What is ISO? Photography Jargon Explained

What is ISO? Just 3 simple letters, but what is it all about? If you are buying a camera or reading articles on camera settings you are bound to come across the term ISO. It forms part of the “exposure triangle” – shutter speed, aperture, ISO. So, if you want to understand your camera properly and not just use it on auto it is important to get a basic grasp of what ISO is and what it does to your photos.

ISO stands for the International Organization for Standardization. Yes, I know the S and O seem to be the wrong way round…. don’t worry about that. It is basically saying that this is an industry standard that applies across photography and is reliable. Before being applied to digital photography, there were standards set out for films that were marked according to an internationally agreed benchmark. These numbers were labelled ISO. So, you could buy ISO 100 film from any manufacturer and know that the 100 meant the same for all of them because it is a standardised number. Equally, you could get ISO 200 or ISO 400, etc and know that each film would respond to light in the same basic way as any other bearing that number.

Clean image at low ISO (ISO 200) on a crop sensor DSLR:

The numbers are all to do with sensitivity. The higher the number, the more sensitive the film is to the light. In digital terms, it works more like a boost. Instead of changing film for a higher ISO you can simply select a new setting on the camera. This new setting boosts the signal received in the sensor – making it effectively more or less sensitive. The lower the number, e.g. ISO 100, the less boost is applied (and the less sensitive it is to the light). The higher the number, e.g. ISO 1600, the more boost is applied (and the more sensitive it is to light). When you double the ISO number you double the boost/sensitivity. When you halve the ISO number you halve the boost/sensitivity. In photographic terms this means that you are going up or down 1 stop then you double or halve the ISO (see post on What is a Stop?).

If there isn’t much light available then you have a few options to enable you to still get a reasonable image. You could widen the aperture (if possible) to let more light in. You could use a longer shutter speed (allowing more light to register). Or, you could boost the ISO so that you can keep a short shutter speed (essential for freezing movement, for example). In some situations you may be able to use flash. However, when that isn’t possible or desirable, boosting ISO enables you to get pictures in low light environments.

High ISO introduces noise – ISO 12800 on crop sensor (compare it with the image above):

The downside of boosting the ISO to a higher number is that this damages image quality. The higher the ISO, the more chance of “noise” (speckles or bands of colours or interference on your image). You get best quality with low ISO (around 100-200). The higher you go, the worse the noise. As the technology progresses, cameras are getting better and better at handling the noise at high ISOs. They can reduce the amount it interferes with the image and allow you to still use the photo. The quality is still less than at a low ISO (for one thing the dynamic range tends to suffer) but at least you can get something.

The difference in ISO performance on full frame and crop sensors:

ISO 12800 noise on crop sensor DSLR (Nikon D7100):

ISO 12800 noise on full frame sensor DSLR (Nikon D750):

Full frame cameras are among the best at working at high ISOs. In some situations it can be a huge bonus. Some events aren’t too tolerant of flash so a usable high ISO is a big plus. It is better to get something slightly less than perfect than nothing at all!

Very close crop of ISO 12800 on crop sensor:

The same on the full frame:

Most of the time you should keep your ISO low if you want to control it yourself. However, don’t be frightened of trying higher ISOs. Maybe do so first when there is no pressure on you to get a great image. Test your camera and see how usable the high ISO images are. It also depends what you will use them for. If you are using them very small online then you will see much less evidence of noise than if you print them out a large size. Also, make sure to try image editing software that can reduce the appearance of noise considerably (e.g. Lightroom, Photoshop, Topaz DeNoise, etc).

ISO 51200 full frame no noise reduction software:

ISO 51200 noise reduction using Topaz DeNoise on very high reduction settings:

As you can see, noise reduction software makes a big difference. The above images are very close crops to show you the detail. So, if you are not cropping and not printing large the effect is even less noticeable. What is ISO? Not something to be afraid of, that’s for sure!

What is ISO? Photography Jargon Explained