Some digital cameras have ISO boost (or ISO adjustment) function. This function allows you to use higher (or sometimes lower) ISO settings than your sensor has in its specifications. In this short article I want to tell you how it works, when it can be useful and how to boost ISO of your camera if your camera don't have this function.

How it works

It is important to know why we want to use higher ISO settings. Mostly it is because we need faster shutter time when taking pictures in a dark place without flash. Sometimes it is because we want to use smaller aperture when working with the depth of field, but I don't think that this is a right place where we can use ISO boosting. ISO boost mostly allows to use ISO that is twice of the maximum of native ISO.

I will explain it on the first situation. If we increase ISO twice, we will have half shutter time for the same aperture. But when using ISO boost, camera cannot set the sensor to ISO we want, because the sensor doesn't support it. So the camera will use lower ISO (half of the value we set) and shutter speed we set. That leads in image that is about 1EV darker than we want so the camera brighten the image.

One example for better understanding: We have camera with maximum native ISO 6400, f/2.8 lens and dark scene we want to take photo of. We set ISO 6400 and f/2.8 and the camera tells us that it needs 1/10 sec. shutter speed. But we need something faster so we use ISO boost and set ISO 12800. Now the camera use 1/20 sec. shutter speed and we can take a photo which looks normal. In fact camera uses ISO 6400 and 1/20 sec. shutter speed so the image which comes from a sensor is 1EV darker than we expected. But camera lightens the image so it looks like it was taken at IS0 12800.

I explained how it works when we use higher ISO values to get faster shutter speed or smaller aperture. But it can work other way. In some cases we may need to use lower ISO values to get slower shutter speed or higher aperture. This works almost the same way, only with little changes - camera took image 1EV brighter and darken it so it looks like made on lower ISO.

Use it or not?

This is hard question. Sometimes it might be helpful when we need to take a photo and cannot use flash or tripod. But we must know that resulting image will have lot of noise which will be hard to remove and because we take photo that is underexposed (or overexposed) we are losing part of dynamic range.

ISO boost without support in camera

It is very easy to simulate the process I explained earlier. We must set ISO to maximum value and exposition compensation to -1EV and than brighten the photo in an editor. We will have same exposition values as with higher ISO (twice the maximum). It is the same that the camera do when using ISO boost. I only recommend to remove the noise before brightening because it will be more noticeable after brightening and it will be little bit harder to remove it.

Examples

I made few examples with my camera. I don't have ISO boost function in my camera, but I used process that I described earlier. Bohužel nemám ve fotoaparátu přímo funkci ISO boost, ale pomohl jsem si výše zmíněným postupem. Nejdříve jsem zkoušel porovnání nativního ISO 12800 a ISO 12800 dosaženého za pomocí ISO 6400 principu ISO boost.

Výřez fotky focené na ISO 12800

Výřez fotky focené na ISO 12800 s použitím
principu rozšířeného rozsahu ISO

Výřez fotky focené na ISO 12800 s následným odšumem.

Výřez fotky focené na ISO 12800 s použitím principu
rozšířeného rozsahu ISO a následným odšumem

Dále jsem udělal stejný snímek na ISO 25600 za pomocí principu rozšířeného rozsahu ISO.

Výřez fotky focené na ISO 25600 s použitím principu rozšířeného
rozsahu ISO

Výřez fotky focené na ISO 25600 s použitím principu rozšířeného
rozsahu ISO a následným odšumem

Nakonec ještě pro srovnání přikládám snímek fotografovaný na ISO 6400

Výřez fotky focené na ISO 6400

Výřez fotky focené na ISO 6400 s následným odšumem