holiday-type snaps and with flash; 200ISO was a little faster and 400ISO film was generally what you used if you were shooting pictures where there was less light.
Flip forward to digital cameras and ISO is still a factor and still relates to light sensitivity but no longer to the sensitivity of the silver-gelatin coating on a roll of film, but the sensitivity of the sensor. And just as you could increase the light sensitivity of your film camera by putting a faster roll of film in it, you can increase the light-gathering properties of your digital camera simply by turning up the ISO setting. But as with the rest of life, away from photography, every action has some reaction and there are compromises to consider.
Raising the ISO means giving up quality. The light-sensitive coating on a roll of film was made up of grains: faster film [higher ISO] meant bigger grains. Bigger meant more visible and photos taken with faster film have the grains far more visbile; hence the term ‘grainy’. In digital terms this all translates to ‘noise’, signal noise. Higher ISO on a digital camera means turning up the ‘gain’ of the chip. That means more signal noise, which you see as speckles in the picture.
Signal to noise ratio is also an issue with digital cameras that use a smaller sensor chip. Most point and shoot cameras use a chip that is 1 & 1/8th of an inch in size. If the camera is 4megapixels in resolution, that means more pixels per square inch. The pixels are smaller and therefore soak up less light each. On a camera with a much bigger chip yet with more resolution, the signal to noise ratio is greater as the pixels are bigger and receive more light each. This is the biggest advantage with full-frame aspect cameras; the chip is bigger and therefore its signal to noise ratio is better and there is less noise. See the links section at the foot of the page for more info on noise.
The pictures in the gallery below will help illustrate this point: check the captions for details about how each was shot, at what ISO and why.
In Conclusion: if increasing the ISO means losing quality, why do it and when?
The answer I give all of my students when they ask this question is; do it when you can’t do anything else. Meaning that, if you have changed the aperture and shutter-speed as much as you can or want to, the ISO is the last tool in the box for getting more light into the camera. Plus, with the excellent and ever-improving low-light/high-ISO performance of digital cameras, increasing the ISO is much less of a problem as time goes on.
Here are some situations where increasing or decreasing the ISO on your camera would be necessary or useful: remember that ISO, shutter-speed and aperture are all linked. Increasing the size of the aperture will increase the speed but when you have run out of aperture, you need to push the ISO……
- You are in a low-light situation and have made the aperture as large as you can/want to and have decreased the shutter-speed as much as you can/want to or, to the point where it is too slow a speed to hand-hold the camera without getting camera-shake. Increase the ISO.
- You are shooting fast action and need to get as fast a shutter-speed on the camera as possible and increasing the size of the aperture has not given you as fast a speed as you need. Increase the ISO.
- You need to get as slow a shutter speed as possible, maybe for shooting blurred motion or a time-exposure. You have made the aperture as small as possible [f/22 for example] and the shutter speed the camera is giving you, to make a good exposure, is not slow enough. You should now reduce the ISO to as low a setting as the camera allows.
- You are shooting a bracketed HDR, where you will overlay multiple frames to increase the dynamic range of the final image. You should decrease the ISO to as low as is prudent to maintain the aperture and shutter-speed settings you need. Overlaying frames for HDR results in increased noise, therefore shooting at as low an ISO as possible reduces noise to a minumum. Reduce the ISO.
- You are shooting a landscape on a tripod. Nothing in the landscape is moving and you want as high-quality an image as you can get. You should reduce the ISO to as low a setting as the camera allows.
N.B. With cameras such as the Nikon D300, that have a 12-bit and 14-bit mode for shooting RAW files, using the camera at its Low-1.0 setting [ISO 100] gives best results when shooting in 14-bit RAW mode. The higher the bits, the more colour definition you get. Using ISO100 in conjunction with 14-but RAW will mean you get the best out of the ISO100 setting. Shooting at Low 1.0 [ISO100] on the Nikon D300 whilst in JPEG mode does not yield noticably different results from shooting at the standard ISO200 setting.
- Digital noise – the Wikipedia entry on this topic.
- Great page at DP Review about digital noise.
- Wiki page on digital photography, a useful page with basic facts about how digital cameras work and with some useful links to other sites.
- Interesting page at The Luminous Landscape website about ‘exposing to the right’, a way o maximising the signal-to-noise ratio in digi9tal cameras. This technique of exposing to get the full benefits of RAW file format and sensor power will be the subject of a longer article here soon.
- Imagenomic – a superb noise-reduction plugin and/or standalone program for removing noise from digital photos.
- Noise Ninja – another excellent noise-reduction program which can also be used as a plugin for Photoshop.