ISO
Written by Abin Alex and Anu Joseph.
Abstract
ISO is a camera setting that sets how much the sensor signal is boosted, so that identical light yields a brighter or darker picture. A low value gives the cleanest image but needs more light; a high value gives a brighter image with more noise. This guide traces the origin of the scale and explains the stop of ISO, base, native and extended ISO, the causes of noise and ways to reduce it, and the choice of ISO in different light. Its central point is that ISO never adds light. It is written for photographers and cinematographers.
Keywords: ISO; noise; native ISO; sensitivity; exposure
- What is ISO, and what does it do?
- What does ISO stand for, and where does it come from?
- How is the ISO scale written, and what is a stop of ISO?
- How does ISO change the brightness of a picture?
- What happens inside the camera when I raise ISO?
- What are base ISO, native ISO and extended ISO?
- What is noise, and what causes it?
- How can I reduce noise in my pictures?
- Which ISO should I use in different light?
- What is Auto ISO, and how do I set ISO on my camera?
- What is the highest ISO I should use?
- How do ISO, aperture and shutter speed work together?
- What is ISO in cinematography, and what are base ISO and exposure index?
- Which ISO suits each kind of photography?
What is ISO, and what does it do?
Short answer. ISO sets the amount of amplification applied to the sensor's signal, so the same scene light can come out brighter or darker. Raising it brightens the picture and adds noise; lowering it keeps the picture cleaner but needs more light. ISO does not produce any extra light.
- ISO is one of the three exposure settings, with aperture and shutter speed.
- Raising ISO makes the picture brighter by amplifying the sensor signal. It does not add light.
- Low ISO, such as 100, gives a clean picture with more detail but needs more light.
- High ISO, such as 3200, gives a brighter picture with more visible noise and less detail.
- The word ISO means the sensitivity to light of a film or a sensor. Digital cameras kept the numbers that film used.
Think of ISO as a volume control. The sensor records a small signal. A low ISO plays it back quietly and cleanly. A high ISO turns the volume up, so you can hear faint things, but the background hiss grows with it.
Cham tipRaise ISO only when aperture and shutter speed cannot give you a bright enough picture.
Cham taskShoot the same scene at ISO 100, 800 and 6400. Compare brightness and noise.
Check yourself: Does raising ISO let more light into the camera?
No. It amplifies the signal from the sensor, so the picture looks brighter.
What does ISO stand for, and where does it come from?
Short answer. ISO is not an abbreviation of "International Standard Organization". It is the short name of the International Organization for Standardization, chosen because the full title would be abbreviated differently in each language. The name derives from the Greek isos, meaning equal. For photography, the same body defines the standard way of stating the light sensitivity of film or a sensor.
- The full name is the International Organization for Standardization. Its short form ISO is the same in every language, and it comes from isos, meaning equal.
- Before ISO, film speed was written in two systems: ASA in the United States and DIN in Germany. They were merged into one ISO scale in 1974.
- Film sold with numbers such as 100, 200, 400 and 800. A lower number meant finer grain and less sensitivity to light.
- Digital cameras cannot be measured the same way as film, but they adopted the same numbers.
- For digital cameras, the standard ISO 12232 describes how to determine the exposure index, ISO speed ratings, standard output sensitivity and recommended exposure index (Imatest, n.d.).
| Period | What was used | Note |
|---|---|---|
| Film era | ASA in the United States, DIN in Germany | Two scales for the same idea |
| 1974 | ISO scale | ASA and DIN combined into one standard number |
| Digital cameras | The same ISO numbers | Measured differently, but similar in meaning |
| Digital standard | ISO 12232 | Defines how digital camera ISO speed is determined |
Table 1. How the ISO number came to photography.
Cham tipISO is defined by a standard, but cameras measure it a little differently, so test your own camera.
Cham taskTake the same picture on two cameras or phones at ISO 800. Compare the brightness.
Check yourself: Is it correct to say ISO means International Standard Organization?
No. It is the International Organization for Standardization, and ISO comes from the Greek word isos, meaning equal.
How is the ISO scale written, and what is a stop of ISO?
