George Theodore, February 27, 2017
ISO Refers to the Sensitivity of Your Camera’s Sensor. Not!
Here’s yet another myth that belongs in the dustbin along with “you get more reach with a lens on an APS-C sensor than with a full-frame sensor”.
ISO in our digital cameras is not the same as ISO in film. Film is produced for a particular sensitivity to light and you can't change that sensitivity midstream through a roll of film. In our digital cameras, we can choose whatever ISO we want whenever we want. But, it has nothing to do with sensitivity. In fact, ISO “happens” after the shutter closes.
Here’s the way it works.
- We increase ISO
- The light falling on the sensor is reduced
- We activate the shutter capturing an underexposed image
- Shutter closes
- The signal from the sensor is amplified to achieve the desired brightness.
Because ISO is applied after exposure, some opine that ISO, therefore, has nothing to do with exposure. Well, perhaps not the same way it did with film but ISO settings do influence how much light falls on the sensor so maybe we can say it affects exposure indirectly.
So, where does noise come in? When the exposure contains fewer photons (underexposure) any noise generated becomes a larger component of the sensor’s final output signal. When that signal is amplified, both “good” and noisy parts of that signal are amplified. The total signal divided by the noisy part is what we refer to “signal to noise ratio (SNR)”. When the image is underexposed, the SNR is low which means we see more noise.
Therefore, there are more artifacts present in a higher ISO image. And, those artifacts are nothing like the grain we saw in high ISO film. Grain was kind of interesting; artifacts are just plain ugly. So, as we always tell our workshop participants: “Shoot at the lowest ISO required”. Of course, we’ll accept noise if that’s the only way we can capture a particular image.
Now, when you hear someone say “ISO changes the sensitivity of the sensor” you can, with confidence, say “no it doesn’t”. Some say, ISO “affects the camera’s sensitivity to light”. That’s not right either. ISO is all about simple amplification or “gain”.