ISO in photography

ISO in Photography

ISO, in Photography, changes the light-gathering ability, or the sensitivity of a sensor. Higher the ISO is raised, brighter an image, and vice versa.

ISO is the third variable that can control the brightness of an image, other two being aperture and shutter-speed.

In the true sense, ISO can not allow more or less light to the sensor, as aperture and shutter-speed can do. Rather, with a higher ISO setting, brightness is achieved by amplification of the electrical signal received.

Exposure triangle, a high ISO photograph

A high ISO image

Exposure triangle, a high ISO photograph

Cropped version

When we double the ISO, keeping aperture and shutter-speed constant, exposure is increased by one stop. And when we halve it (aperture and shutter-speed remaining the same), total exposure is reduced by one stop.

When we are unable to get brightness through the different settings of aperture and shutter-speed, we resort to higher ISO to increase brightness. ISO is a function of the sensor or film of a camera, and nothing to do with the lens. It is unitless, and denoted by ISO 100, 200, 6400 etc.

ISO 200 will brighten an image by one stop than what can be achieved by ISO 100. Again, ISO 3200 will reduce exposure by one stop than ISO 6400; in both cases aperture and shutter-speed are assumed to be constant. The following ISO values are said to be one stop brighter than its previous value :
... 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200 ...

Since brightness through increased ISO is achieved via amplication of the light signals, higher ISO produces some amount of grains, which is also known as noise in photography. Higher the ISO, more the noise. Due to this we always want to keep our ISO to the lowest.

Suppose in a low-light scenario; we want to photograph a moving kid. After widening the aperture to the maximum limit (say f/2.8) of the lens, we are getting a correct exposure, but with a shutter-speed of 150 sec., which is not sufficient to freeze the moving kid. We don't want motion-blur; so a faster shutter is to be applied. But this will darken the image.

In this situation, we check our present ISO (say ISO 400 in that situation), and double it (ISO 800). We shall now observe that the scene is over-exposed by 1 stop due to the increased ISO. Now we dial-in a one stop of faster shutter-speed 1100 sec. So exposure is reduced by one stop, and the over-all exposure is normalised. With a shutter-speed of 1100 sec. the problem may be solved, but we gain some more noise in the photograph.

Exposure triangle, a high ISO photograph

A high ISO image

Exposure triangle, a high ISO photograph

Cropped version

Note : 1100 sec. may not be sufficient again to freeze the kid in the photograph [depends on speed of the subject, lens to subject distance etc.]. In that case, we have to try one stop faster shutter 1200 sec. For that we must increase ISO by another one stop (ISO 1600).

Before changing ISO:

After changing ISO:

Exposure triangle, a high ISO photograph

A high ISO image

Exposure triangle, a high ISO photograph

Cropped version

Above is an example (with its cropped version) which was shot in very low-light with ISO 6400. Check the cropped version to see how badly the noise is dominating here, and why we should avoid higher ISO if situation permits. I could have lowered the shutter-speed from 160 sec. to 130 sec. , so that the ISO could be 3200 with less noise. But 130 sec. shutter-speed could have captured some motion-blur as the subjects were moving. This is the main purpose of ISO; to help the shutter-speed fast enough to avoid motion-blur.

Different ISO on same image

ISO 64 ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200 ISO 6400 ISO 12800 ISO 25600 Original Hover to zoom
ISO 64 ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200 ISO 6400 ISO 12800 ISO 25600 Cropped Hover to zoom

The same scenario was shot with ten different ISOs, namely, ISO 64, 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600. All ten images are displayed above one by one alongwith their cropped versions.

Why higher ISO did not produce brighter images in the above examples ? Because when I increased the ISO by one stop, shutter-speed was set by the camera one stop faster (shot on aperture priority, it being f/8 in each image). Thus the exposure was equalised for all the shots. No noise reduction was done.

Our aim will be to keep the ISO as low as possible to avoid noise. We should raise it only when situation demands. If you are using tripod, and nothing is moving in the scenario, then use lowest possible ISO, and lengthen shutter-speed accordingly.

When noise is dominant in an image, detail is also lost, and the image quality reduces severely. In photography, you will find a term 'NR', which means Noise Reduction. In post-processing, noise can be reduced by softwares, but in this process, you will also lose details in the image. High ISO also reduces the dynamic range of a photograph.

In a nutshell, effects of high ISO images are:

In spite of the above facts, ISO in photography has very important role. It is our friend, in low-light, birding, wildlife, sports and astro photography. With rising technology, camera sensors of this era can produce almost clean images even with ISO 6400 or more. Only a wise use of ISO can result in good photographs even in low-light.

bird in flight in low light ISO example

f/5,   11250 sec. , ISO-2000, FL-122mm eqv., Nikon d810, Nikkor 70-300mm, Jaipur, Rajasthan, India 2015.

It was a Sunset in Jalmahal point at Jaipur. To freeze the motion of this flying bird, I used 11250 sec. shutter-speed. But due to low-light, ISO of 2000 was used to compensate that fast shutter-speed. Noise reduction was used on this photograph.

{/* We shall discuss the other terms, related to ISO, such as Auto-ISO, ISO invariance, Signal-to-Noise Ratio(SNR) etc. in a separate page if time permits. */} Modern cameras are smart enough to handle higher ISO with less noise, so never hesitate to raise the ISO if it is required.

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