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.
A high ISO image
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 1⁄50 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
1⁄100 sec.
So exposure is reduced by one stop, and the over-all exposure
is normalised. With a shutter-speed of 1⁄100 sec. the
problem may be solved,
but we gain some more noise in the photograph.
A high ISO image
Cropped version
Note : 1⁄100 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
1⁄200 sec.
For that we must increase ISO by
another one stop (ISO 1600).
Before changing ISO:
● aperture f/2.8, shutter-speed
1⁄50 sec., ISO 400.
● Image is neither over-exposed nor under-exposed.
● With shutter-speed 1⁄50 sec. ,
there may be motion-blur.
● Use a faster shutter-speed, but image will be darker.
After changing ISO:
● aperture f/2.8, shutter-speed 1⁄100 sec., ISO 800.
● Using 1 stop faster speed will darken the image by 1 stop.
● Using 1 stop higher ISO will brighten the image by 1 stop.
● So overall exposure remains same.
● With 1⁄100 sec. shutter-speed motion of the kid may be freezed.
● But we gain some more noise in the final photograph.
A high ISO image
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 1⁄60 sec.
to 1⁄30 sec. , so that the ISO could be 3200 with less noise.
But 1⁄30 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
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:
Noisy image, grains will be dominanting the pixels.
Colour noise may ruin original colours of the subject.
Less details will be photographed.
Reduced dynamic range in the image.
Overall image quality will be decreased.
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.
f/5, 1⁄1250 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
1⁄1250 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.
