The third element of the exposure triangle is ISO, this is the only one that is directly related to the performance of your camera body rather than the lens. Very simply your ISO setting controls your sensor in your camera to absorb light during an exposure. Typical ISO settings you’re likely to see are 100, 400, 1600 and 3200; this value can go all the way up to 20,000 depending on the quality of your camera. Generally more expensive cameras will give you a higher ISO ability but beware higher ISO is not always better!
Why do my night photos look so “grainy”?
With a higher ISO increasing your sensor sensitivity to light it makes sense that in low light conditions you will need to push your ISO setting higher but this has a dramatic effect on the quality of your image. To make your sensor more sensitive your camera needs to amplify the signal from your sensor meaning each pixel is able to absorb more light per exposure and therefore giving you a brighter image. But, as the signal is amplified it gets distorted just like when you used to turn your speakers up to 11 like Spinal Tap the quality of the music is reduced. It is exactly the same with ISO but rather than distortion you get something call “noise” (not grain but more on that another time). Noise is when you start to lose quality in an image especially in the shadows and the level of detail that can be seen is reduced.
With the camera on a tripod and the camera set to its lowest ISO of 100 (note the shutter speed of 1/5th second and aperture of f/7.1) we get a properly exposed image of He-Man.
If you zoom into 1:1 scale you can make out all the detail in the plastic and the clarity image is excellent.
When you increase the ISO from 100 to 3200 (but keeping shutter speed and aperture the same) you can see the image is blown out due to the sensor being more sensitive to light. To compensate for this we need to restrict the amount of light coming into the camera, so we can either speed up the shutter speed or narrow the aperture, in this case we increase the shutter from 1/5th to 1/20th of second giving a properly exposed image.
If we look again at 1:1 scale we see that the quality of the image is starting to reduce due to the distortion of the signal from the sensor.
If we take it to the extreme of 25,600 ISO (and increase the shutter speed to 1/320th to compensate for the extra sensitivity) you can see the quality of the image in massively reduced due to the additional noise in the image.
Different cameras handle ISO settings differently depending on if it is crop or full frame sensor, what brand or how much light is available. For example my first D3000 looks like Swiss cheese at 1600 in low light but my D600 performance is much better due to it having a larger full frame sensor.
The three golden rules are:-
1. ISO is controlled in the camera not the lens
2. Increase your ISO increases your sensor’s sensitivity and ability to absorb light resulting in a brighter image.
3. Increased ISO reduces image quality due to noise (not grain)