ISO — Why Turning Up the Sensitivity Has a Price
ISO can rescue a photograph — but it doesn't give you light for free.
Photography is fundamentally about collecting light.
Sometimes we have plenty of it. Outdoors on a bright summer afternoon, the problem may actually be keeping light out of the camera.
At other times, light becomes precious. We might be photographing indoors, at dusk, inside a church, at a concert, on a dull winter afternoon, or trying to photograph wildlife when the light is beginning to disappear.
We can open the aperture.
We can use a slower shutter speed.
But eventually we reach a problem.
Open the aperture too far and we may lose the depth of field we want. Slow the shutter too much and either the camera moves or the subject does.
There is another control available:
ISO.
Turn up the ISO and suddenly that photograph that seemed impossible becomes possible.
ISO 400.
ISO 800.
ISO 1600.
ISO 3200.
ISO 6400.
Modern cameras may offer settings far beyond these.
It can feel as though we have somehow made the camera more sensitive to light.
But there is an important catch.
Increasing ISO doesn't create any more light.
And that is why ISO always comes with a price.
The Third Part of the Exposure Triangle
Photography is often taught using the idea of an exposure triangle:
aperture;
shutter speed;
ISO.
It is a useful starting point, although the three controls do rather different things.
Aperture changes how much light reaches the sensor.
Shutter speed changes how long the sensor collects that light.
ISO is different.
Changing ISO does not cause more photons to arrive at the camera.
Instead, in simplified terms, it changes how strongly the camera amplifies and processes the signal produced from the light that has already been collected.
That distinction is extremely important.
Suppose I photograph something at:
ISO 100
and then photograph the same subject at:
ISO 3200.
The second photograph may appear much brighter if I leave everything else unchanged.
But the sensor hasn't magically collected 32 times as much light.
The camera has done more with the signal that it received.
Unfortunately, it also has to deal with the uncertainty and noise associated with that signal.
A Simple ISO Experiment
This is one of those photographic ideas that is far easier to understand by actually doing it.
Find a subject containing plenty of fine detail.
For example:
a bookshelf;
newspaper print;
a person's face;
fabric;
a feather;
a model;
a plant;
a detailed ornament;
a circuit board;
or even a collection of photographic equipment.
Put the camera on a tripod.
Ideally use RAW, although JPEG will still demonstrate the principle.
Then photograph exactly the same scene at several ISO settings.
For example:
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
ISO 3200
ISO 6400
ISO 12800
But there is an important point about how we conduct the experiment.
If we simply increase ISO while leaving everything else unchanged, the photographs will become progressively brighter.
That isn't really the comparison we want.
Instead, we want approximately the same final exposure while changing ISO.
Keeping the Brightness Similar
Suppose our first photograph is:
ISO 100
1/4 second
f/8
We could then approximately double ISO and halve the exposure time each time:
ISO 200 — 1/8 second
ISO 400 — 1/15 second
ISO 800 — 1/30 second
ISO 1600 — 1/60 second
ISO 3200 — 1/125 second
ISO 6400 — 1/250 second
The precise shutter speeds offered by a camera may vary slightly, but the principle is what matters.
The aperture remains at f/8.
The photographs should therefore have broadly similar brightness.
But something extremely important has changed.
At ISO 6400, the sensor receives light for a tiny fraction of the time it did at ISO 100.
We have traded light collection for amplification.
Now enlarge the photographs on a computer.
That is where the experiment becomes interesting.
Don't Compare the Whole Photograph
At normal social-media size, the photographs may initially look surprisingly similar.
Modern cameras are remarkably good.
Instead, zoom into perhaps 100%.
Look particularly at:
dark shadows;
smooth walls;
skin;
blue sky;
fine textures;
hair;
feathers;
fabric;
small lettering.
The differences should gradually become visible.
At low ISO, the photograph will normally appear cleaner.
As ISO rises, you may begin to see a grainy or speckled appearance.
This is what photographers commonly call noise.
What Does Noise Look Like?
Noise does not necessarily appear as one simple effect.
You may notice variations in brightness between neighbouring pixels.
At very high ISO you may also see unwanted colour variations.
A dark grey wall that should look smooth might contain tiny coloured speckles.
Fine detail can begin to disappear.
The camera's noise-reduction software may then try to hide those variations.
That creates another compromise.
Strong noise reduction can make the image look smoother, but it may also remove genuine detail.
Hair, feathers, grass and fabric can start looking slightly artificial.
So there are really two things worth examining:
the noise itself and what the camera has done to remove it.