Short answer. ISO values are numbers like 100, 200, 400. Each full stop doubles the number and doubles the brightness, so ISO 200 is a stop brighter than ISO 100, and ISO 6400 is six stops brighter than ISO 100. Cameras also provide half-stop and third-stop steps for finer control.
- Full stops: 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200.
- One stop up doubles the sensitivity. One stop down halves it.
- From 100 to 400 is two stops. From 400 to 6400 is four stops.
- Half stops add one value between full stops, such as 140 between 100 and 200. Each half step is about 1.41 times.
- Third stops add two values between full stops, such as 125 and 160 between 100 and 200. Each third step is about 1.26 times. Most cameras use third stops by default.
- Some printed charts show rounded numbers, such as 12500 instead of 12800.
| Full stop | Third stops after it | Half stop after it |
|---|---|---|
| 100 | 125, 160 | 140 |
| 200 | 250, 320 | 280 |
| 400 | 500, 640 | 560 |
| 800 | 1000, 1250 | 1100 |
| 1600 | 2000, 2500 | 2200 |
| 3200 | 4000, 5000 | 4400 |
| 6400 | 8000, 10000 | 8800 |
| 12800 | 16000, 20000 | 17600 |
| 25600 | 32000, 40000 | 35200 |
| 51200 |
Table 2. The ISO scale in full, third and half stops.
Cham tipDoubling the ISO number doubles the sensitivity. ISO 100 to 200 is one stop.
Cham taskList the full ISO stops from 100 to 6400.
Check yourself: How many stops is it from ISO 100 to ISO 800?
Three stops: 100 to 200, 200 to 400, 400 to 800.
How does ISO change the brightness of a picture?
Short answer. With aperture and shutter speed held constant, raising ISO brightens the picture and lowering it darkens the picture. ISO does not alter how much light enters the camera; it alters how strongly the recorded signal is boosted afterwards. So choose aperture and shutter speed for the look you want, and use ISO to set brightness.
- Same aperture f/8 and shutter speed 1/250 s: ISO 100 is dark, ISO 200 is slightly brighter, ISO 800 is brighter, ISO 3200 is much brighter.
- Each stop of ISO doubles the brightness.
- The light that reaches the sensor is set only by the aperture and the shutter speed.
- ISO and the light needed have a reciprocal relationship: the lower the ISO, the more light the picture needs.
- Raising ISO to rescue a dark picture also raises noise. Whenever you can, add light or slow the shutter first.
| ISO | Brightness | Noise |
|---|---|---|
| 100 | Darker | Low |
| 200 | Slightly brighter | Low |
| 400 | Brighter | Low |
| 800 | Brighter | Slight |
| 1600 | Brighter | More visible |
| 3200 | Much brighter | High |
Table 3. The same scene at different ISO values, with aperture and shutter speed fixed..
Cham tipRaising ISO brightens the picture but adds noise.
Cham taskIn Manual, keep shutter speed and aperture fixed and shoot at ISO 100, 400 and 1600. Compare.
Check yourself: Aperture and shutter speed are fixed. You go from ISO 200 to ISO 800. How many stops brighter is the picture?
Two stops brighter, four times the brightness.
What happens inside the camera when I raise ISO?
Short answer. The sensor turns light (photons) into a weak electrical signal (electrons). The camera amplifies that signal and a converter turns it into digital numbers. A higher ISO means more amplification before conversion. The sensor does not become more sensitive, and the amplifier raises the noise along with the signal.
- Step 1: the sensor turns light into electrons.
- Step 2: an amplifier raises the signal. A higher ISO means more amplification, and a lower ISO means less.
- Step 3: an analog to digital converter, called the ADC, turns the voltages into numbers, and the processor saves the file.
- The sensor keeps the same sensitivity at every ISO. Only the amplification changes.
- In low light the signal is weak, so it needs more amplification, and the noise becomes more visible.
A strong signal, from bright light, needs little amplification. A weak signal, from dim light, is amplified much more, and the small errors mixed into it are amplified with it.
Cham tipHigher ISO amplifies the signal. It does not collect more light.
Cham taskShoot a dark room at ISO 100 and brighten it on the computer. Compare it with a shot at ISO 1600.
Check yourself: At which stage of the signal path does ISO act?
At the amplifier, before the analog to digital converter.
