ISO - Explained

 

You can find numerous confusing videos about ISO and some misguiding advice. But It is not too difficult to understand in principle. The devil is in the detail, however, and the best setting is still a compromise.

Let us summarize, what actually happens in photography. For simplicity, we ignore color and talk only about brightness.

• The scene in front of you has different grades of brightness called light values. In theory, it goes down to black, but we’re only interested in a range from the darkest objects to the brightest objects we want to show. Those objects should have some texture and details.
• The scene is projected onto the sensor, and the light values become light per area, which we call exposure values. The exposure value of a given light value depends solely on the aperture and the exposure time, not the sensor or the ISO. 
• The sensor can capture values from a minimum to a maximum. The minimum is determined by the noise because the signal must be stronger than the noise to be useful, the maximum is the capacity of the sensor pixels. The total range is the dynamic range of the sensor.
• The sensor works best in the middle of its range. That’s where your main subject should be exposed to. It could be that the brightest areas in the scene exceed the sensor maximum. Then they will show no details. It is an artistic decision if you let that happen or not. Likewise, with the darkest areas in the scene. Ideally, all values of your image fit within the sensor range.
• The exposure values are ultimately transferred to JPEG values of the final image, or whatever output format you use. If this is done in the camera or via a RAW file on the computer does not matter. A lot of artistic tweaking can happen in that process. You could spread the darker or the brighter values to give them more room and thus more details. Or you can simply drag all values to the dark or bright side to change the exposure, or push them towards the edges to gain contrast.

• Now, suppose the brightest exposure value reaches only the middle of the sensor range because you cannot increase exposure time or aperture enough. Then, you can stretch the values to the maximum. This is like amplifying all values. And that’s, what ISO does. The amplification can be done on the analog signal or on the digital values. On most cameras, it does not matter much to the end result if you simply brighten the values of the RAW file or set ISO in the camera and let it do the amplification. Some cameras, however, have a dual readout which changes the sensor range for higher ISO settings. In general, it seems better to set the ISO in the camera to get the brightest values close to the maximum. This will also generate a correct preview in the camera.
• Of course, the noise will be amplified too. Thus, an underexposed image, brightened up by ISO setting or in post, will look worse than a correctly exposed image. For best results, get as much light as possible. 
• The effect of high ISO and its amplification is simply a loss of dynamic range. The loss is not at the high end, but in the low end, where darker areas are now sprinkled with noise. You can see that in the image above if you look into the dark areas where texture is lost. Instead of using the sensor from its minimum to its maximum, you use it only from the minimum to the brightest exposure value of your scene, which is then amplified to the maximal sensor value. The exposure values marked in red are lost.

We need to be sure that we understand the effect of reduced dynamic range and high ISO completely. Assume you have a face in good light and compare it to low light where you have to increase the ISO. The exposure values of this face will be in the middle of your dynamic range. But it will sit much closer to the noise level in the high ISO setup. Thus, you will detect more noise in the face in higher ISO.

Higher ISO (or pushed underexposure) lowers the dynamic range at the low end, which is determined by the useful noise ratio. But high ISO shows more noise everywhere.

In summary, it is always best to use the base ISO of your camera and to use the full range of your sensor by proper exposure. Then you will get the highest dynamic range. 

But modern cameras have a very high dynamic range. So, it is better to set a higher ISO and a faster shutter speed to get sharp images than to spoil the image by motion blur or camera shake. Moreover, modern software can remove much of the noise easily and still get clean and detailed images. On a modern full frame camera, I have no issues up to ISO1600 for normal scenes if necessary.