Why Shadows Reveal the Problem
One particularly useful experiment is to examine the darkest parts of the photograph.
Noise often becomes much more obvious there.
This gives us an important practical lesson.
A photograph taken at high ISO with a reasonably good exposure can sometimes look considerably better than a badly underexposed photograph that we later brighten dramatically during editing.
This is why photographers should be cautious about simply saying:
"Always use the lowest possible ISO."
It sounds sensible, but it is incomplete advice.
The real objective is to collect enough light while still achieving the shutter speed and aperture needed for the photograph.
ISO and Dynamic Range
Noise is not the only issue.
Increasing ISO can also reduce the amount of highlight information we can preserve.
This introduces another important photographic idea:
dynamic range.
Imagine photographing someone standing beside a bright window.
The face may be comparatively dark while the outside scene is extremely bright.
A camera has to record both.
At lower ISO settings, many modern cameras can capture an astonishing range between dark shadows and bright highlights.
As ISO rises, that usable range generally becomes smaller.
Eventually the bright parts of the photograph may become completely white.
Once detail has been clipped from a highlight, there may be nothing to recover later.
A white wedding dress, clouds, pale skin or reflections from water can therefore become particularly challenging.
Try a Dynamic-Range Experiment
Set up a scene containing both very bright and very dark areas.
A room containing a window can work extremely well.
Include:
something dark in shadow;
a mid-tone object;
something pale;
the bright window.
Take a sequence at different ISO values while adjusting shutter speed to maintain broadly similar exposure.
Then examine the files in editing software.
Try reducing the highlights.
Try lifting the shadows.
Compare the ISO 100 file with ISO 3200 or ISO 6400.
The differences can be surprisingly revealing.
This is much more useful than simply being told:
"High ISO is noisy."
You can actually see what information the camera has retained.
Modern Cameras Have Changed the Rules
This is an area where photographic advice can become outdated remarkably quickly.
There was a time when ISO 800 could be treated with considerable suspicion.
ISO 1600 might have been something used only when absolutely necessary.
Modern digital cameras have changed that dramatically.
Improved sensor technology, better electronics and sophisticated image processing mean that ISO settings that once produced rather unpleasant photographs can now produce perfectly usable images.
With a modern full-frame camera, ISO 3200 may be entirely unremarkable for many purposes.
ISO 6400 can often be very usable.
Sometimes considerably higher settings are worthwhile.
But there is no universal number at which photographs suddenly become "bad".
It depends upon:
the camera;
sensor size and design;
exposure;
subject;
processing;
output size;
how much the photograph is cropped;
and what the photograph is actually for.
A photograph intended for Instagram is very different from one intended for a large exhibition print.
A Noisy Photograph Can Be Better Than No Photograph
This is perhaps the most important practical lesson.
Imagine I am photographing a musician performing indoors.
I could use:
ISO 400
1/30 second
The image might have very little noise.
Unfortunately, the musician's hands may be blurred.
Alternatively:
ISO 3200
1/250 second
might freeze the movement.
Which is the better photograph?
Almost certainly the second.
A technically clean photograph of a blurred subject isn't necessarily useful.
The same applies to sport, wildlife, children, sailing and many other moving subjects.
I would much rather have a sharp photograph containing a little noise than a beautifully noise-free photograph in which the important subject is blurred.
This Is Where Auto ISO Becomes Very Clever
For many years I think photographers sometimes treated automatic settings as though using them meant surrendering photographic skill.
That isn't necessarily true.
Modern Auto ISO can be extraordinarily useful.
The photographer can still make the important creative decisions.
Suppose I am photographing sailing.
I might decide that I need at least:
1/1000 second
to freeze movement.
I may choose:
f/5.6
because that gives me the depth of field I want.
I can then allow Auto ISO to compensate as the light changes.
Bright sunshine?
The camera might select ISO 100.
A cloud passes overhead?
Perhaps ISO 400.
The boat enters a darker patch beside trees?
Perhaps ISO 1000.
The photographer has not surrendered control.
Quite the opposite.
I have decided that shutter speed and aperture matter more than ISO for this particular photograph.
The camera is simply handling the variable that I am most willing to compromise.
That is a very different way of thinking about automatic exposure.
Set Limits on Auto ISO
Many cameras allow Auto ISO to be customised.
This is worth investigating.
You may be able to specify:
minimum ISO;
maximum ISO;
minimum shutter speed;
how aggressively the camera raises ISO.
For example, I might decide that ISO 6400 is perfectly acceptable on a particular camera.
I can therefore tell the camera:
Use anything between ISO 100 and ISO 6400 if necessary.