What are base ISO, native ISO and extended ISO?
Short answer. Base ISO, or native ISO, is the camera's lowest regular ISO. It needs the least amplification and produces the cleanest image, and is typically ISO 100 or 200, or 50 on some cameras. Anything higher uses more amplification. Extended settings labelled HI or LO lie outside the normal range, are produced by software and reduce quality.
- Native ISO: the electrons collected in the sensor are sent to the converter with no added amplification. This gives the highest image quality.
- Amplified ISO: the voltage is increased before conversion. More amplification means more noise.
- Simulated ISO: values produced by a software algorithm, including the HI and LO extended settings. They are mimicked, reduce quality, and are best avoided.
- Auto ISO is not simulated ISO. Auto only lets the camera choose among the real values for you.
- For everyday shooting, Nikon describes ISO 200 to 1600 as the range where most cameras give good results (Nikon USA, n.d.).
| Type | How it is made | Quality |
|---|---|---|
| Native or base ISO | No added amplification | Highest quality |
| Amplified ISO | Sensor signal boosted before conversion | Noise grows with each stop |
| Simulated ISO (HI, LO) | Software extends the range | Reduced quality, avoid |
Table 4. Types of ISO. The camera still records real values at Auto ISO.
Cham tipStart at your camera's base ISO for the cleanest picture. Use extended settings only when you must.
Cham taskFind the base ISO of your camera in its manual.
Check yourself: Is Auto ISO a simulated ISO?
No. Auto ISO lets the camera choose among real ISO values. Simulated ISO means the extended HI and LO settings made by software.
What is noise, and what causes it?
Short answer. Noise appears as random grain or coloured speckles. It has three sources: the random way light arrives, called photon shot noise; heat in the sensor, called dark noise; and the electronics that read and amplify the signal, called read noise. High ISO makes noise easier to see because the signal and the noise are boosted together.
- Signal to noise ratio, SNR, compares the strength of the signal with the noise. A high SNR means low visible noise. A low SNR means high visible noise.
- Photon shot noise: photons arrive at a pixel at random, so the noise equals the square root of the number of photons. It is a natural kind of grain.
- Dark noise: a small leak of current in the sensor. A hotter sensor produces more.
- Read noise: added when the signal is read out and converted, including after each ISO amplification.
- Noise does not come from ISO alone. The subject, pixel size, in-camera noise reduction and RAW or JPEG all change it.
Dark parts of a picture have the lowest signal to noise ratio, because fewer photons reach those pixels. This is why noise shows first in shadows.
| Photons at a pixel | Shot noise (square root) | Signal to noise ratio |
|---|---|---|
| 100 | 10 | 10 |
| 10,000 | 100 | 100 |
| 1,000,000 | 1,000 | 1,000 |
Table 5. Photon shot noise is the square root of the signal, so more light gives a cleaner signal.
Cham tipNoise shows first in the shadows. Check the dark areas at 100 percent.
Cham taskZoom to 100 percent on the shadows of an ISO 3200 photo and describe the noise.
Check yourself: Which areas of a picture show noise first, bright or dark?
Dark areas, because fewer photons reach them and the signal to noise ratio is lower.
How can I reduce noise in my pictures?
Short answer. Choose the lowest ISO that still gives a good picture, supply more light where you can, put the camera on a tripod in dim conditions so a slower shutter is possible, shoot RAW, and apply noise reduction in camera or in editing. Get the exposure right at the time of shooting, since brightening a dark picture later also lifts the noise.
- Use the lowest ISO possible.
- Get enough light: add a lamp, move to a window, open the aperture.
- Use a tripod in low light, so a slower shutter can replace a higher ISO.
- Shoot in RAW, which gives better editing with less damage.
- Use noise reduction, in the camera (for example the High ISO NR setting) or in post.
- More noise reduction removes fine detail, so use it with care.
| Tip | Why it works |
|---|---|
| Lowest ISO possible | Less amplification, less noise |
| More light | A stronger signal gives a higher signal to noise ratio |
| Tripod in low light | Allows a slower shutter and a lower ISO |
| Shoot RAW | More data to correct in editing |
| Noise reduction | Smooths grain, at some cost to detail |
Table 6. Five ways to reduce noise.
