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What is ISO and how it affects your DSLR photos.

If your low light photography pictures are coming out too dark or too blurry, ISO might be the solution for you. Back in the film days, ISO was known as the sensitivity of your film to light. Bigger the number, more sensitive it was and thus the photographer could take better photos in low light. In digital photography, it increases the sensitivity of your DSLR towards the light. Higher the ISO, the brighter and grainier images will look. Technically speaking, it increases the gain of the sensor which means amplifying the signal value.

Iso settings on a dslr
Pentax K5-II used to take high ISO photos
Value of ISO forms an integral part of the exposure triangle, where other two factors include aperture and shutter speed. Below are a few example shots showing the impact of ISO on photos taken in low light conditions. All the other factors such as aperture, shutter speed, metering mode and focal length were kept constant while taking these photos.
DSLR settings: 
Aperture: F/3.5
Shutter speed: 1/15 seconds
DSLR: Pentax K5-II
Focal length: 18 mm


DSLR photo taken at 160 ISO
ISO: 160

DSLR photo taken at 800 ISO
ISO: 800

DSLR photo taken at 4000 ISO
ISO: 4000


DSLR photo taken at 12800 ISO
ISO: 12800


From these 4 photos,  as a beginner photographer or camera owner, you can easily spot the effect of an increase in the value of ISO. As the ISO goes higher, the image keeps getting brighter and brighter, ultimately gets way overexposed. At the same time, if you zoom in, you will notice the images get noisy as well. Here noisy means a lot of grain, some people like the grainy look though but it does not look that appealing in people photos.

 When do you increase the ISO?

When your shutter speed and aperture is not able to bring in enough light, that's where ISO comes to the rescue. Photographing a high-speed action? You will need higher shutter speed which will lead to loss of light. This can be compensated by an increase in the value of ISO which increases the gain. Photographing a low light indoors event, this is where you might have to bump up the ISO. Taking photos of moving bird/animal during evening hours, you will need an above normal shutter speed because the animal might move. This leads to less light coming in which can be balanced by increasing the value of ISO to boost sensor sensitivity to light. 
If a photo can be properly exposed and captured by changing the value of aperture and shutter speed only, then do not increase your ISO. ISO could be termed as the least preferred method to reach a perfect exposure in the photo.
Also, some cameras have a functionality called Auto ISO. This means you are leaving the ISO selection part to the camera. You will define the shutter speed and aperture, or either of them, the camera will figure out the value of ISO depending on the available light. With technology advancements allowing fairly high usable values of ISO, some professional photographers love to use Auto ISO in Manual mode.

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