ISO

Learning goal

You will be able to explain that ISO doesn't bring more light to the sensor but strengthens the signal of the light gathered. You will tell apart that the real source of grain is too little light and how a high ISO makes it visible, work out that each step in the ISO series shows the result twice as bright, find your camera's base ISO and the highest ISO you can accept on your own camera, know the difference between brightening a dark photo afterward and shooting at the right ISO, and record faces in dim light, especially dark skin, without leaving them in blotchy grain.

The question

When you look the next day at the photos you took in dim light at a wedding hall at night, it looks as if there is a thin layer of sand over the faces. Small red and green dots swarm on the dark walls, and the faces in the shadows seem to have stayed behind a television's static. The photos you took with the same phone in daylight, on the other hand, are smooth and clean.

Why do photos come out grainy in the dark? Because in the dark little light reaches the sensor, and the camera has to strengthen the signal to turn this little light into a bright enough photo. The setting for this strengthening is ISO. In this lesson we will talk about what ISO really does, where grain comes from, and when you shouldn't be afraid to raise ISO.

Context

In the bucket comparison we placed ISO not at the tap but at the reading needle beside the bucket: the water stays the same, only the value read is enlarged. The aperture and shutter speed gather the light; ISO decides how bright to show the gathered light in the photo. In Lesson 4.7 we introduced ISO as how sensitively the sensor responds to light. This lesson refines that definition: the sensor itself doesn't change, what changes is how much the signal it gives is strengthened.

The name ISO comes from the standard that measured the light sensitivity of films in the film era; it is the abbreviation of the international organization that publishes the standard. In the film era, an ISO 100 film and an ISO 400 film really carried different chemicals, and the second was grainier. Digital cameras keep using the same numbers, so a photographer knows that at ISO 400 the camera will behave roughly like ISO 400 film. The word grain also comes from these grains of the film era.

Simple explanation

Does ISO increase light, or the signal?

ISO doesn't increase light. Only the aperture, the shutter speed and the scene's own brightness decide the amount of light reaching the sensor. ISO, on the other hand, strengthens the signal the sensor records. At ISO 100 the signal is strengthened a little; at ISO 3200, a lot. In the end a photo taken at ISO 3200 looks much brighter than one taken at ISO 100 in the same light; but the light filling the buckets is the same.

ISO also moves in a series: 100, 200, 400, 800, 1600, 3200, 6400. In this series each step shows the result twice as bright. That is why ISO speaks the same language as the aperture and shutter speed: when you raise ISO by a step, you can keep the same brightness by shortening the shutter speed by a step. Every camera has a lowest ISO value at which it gives the cleanest result; this is called the base ISO, and on most cameras it is around 100. Some cameras come with lower or higher base values; the camera's manual tells you.

Where does grain come from?

Light arrives as small particles, and these particles fall on the sensor not like even rain but like irregular drops. In plenty of light this irregularity stays insignificant next to the total amount. In little light, on the other hand, because the number of drops falling into each bucket is small, the differences between neighboring buckets become noticeable; these differences show up in the photo as grain. The sensor's own electronic noise is added to this. So the real source of grain is too little light.

A high ISO doesn't change this situation; while strengthening the low-light signal, it also makes the grain inside it visible. Grain doesn't look the same everywhere either. Luminance noise is like fine grain sprinkled over the photo and resembles film grain; if it is moderate, it may not bother you. Color noise, on the other hand, appears in the shadows as red, green and purple blotches and is more disturbing. Grain is noticed most in the shadows and on plain colored surfaces, for example a wall or a sky.

Shoot dark and brighten later, or raise ISO?

Many photographers who are afraid of ISO keep ISO low in dim light, take a dark photo and brighten it in editing. This is usually not a good idea. On many cameras, strengthening the signal at the moment of shooting, that is choosing the right ISO, gives a less grainy result than brightening a dark frame by the same amount afterward. When you brighten a dark frame, the sensor's electronic noise grows along with the signal; also, Lesson 7.3 showed that opening up shadows has its own limits.

That is why in dim light you first open the aperture, then bring the shutter speed to the longest time that can still freeze movement; if the light still isn't enough, you raise ISO. A sharp but slightly grainy photo is almost always more valuable than a clean but blurred or pitch-black one. The best way to find the highest ISO you can accept on your own camera is to photograph the same scene at different ISOs and compare them zoomed in on the screen. This limit varies greatly from camera to camera; cameras with big sensors usually give a cleaner result at the same ISO.

How does grain affect skin?

Grain shows most in the shadows, and dark-skinned faces, especially in dim light, can fall into the dark areas of the photo. That is why a dark face shot at a low ISO and brightened afterward can be covered with the red and green blotches of color noise; the skin looks blotchy and spoiled. Shooting at the right ISO fills the face's buckets with enough signal at the moment of shooting and records the skin's color more cleanly. Setting exposure by the face is the most reliable guide here too.