Cham tipAdd light before you raise ISO. A lamp or a window often helps more than noise reduction.
Cham taskTake a low-light shot at ISO 3200. Reshoot it with a lamp at ISO 400 and compare.
Check yourself: You can use a tripod and the subject is still. Should you raise ISO or slow the shutter?
Slow the shutter and keep the ISO low.
Which ISO should I use in different light?
Short answer. Pick the lowest ISO that delivers the picture you want: ISO 100 in bright sun, 200 to 400 in cloud or shade, 800 to 1600 indoors, 1600 to 3200 for action in poor light, and 3200 or above only when unavoidable. These are starting points, not rules.
- Bright daylight: ISO 100 gives the best quality, the most detail and the cleanest result.
- Cloudy or shade: ISO 200 to 400 adds sensitivity and keeps the quality.
- Indoors: ISO 800 to 1600 gives more sensitivity in low light.
- Action and sports: ISO 1600 to 3200 lets you use a faster shutter, so motion blur is reduced.
- Night or very low light: ISO 3200 and above, only as needed, with more visible noise.
- In shade, raise ISO to protect the shutter speed: ISO 200 at 1/125 s may blur a moving lizard, while ISO 1600 at 1/1000 s keeps it clear.
| Situation | Starting ISO | What you get |
|---|---|---|
| Bright daylight | 100 | Best quality, cleanest result |
| Cloudy or shade | 200 to 400 | Keeps quality, adds sensitivity |
| Indoors, low light | 800 to 1600 | More sensitivity, some noise |
| Action, sports | 1600 to 3200 | Faster shutter, less motion blur |
| Night, very low light | 3200 and above | Use only as needed, more noise |
Table 7. Starting ISO values.
Cham tipAs a starting point, use ISO 100 to 400 in daylight and 800 to 3200 indoors.
Cham taskShoot a window-lit room and an evening street. Write the ISO you chose for each.
Check yourself: A moving subject is blurred in shade at ISO 200 and 1/125 s. What can you change?
Raise the ISO, for example to 1600, so you can use a faster shutter such as 1/1000 s.
What is Auto ISO, and how do I set ISO on my camera?
Short answer. You set ISO through the camera menu, an ISO button with a dial, or a dedicated ISO wheel. With Auto ISO you fix a minimum and a maximum, and the camera chooses within that range as the light changes. A good approach is to choose aperture and shutter speed yourself and let Auto ISO handle the rest.
- Open the ISO section of the menu, or press the ISO button and turn the dial. Some cameras have an ISO wheel.
- Auto ISO: you set the limits and the camera chooses the best value inside them, for example Auto ISO range 100 to 6400.
- Many cameras let you set the maximum ISO and the shutter speed at which the camera starts raising ISO.
- A lower maximum protects quality but may not be enough in very low light. A higher maximum gives more flexibility but more noise.
- Try this: set aperture and shutter speed, let Auto ISO adjust, and check sharpness first.
- Always review the picture and check exposure, detail and noise.
| Setting | What it does | Effect |
|---|---|---|
| Minimum ISO | Lowest value Auto may use | Usually the base ISO, such as 100 |
| Maximum ISO | Highest value Auto may use | Lower: cleaner but may be too dark. Higher: brighter but more noise |
| Minimum shutter speed | Speed below which Auto raises ISO (on many cameras) | Protects against blur |
Table 8. The three Auto ISO settings.
Cham tipSet Auto ISO with a maximum of 3200 and a minimum shutter speed of 1/125 s, then watch the light, not the ISO.
Cham taskTurn on Auto ISO with those limits. Walk from a bright place to a dark one and watch the ISO change.
Check yourself: What two limits do you set for Auto ISO?
A minimum ISO and a maximum ISO. The camera chooses inside that range.
What is the highest ISO I should use?
Short answer. No single limit applies to everyone. It varies with your camera, the light, how fast the subject moves and how much noise you are willing to accept. Test your camera at each ISO and decide on a ceiling you are comfortable with. As a rough starting guide in our course, take the camera's highest ISO and divide it by four.
- Lower maximum, such as 800: cleaner image, less flexibility in very low light.
