Entry photographers tend to use low ISO to make sure they get the Highest Image Quality. However, there are 4 conditions that may require the use of High ISO. Let’s check them out.
(1) When you want to freeze a motion
When you are shooting photos of a flying bird, a moving race car or a running dog, you need a high shutter speed, sometimes may go up to 1/1000s, to freeze the motion, or you will get a blur image. In this case, especially in a low light environment, it is not enough to only widen the aperture. ISO will be the tool you have to use.
By increasing the ISO from 100 to 800, you will find your shutter speed dramatically rises up. Some people may worry the noise caused by high ISO. However it is better being noisy than a blur image. The former can be fixed by post-editing while the latter is not easy to correct.
(2) When you are shooting star photo
If you are a star photographer, you will know that without using a star tracker, you should limit the shutter speed to 25 to 30 seconds. Otherwise you will find little tails after each star because the Earth is constantly moving.
So in order to have a correct exposure of a star photo under the night sky, we should use the largest aperture we have. While the aperture and shutter speed are rather “fixed”, the only option to change the exposure is the ISO. Depending on the light of the sky and surrounding (also the aperture you are using), sometime we need to use ISO as high as ISO3200 to make the stars and sky clearly shown on the photo.
(3) When you are handheld shooting photo in low light
If you are taking photos in a low-light environment without a tripod, you have to keep your shutter speed equal or faster than Safe Shutter Speed (Refer to this tutorial). Sometimes even the largest aperture (or the aperture you desired) does not allow enough light getting past the lens to maintain an appropriate shutter speed. Then we need to raise up the ISO in order to have a sharp and clear image.
(4) When you are using tele lens
As you may already know the Safe Shutter Speed is calculated based on 1 / focal length. That means the longer the focal length (e.g. 200mm), the faster the shutter speed is needed. An effective way to increase the shutter speed is to use a higher ISO. Under bright sunlight, even a small increase of ISO can boost up the shutter speed a lot. Moreover, tele lens is very sensitive to movement. a small amount of object movement will be apparently shown on your photo. So it is a good idea to use high ISO to keep the shutter speed fast.
Aperture, shutter speed and ISO are three most important elements of exposure. You must be very clear on when and how to adjust these settings on different conditions. If high ISO is needed, do not be afraid to use it.
Author: Alex Tam