What can you do if light levels are low, and your shutter speed is too slow for you to hand-hold the camera? But you are already on the widest aperture possible, or you don’t want to make your aperture any wider for artistic reasons.
How can you still take the shot and not risk it being blurry due to a slow shutter speed? The answer is something called ISO, which is named after the International Organization for Standardization. They chose this term to describe themselves in all languages, as it comes from the Greek word 'ISOS' which literally means 'equal - and their international standards promote equality and fairness in trade and commerce.
Anyway, that's enough of a history lesson! So, increasing your ISO setting results in a brighter image, without needing to let in more light by changing aperture or shutter speed - which means you can shoot at slower shutter speeds in low light situations.
Without getting too technical, the same amount of light hits the sensor - but then the electronic signal between the sensor and the SD card is amplified, to create a brighter image.
For example, I want to use an aperture of f8, but in a specific set of lighting conditions, at 200 ISO my shutter speed is 1/10 of a second - which is far to slow to hand-hold.
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By increasing my ISO to 2000, I can now use a shutter speed of 1/100, which is fine for hand-holding, and the shot will still be correctly exposed.
The base level ISO measurement on most cameras is 100, though on some it can be slightly less (usually 64) or slightly more (up to 200). The standard ISO increments are 100, 200, 400, 800, 1,600, 3,200 and 6,400 - although some cameras have intermediate levels as well, and some go much higher, up to 25,600 for example.
This is a very useful tool, but it does affect image quality, as the process of amplifying the signal causes 'noise' - much like turning up the music volume until the loudspeaker starts to buzz!
This 'noise' or 'grain' as it is also known is especially noticeable on smaller sensors, and gets progressively less of an issue with larger sensors.
The best way to find out what level of grain is acceptable on your specific camera is to take a series of shots of the same thing at all of the different ISO settings, and then look at them at about 200% or more on your computer.
But as a rough guide, an APS-C or MFT sensor can happily go up to 1,600 at least, and usually 3,200 for APS-C, without any major issues, and full frame can go much higher - whereas smaller sensors such as those found in bridge or compact cameras have far less tolerance, and I'd always recommend keeping the ISO as low as possible on these.
The bottom line though is that it is always better to have noise than blur, especially as there are now some excellent 'denoise' software options available. So if you need to put the ISO up in order to get a fast enough shutter speed, then don’t be scared to do so.
It’s worth noting here that cameras are often set to Auto ISO when they come out of the box, even if you are using a manual or priority setting. Always change to manual ISO, starting at the lowest level, as the auto option may choose a higher ISO than is actually necessary.
More importantly though - using manual ISO will make it much easier to understand the relationship between aperture, shutter speed and ISO when you are learning.
So, in summary - if you can’t get a fast enough shutter speed (and your aperture is as wide as it will go, or you need a narrower one for artistic reasons) – then put the ISO up until your shutter speed comes back to at least 1/100, or whatever speed you need for the shot you’re taking. Bearing in mind that moving subjects need faster shutter speeds.
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