Phones and many cameras reduce grain on their own. This noise reduction softens grain but also wipes out fine texture; faces taken at a high ISO can look waxy and plastic because of it. On light skin this flattening can make the skin look like porcelain, and on dark skin like a blotchy, blurry surface. When using the noise reduction slider in editing, zoom in on the face and look: leaving a little grain often looks more natural than a face whose texture has been wiped away.

Visual evidence

One evening, in a dim folk music café, you photograph a man playing the saz with a camera. The light comes only from a few yellow bulbs on the ceiling. The aperture is at f/2.8, the shutter speed at 1/60; to freeze his hand and the fingers touching the strings, you can't use a longer time. First you leave ISO at 100. The photo is almost pitch black. When you brighten it in editing, the man's face can be seen, but his dark wheat-colored skin is covered with red and green dots, and the dark wall behind has turned into seething static.

Then you raise ISO to 3200 and shoot again; that is five steps, a signal thirty-two times stronger. The photo is at the right brightness at the moment of shooting. There is fine grain on the face, like film grain, but no color blotches. His mustache, the lines around his eyes and his fingers pressing the strings of the saz can be made out; his skin stands in its own warm color. The same light, the same aperture, the same time; the difference is whether the signal was strengthened at the moment of shooting or afterward.

Common mistakes

Summary

ISO doesn't bring more light to the sensor; it strengthens the signal of the light gathered, and each step in the series shows the result twice as bright. The real source of grain is too little light, and a high ISO makes it visible; grain appears most in the shadows, as luminance noise and color noise. In dim light, taking light first from the aperture and shutter speed and raising ISO if needed often gives a cleaner result than brightening a dark frame afterward; shooting faces at the right ISO with exposure set by the face, and using noise reduction in moderation, keeps skin in every tone cleaner and more natural.

Mini assignment

In this assignment you will find your own camera's grain limit; a mode where you can choose ISO by hand is enough.

1. In a dim room, photograph a scene with dark and light areas at the same aperture at ISO 100, 400, 1600 and 6400; keep the brightness constant by shortening the shutter speed at each step accordingly.
2. Zoom in on the four frames on the screen and note at which ISO the grain starts to become disturbing.
3. Photograph the same scene dark at ISO 100, brighten it in editing and compare it with the ISO 1600 frame.
4. Photograph a friend's face in dim light at a suitable ISO, and compare the texture of the skin with noise reduction set low and high.

Check yourself

• Can you say with a number the highest ISO you can accept on your own camera?
• Between the frame shot dark and brightened and the frame shot at the right ISO, which is cleaner?
• Are there any color blotches or waxy flatness left on your friend's face?

Glossary

Mini quiz

1. What does raising ISO change?

• A) The amount of light reaching the sensor
• B) The size of the aperture
• C) How much the signal of the gathered light is strengthened
• D) The length of the shutter speed

2. What is the real source of grain?

• A) Too little light and the sensor's own electronic noise
• B) Too much light
• C) A wide aperture
• D) A long focal length

3. How does going from ISO 400 to ISO 800 affect the result?

• A) It doesn't change it
• B) It darkens it by half
• C) It brings twice as much light to the sensor
• D) It shows the result twice as bright

4. To reduce color blotches on a dark-skinned face shot in dim light, which gives a better result?

• A) Shooting dark at ISO 100 and brightening it all the way in editing
• B) Choosing the right ISO and setting exposure by the face at the moment of shooting
• C) Raising noise reduction to its highest
• D) Turning the photo black and white

Answers

1. C — ISO doesn't change the light the sensor gathers; it only decides how much to strengthen that light's signal.
2. A — In little light the drops falling into the buckets become irregular and the sensor's noise is added; a high ISO makes this grain visible.
3. D — In the ISO series each step strengthens the signal twice over; the light reaching the sensor stays the same, but the result looks twice as bright.
4. B — Strengthening the signal at the moment of shooting is often cleaner than brightening a dark frame afterward; the tone of the face is recorded without blotches.

Summary card

• ISO doesn't bring light; it strengthens the signal of the light gathered.
• The series 100, 200, 400, 800: each step twice as bright.
• The source of grain is too little light; a high ISO makes it visible.
• In dim light, the order: open the aperture, lengthen the time as far as still freezes, raise ISO if needed.
• Don't shoot dark and brighten; record the face at the right ISO, and use noise reduction in moderation.

Bridge to the next lesson

Now you know all three corners of the exposure triangle one by one: the aperture decides light and sharpness, the shutter speed light and movement, and ISO the signal and grain. But how do the three work together? The same brightness can be reached by many different paths. The next lesson shows how these three settings balance each other, and why choosing between them is a creative decision.