I found the idea of “stops” the most complicated part of getting to grips with photography – I can still remember now how my face glazed over when it was first explained to me.
Crudely put, a “stop” involves the doubling or halving of the amount of light hitting the sensor (or in the case of ISO, how that light is used). For instance, if we had our shutter speed set to 1 second, changing it to 1/2s would let half as much light in, changing it to 2s would let twice as much light in.
ISO stops works in the same straightforward, linear way as shutter speed – ISO200 is twice as sensitive as ISO100 but only half as much so as ISO400 – but the aperture scale seems to make less sense initially. Changing the aperture size also affects the amount of light hitting the sensor, however rather than changing in multiples or 2 they change in multiples of the square root of 2 (approximately 1.4) as follows f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 etc.
When the aperture is stopped down (changed to a smaller opening by increasing the f-stop) less light will enter the camera. In order to obtain the same exposure, the amount of light entering the camera will need to be balanced by slowing the shutter speed down (so that the shutter is open for longer), or the sensitivity of the sensor will need increasing (by increasing the ISO).
Obviously if you’re using one of the camera’s creative modes it will take care of some of this for you – for instance if you are in Aperture Priority mode and you change the aperture size the camera will automatically adjust the shutter speed in order to attempt to get what it thinks is the correct exposure.
Stops in action
Let’s say we’re taking a picture in the early evening which requires a short depth of field, such as portrait, and we’ve got our aperture set to f/2.8. Our camera, which is set to ISO100, has decided that a shutter speed of 1/100s is required for the correct exposure. Say we now want to take a picture that requires a greater depth of field, a landscape for instance – we might set the aperture to f/11 for this.
If we do this however we’ve restricted the amount of light available to the sensor by 4 stops – and because there is now going to be less light hitting the sensor the camera knows it needs to adjust the shutter speed to let 4 stops more of light in. If we follow the progression of stops in shutter speed from 1/100s, dividing by 2 each time, we can see that a shutter speed of 1/6s would be selected by the camera – 100/2=50, 50/2=25, 25/2=12.5 (although this will be rounded to 12), 12/2=6.
Unless we’re using a lens with image stabilizing (or a tripod) a shutter speed of 1/6s is going to result in a pretty blurred image – so we need to decide, do we really want f/11? – using a larger aperture, such as f/8 would give us a better shutter speed – or can we sacrifice some quality by changing the ISO to make the sensor more sensitive to the available light. Changing the ISO to 400 adjusts the whole equation by 2 stops again (100×2=200, 200×2=400) and means we’d now have a shutter speed of 1/25s. Still perhaps too slow for a hand held shot, but hopefully by now you get the idea :)
Other examples
The following settings would all produce shots with a the same exposure:-
a) 1/100s, f/11, ISO100
b) 1/200s, f/8, ISO100
c) 1/100s, f/22, ISO400
d) 1/50s, f/22, ISO200
Can you see why?
