This might seem basic stuff from Digital Camera World but as we are just starting our new term of courses and I kicked off with our Understanding Your Digital Camera on Tuesday I am thinking about ways to best explain things like ISO. One of the great things about teaching photography is that it constantly forces me to reconsider my approach to the technical and visual and how best to convey that to my students.
What is ISO?
The camera’s ISO setting is its sensitivity to light. The higher the ISO, the more sensitive it is. This is measured according to international standards, so ISO100 on one camera will be exactly the same as ISO100 on another.
Each ISO setting is double the one before: if you increase the ISO from 100 to 200, you double the camera’s sensitivity; and if you increase it from 200 to 400, you double it again. This carries on through the ISO scale.
This is deliberate. The ISO settings are designed to double (or halve) the exposure in the same way that the lens aperture settings and shutter speed settings are, and this is why the lens aperture, shutter speed and ISO are often described as the ‘exposure triangle’.
For example, if you want to use a faster shutter speed without changing the aperture, you could increase the ISO instead.
This relationship between lens aperture, shutter speed and ISO could quickly get complicated, but there are drawbacks to changing the ISO which mean that in practice you tend to change the ISO only when you have to.
There is more to read if you need
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Oxford School of Photography 
Shooting wildlife with a long – not particularly fast – lens, hand-held or on a monopod, often in the early morning or evening, I found that using shutter priority to get the fast shutter speed I needed, but letting the ISO float was by far the most convenient technique. When light is poor, the camera will select a wide aperture, and increase the ISO only as far as is necessary for the correct exposure. On my D600 you set a base ISO rating and you can set a maximum that you allow it to float to if you wish.
Of course, if you need a narrower aperture to manage depth of field, then you’d have to set a higher ISO manually – as it says, it can quickly get complicated.