1. What is ISO?
Originally ISO related to film speed; it refers to the sensitivity of the emulsion to light. Digital sensors work in a similar, but different way. With digital sensors, when shooting at lower ISO settings the sensor appears to be less sensitive to light and image quality is better, while at higher settings noise becomes more apparent as the sensor becomes more sensitive to light. The reality, however, is that digital cameras have a single base ISO setting, which is often ISO 100, and when increased or decreased, a software algorithm simulates an effective sensitivity of the sensor.
2. Test your camera’s ISO performance
The best ways to identify the highest ISO setting you’re happy to use is to do an ISO test. Set your camera up on a tripod in aperture-priority mode and take a series of shots at all the native ISO settings. Shoot in raw and load the images into Lightroom or your preferred raw-editing software and make sure all noise reduction is switched off. From this you can see exactly where image quality becomes unusable.
3. The exposure triangle
Working with the simple idea that digital ISO levels are the same as film ISO levels, ISO is part of the exposure triangle that also includes aperture and shutter speed, the combination of which dictates whether an image is correctly under or overexposed.
A stop with shutter speed and ISO is easy to understand; you simply double or halve the number. So you might go from ISO 100 to 200 to 400 and so on, while with shutter speed you could go from 1/125sec to 1/60sec to 1/30sec, etc. Aperture is however different, and the most common are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22.
4. Turn off long-exposure noise reduction when shooting in raw
When shooting in raw it pays to turn off long-exposure noise reduction because the processing time can be as long as the exposure itself. So, a two-minute exposure would require roughly two minutes of processing. Noise reduction in this situation is best applied to images in post-production – for more control and to save time when shooting.
5. Effect of high ISO on sharpness
As ISO is increased you will experience more grain in images. This grain will effectively break up sharp subject edges and reduce fine detail, which has the effect of softening images. There will always be times when you have to shoot at extremely high settings, but the trade-off here is achieving the desired combination of shutter speed and aperture at the expense of overall image quality.
6. Reciprocity failure no longer applies
Reciprocity is the idea that all exposures are mathematically equal, so if you increase ISO, for example, by two stops, you need to decrease the shutter speed or aperture by two stops or by one stop each to maintain the same exposure. With film, in general, reciprocity only works with exposures between 1sec and 1/1000sec owing to the limitations in film emulsions. But with digital, reciprocity works at all shutter speeds without detail problems or colour shifts.
7. ISO invariant sensors
At a basic level, ISO invariance is when you can achieve a near-identical level of noise regardless of whether you increase ISO in-camera or lighten an underexposed image in post-processing. So why would you do this? The reason is simply that at lower ISO settings sensors achieve a larger dynamic range, so you can ultimately capture more detail throughout the scene. The downside, however, of shooting at ISO 800 rather than 3200, for instance, is that you won’t be able to review your underexposed images on the LCD to check composition and other things. A quick internet search will tell you if your camera is ISO invariant.
8. Use a prime and shoot wide open
‘Nifty fifties’ are not only often affordable and provide excellent image quality, but their fast maximum aperture of either f/1.8 or f/1.4 also means you can allow more light to enter the lens. So, with this you’ll get beautifully out-of-focus backgrounds and in low-light situations, lower ISO and less high-ISO noise – which is a true win-win situation.
9. Get more light with wideangle lenses
Wideangle lenses by their nature are fast lenses because their wide field of view draws in more of the scene in front of them and more light. Wide aperture settings will increase their light-gathering capabilities, but if you want to keep ISO settings lower you’ll be able to do so more effectively with a 28mm lens than an 85mm at the same aperture setting, for instance.
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10. When to use Auto ISO
Auto ISO is a handy feature, but one that must be used with caution as it may cause problems. With Auto ISO, on most cameras, you set the minimum and maximum settings, such as ISO 100-1600, and the slowest shutter speed you’re willing to use, for example, 1/125sec. The camera will then use the lowest ISO setting possible to maintain at least that shutter speed or faster. Auto ISO should only be used for handheld shooting, and certainly not when using a tripod or ND filters because you’ll end up shooting at higher settings than necessary and will not achieve the desired results.