What it is
A brightness setting. The aperture and shutter speed decide how much light reaches the sensor; ISO decides how bright the picture made from that light looks.
Chapter 2Part 1: Exposure
ISO and
image noise
ISO is your camera's brightness setting. It doesn't let in more light: it brightens the light the sensor already caught. That lets you use a faster shutter when light is low, at the cost of more visible noise.
ISO in a minute
The numbers
100, 200, 400, 800, 1600… Each doubling is one stop, so the picture looks twice as bright. Most cameras also offer the 1/3-stop values in between.
Why you raise it
When there isn't enough light for the shutter speed and aperture you need, like a fast shutter to freeze movement indoors or at night.
What it costs
More visible noise: grain and blotchy colour, because there was little light to begin with. And bright areas, like a sky or a lamp, turn pure white sooner.
Try the ISO trade
Watch the flying bird. Keep the shutter fixed, then let it get faster as you raise ISO. The scene is an illustration, not a camera test.
f/41/30ISO100
- Picture brightness
- Balanced
- Light captured
- 100%
- Motion
- Blurred
Noise depends on the light captured, the camera and processing. The grain shown here is a simplified visual guide.
When ISO goes up
Keep 1/30 s. You catch the same light, but the picture gets brighter.
1003200
One stop per step.
What changed?
At ISO 100, 1/30 s gives us a starting picture.
How the settings connect
Every photo is made in three steps. Step 1 decides how much light is caught, step 2 records it, and step 3, ISO, decides how bright the result looks. Pick a change to follow it through.
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Catch the light
Aperture is how wide the lens opens. Shutter speed is how long it stays open. Together they decide how much light reaches the sensor.
They also shape the picture: aperture sets the background blur, shutter speed sets the motion blur.
1/30 s → 1/250 s About 3 stops less light is caught, and the movement freezes.
No change Same opening, same time, same light.
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Record it
The sensor records the light it caught. More light gives a cleaner picture. Less light lets noise show through.
The camera's electronics add a little noise of their own too. Base ISO below explains how some cameras reduce it.
Less light The recording is noisier underneath.
Same light The recording is no cleaner than before.
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Brighten it
ISO sets how bright the result looks. It doesn't add light: it turns up what was recorded, like the volume on a quiet song.
Turning it up makes noise easier to see, and bright areas turn white sooner.
ISO 100 → 800 3 stops brighter, back to where it started, with more visible grain.
ISO 100 → 400 2 stops brighter. A bright sky may now turn pure white.
In short: the light you catch decides how clean the photo can be. ISO decides how bright it looks.
Choosing ISO
In three steps
- Set what the picture needs first. A shutter speed fast enough for the movement, and an aperture for the depth of field you want.
- Then set the ISO that makes it bright enough. In good light that's often ISO 100 or 200. Indoors or at night it may be 1600 or more.
- Check the photo. If the noise bothers you, add light, open the aperture, or slow the shutter if the subject allows.
Or let Auto ISO do step 2
Set the shutter speed and aperture yourself, and the camera picks the ISO for each shot. It's ideal when the light keeps changing, like moving between rooms or in and out of shade. Most cameras let you set a highest ISO it may use.
Two common mistakes
ISO 100 at all costs
In dim light this forces a slow shutter and blurs a moving subject. A sharp photo with some grain beats a clean, blurry one.
All grain is a failure
Check the whole photo at the size you will share it. A little grain often matters far less than it looks when zoomed in.
Going further
Base ISO and dual native ISO
You don't need this to take good photos. It explains the "base ISO" and "dual native ISO" you'll see in camera specs.
Base ISO
A setting the sensor is designed around, where it gives its best balance of low noise and highlight detail. The lowest base keeps the most highlight detail. It's often ISO 100 for photos, and higher in video log modes.
Native range
All the standard ISO settings, such as 100–64,000. It's a range to choose from, not thousands of bases.
