Photography notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Digital

On digital ISO

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

tl;dr:

• higher ISO means more gain during readout of the sensor.

• Which can be seen as more sensitivity,

since it's just amplification, it increases the amount of signal as much as the noise

...but it's a little more interesting than that due to the pragmatics of photography (and some subtleties of electronics)

• You generally want to set it as low as sensible for a situation, or leave it on auto

ISO in film indicates the grain size in photo rolls (see ISO 5800). This amounts to: Smaller grains bring out finer detail but require more light to react for the same amount of image; larger grains is coarser but more sensitive.

ISO in digital photography (see also ISO 12232) is different. It refers to the amplification used during the (at this stage still analog) readout of sensor rows - basically, what gain to use before feeding the signal to the ADC. ...okay, but what does that do? What does it amount to in practice?

Given a sensor with an image currently in it, the only change it would really make to the readout is brightness, not signal to noise, or quality in any way.

In that sense, it has no direct effect on the amount of light accumulated in the sensor. However, since it is one of the physical parameters the camera chooses (alongside aperture and shutter time), it can choose to trade off one for another.

For example, in the dark, a camera on full auto is likely to choose a wide open aperture (for the most light), and then choose a higher ISO if that means the shutter time can be lowered to not introduce too much motion blur from shaking hands.

There are other such tradeoffs, e.g. controlled via modes -- for example, portrait mode tries to open the aperture so the background is blurred, sports mode aiming for short shutter time so you get minimal motion blur, but at the cost of noise, and more. But most of these are explained mostly in terms of the aperture/shutter tradeoff, and ISO choice is relatively unrelated, and can be explained as "as low as is sensible for the light level".

Still, you can play with it.

Note that the physical parameters are chosen with the sensor in mind - to not saturate its cells (over-exposure) {{comment|(also other constraints, like avoiding underexposure, signal falling into the noise floor, and in general also tries to use much of storage range it has (also to avoid unnecessary quantization, though this is less important.)

As such, when you force a high ISO (i.e. high gain) but leave everything else auto, you will effectively force a camera to choose a lower shutter time and/or smaller aperture.

Which means less actual light being used to form the image, which implies lower signal-to-noise (because a noise floor is basically a constant). The noise is usually still relatively low, but the noise can become noticeable e.g. in lower-light conditions.

Similarly, when you force a low ISO, the camera must plan for more light coming in, often meaning a longer exposure time. On a tripod this can mean nicely low-noise images, while in your hands it typically means shaky-hand blur.

In a practical sense: when you have a lot of light, somewhat low ISO gives an image with less noise. When you have little light, high ISO lets you bring out what's there, with inevitable noise. Leaving it on auto tends to do something sensible.