That creates an extremely powerful semi-automatic system.
The important point is that I have decided what compromises are acceptable before taking the photograph.
Different Subjects Need Different Priorities
There is no single correct ISO strategy.
Landscape
If the camera is on a tripod and nothing is moving, I can often use a low ISO and simply lengthen the exposure.
There may be little reason to use ISO 6400.
Sport
Shutter speed becomes critical.
Increasing ISO may be essential.
Wildlife
The same problem appears.
A bird will not stay still merely because the photographer wants to use ISO 100.
Portraits
Moderate ISO may allow a sufficiently fast shutter speed to prevent tiny movements from softening the face.
Concerts and theatre
Flash may be inappropriate or prohibited.
High ISO becomes one of the photographer's most useful tools.
Macro photography
This becomes particularly interesting.
We may want a small aperture to increase depth of field, but that reduces the amount of light reaching the sensor.
Higher ISO may therefore help maintain a practical shutter speed.
Every situation involves choices.
What About Flash?
There is another solution to insufficient light:
add more light.
Instead of increasing ISO, we might use:
flash;
continuous LED lighting;
studio lights;
reflectors;
or simply move the subject closer to a window.
This brings us back to the central idea.
ISO does not create light.
If I can physically provide more light, I may be able to use:
ISO 100 instead of ISO 3200.
That may improve both image quality and dynamic range.
But adding light isn't always possible.
I cannot conveniently illuminate an entire sailing race.
I cannot necessarily use flash during a concert.
I probably don't want to fire a powerful flash at wildlife.
ISO exists partly because photography happens in the real world rather than under perfect studio conditions.
An Experiment With Your Own Camera Is Better Than an Internet Argument
Photographers can spend an extraordinary amount of time debating whether a particular camera is "usable" at ISO 3200, 6400 or 12800.
There is a much better approach.
Test it.
Put your own camera on a tripod.
Photograph a realistic subject.
Use the lenses you normally use.
Process the files using your normal software.
Then view them at the size at which you actually publish or print photographs.
Ask yourself:
At what point does the loss of quality actually matter to me?
That answer is far more useful than somebody else's arbitrary ISO limit.
You might discover that you have been unnecessarily frightened of ISO 3200.
Or you might discover that ISO 12800 is acceptable for a small web image but not for a large print.
That is useful knowledge because it is knowledge about your equipment and your photography.
The Camera Matters — But So Does the Photograph
It is very easy for photography to become obsessed with technical perfection.
Perfect sharpness.
Minimum noise.
Maximum dynamic range.
Lowest ISO.
But photographs are not laboratory measurements.
Imagine photographing an extraordinary moment at ISO 12800.
The image contains noise.
Now imagine not taking the photograph because you insisted upon ISO 100.
Which produces the better photograph?
Sometimes technical quality matters enormously.
At other times, capturing the moment matters far more.
Understanding ISO gives us the ability to make that decision deliberately.
A Useful Challenge
Try this with your own camera.
Choose one detailed subject and photograph it at:
ISO 100
ISO 400
ISO 1600
ISO 6400
ISO 12800
Keep the aperture constant and adjust shutter speed to maintain similar brightness.
Then create two comparisons.
First, examine each photograph at 100% magnification.
Second, reduce every photograph to normal social-media size.
You may be surprised by how different your judgement becomes.
Then repeat the experiment in a scene containing deep shadows and bright highlights.
Finally, decide for yourself:
What is the highest ISO I would happily use on this camera?
There is no universal correct answer.
Conclusion — ISO Is a Compromise, Not an Enemy
ISO is sometimes presented almost as something photographers should avoid.
Keep ISO low.
Never go above ISO 800.
High ISO ruins photographs.
That advice is far too simplistic for modern photography.
Low ISO generally gives us excellent image quality, low noise and strong dynamic range.
But photography is not always conducted in bright sunshine with a stationary subject and a tripod.
Sometimes we need shutter speed.
Sometimes we need depth of field.
Sometimes the light disappears.
And sometimes the photograph matters more than technical perfection.
That is when increasing ISO becomes enormously valuable.
The important skill is not learning to avoid high ISO.
It is understanding when the advantages of raising ISO are greater than the price we pay for doing it.
Modern cameras have made that price remarkably small compared with earlier generations of digital photography.
So experiment with your own camera.
Discover where its limits really are.
And the next time the light begins to disappear, don't automatically put the camera away.
You may have far more photographic capability left than you realise.
ISO can rescue a photograph — but it doesn't give you light for free.
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