Extended ISO
Extra settings outside the native range, often marked Lo or Hi. A lower number isn't a better base: the Nikon Z6III's ISO 50 equivalent is extended, and its standard floor is ISO 100.
Why some cameras have two or three
Each pixel collects light as electric charge, and the camera turns that charge into a signal. Many sensors can do this at two strengths, called conversion gain. Neither strength is best for every scene.
Low gain: the lower base
Each pixel holds a lot of charge before it fills up, so bright highlights keep their detail.
Gives up: the readout adds more noise in dark areas.
High gain: the higher base
Each bit of light makes a bigger signal at the pixel, before the rest of the electronics adds its noise, so shadows come out cleaner.
Gives up: the pixel fills sooner, leaving less room for highlights.
In dim light you're already at a high ISO, and the pixels never come close to filling up. The low-gain reading's extra highlight room goes unused, so switching to high gain costs little and lowers the noise from the electronics. That's why noise often drops suddenly at the second base. A third base adds a step in between, keeping the sensor nearer its best reading across more of the ISO range.
Video cameras advertise this most because video leaves ISO as the setting you can change: the shutter usually stays near twice the frame rate (1/50 s at 25 fps) so motion looks natural, and the aperture is chosen for the look.
The bases change with the recording mode
Try a real example: LUMIX S5IIX
Switch the picture profile. Panasonic lists a different pair of base sensitivities for each one.
Base sensitivities in Normal
ISO 100ISO 640
ISO 100ISO 12,800
In Normal, the two bases are ISO 100 and 640. Panasonic can select the low or high circuit automatically.
The scale places ISO values by stops; distance on screen is not measured noise or image quality. V-Log’s ISO 320 and Normal’s ISO 50 are listed by Panasonic as extended options.
| More examples | Recording mode | Base ISOs listed by the maker |
|---|---|---|
| Sony FX3 | Cine EI, S-Log3 | 800 and 12,800 |
| Canon EOS C400 | Canon Log 2/3 or RAW | 800, 3,200 and 12,800 |
| Canon EOS C80 | Canon Log 2/3 or RAW | 800, 3,200 and 12,800 |
Examples only: look up your own camera and recording mode in its manual. Panasonic explains its dual circuit.
Using a second or third base
A higher base lowers the noise the camera's electronics add. It can't add light, and it can't rescue a highlight that's already white.
- In bright light, use the lowest base. It keeps the most highlight detail.
- In dim light, try the next base up. Keep the shutter speed and aperture the same, and compare the shadows.
- Just below a base? Try the base itself. On the S5IIX in Normal, ISO 640 can look cleaner than ISO 500, because 640 switches to the high-gain circuit. Read-noise charts such as Photons to Photos show where a camera's noise drops.
- Settings between bases are ordinary ISO settings. Some cameras let you choose which circuit to use: the S5IIX offers Auto, Low and High.
Try it yourself
No schedule: pick any of these when you have your camera or phone with you. Each takes a few minutes.
- Climb the ISO ladderIn manual mode, keep the aperture and shutter speed fixed. Photograph one still scene at ISO 100, 400 and 1600.Look for: each step brightens the picture, and bright areas turn white sooner. Zoom in to compare the grain.
- Blur or grain?Photograph someone moving indoors twice: low ISO with a slow shutter, then a faster shutter with ISO raised until it's bright again.Look for: which one you'd keep, at the size you'll share it.
- Walk with Auto ISOTurn on Auto ISO and move from a bright place to a dim one, like from outside to indoors.Look for: the ISO number rising as the light drops.
- Choose on purposeShoot outdoors, indoors and a moving subject. Pick the ISO yourself before each shot.Look for: whether you can say why you chose each one.
- Find your camera’s basesIn your camera’s manual, look up the exact recording mode and picture profile. Write down the base values and any extended settings.Look for: whether the values change when you switch profiles.
Check yourself
One question at a time. Your next question changes based on your answer.
Question 1