- Balanced maximum, such as 3200: good for most situations.
- Higher maximum, such as 12800: more flexibility for fast action or very low light, with more visible noise.
- Rough guide: highest ISO of the camera divided by 4. If the highest ISO is 3200, a safe value is 800.
- Treat the guide as a starting point and test. Pixel size and noise reduction change the result.
- Avoid the extended HI settings. They are simulated and reduce quality.
| Maximum ISO | Result | Best for |
|---|---|---|
| 800 (lower) | Cleaner, less flexible | Controlled light |
| 3200 (balanced) | Good balance, acceptable noise | Most situations |
| 12800 (higher) | More flexibility, more noise | Fast action, very low light |
Table 9. Choosing a maximum ISO.
Cham tipFind your own limit. Take test shots at each ISO and pick the highest one you accept.
Cham taskShoot a dark scene at ISO 800, 1600, 3200, 6400 and 12800. Choose your highest usable ISO.
Check yourself: By the rough guide, what is a safe ISO for a camera whose highest ISO is 6400?
About 1600, which is 6400 divided by 4.
How do ISO, aperture and shutter speed work together?
Short answer. All three settings control brightness together. When you change one, adjust another by the opposite amount to hold the exposure steady. Pick shutter speed first for motion, aperture first for depth, and use ISO to set the brightness.
- Choose shutter speed for motion. See shutter speed.
- Choose aperture for depth. See aperture and depth of field.
- Adjust ISO for brightness, noise and quality.
- One stop of any setting is balanced by one stop of another: ISO 400 at 1/500 s matches ISO 100 at 1/125 s.
- Different combinations can give the same brightness but not the same picture.
| Look | ISO | Aperture | Shutter | Brightness |
|---|---|---|---|---|
| Base | 100 | f/4 | 1/125 s | Reference |
| Faster shutter | 400 | f/4 | 1/500 s | Same |
| Deeper focus | 400 | f/8 | 1/125 s | Same |
| Frozen action, shallow depth | 400 | f/2.8 | 1/1000 s | Same |
Table 10. Combinations that give about the same brightness. Check each by counting the stops.
Cham tipChange one setting, then balance it with another so the exposure stays the same.
Cham taskStart with a correct exposure. Change the shutter speed by two stops and restore the exposure with ISO and aperture.
Check yourself: ISO 100, f/4, 1/125 s is correct. You raise ISO to 400. What shutter speed keeps the brightness at f/4?
1/500 s, because ISO went up two stops and the shutter must go two stops faster.
What is ISO in cinematography, and what are base ISO and exposure index?
Short answer. Cinema cameras have a base ISO, the native sensitivity that gives the cleanest image, and an exposure index (EI), the brightness you choose to expose to. Changing EI does not change how the sensor records. It changes the brightness you monitor and the metadata in the file, so post-production can adjust the gain. Shooting with EI equal to base ISO gives the least noise and the most latitude.
- In Cine EI mode, the recording sensitivity stays at base ISO whatever EI you choose (Sony, n.d.).
- Setting EI below base ISO gives more room to recover dark scenes in post, where the gain is reduced (Sony, n.d.).
- EI can be stored in the file metadata, so editing software can add or reduce gain automatically on import (Sony, n.d.).
- Dual base ISO gives a sensor two native sensitivities. Sony VENICE has a low base of ISO 500 for brighter light and a high base of ISO 2500 for lower light (Sony Cinema Line, n.d.).
- Each base setting gives a clean image, because neither adds gain. Both give the same wide dynamic range, a total of 15 stops on the VENICE (Sony Cinema Line, n.d.).
- Gain based brightening adds noise in proportion to the amount of gain.
- Always read the maker's base ISO for the camera you use. They differ between cameras.
| Term | What it means in cinema |
|---|---|
| Base ISO (native) | The sensitivity with no added gain and the cleanest image |
| Dual base ISO | Two native sensitivities, one for bright light and one for low light |
| Exposure index (EI) | The brightness you plan to expose for, stored in the metadata |
| EI below base ISO | Brighter monitoring, more room to recover shadows |
| Gain | Amplification that adds noise in proportion to the amount |
Table 11. Cinema ISO terms.
Cham tipOn cinema cameras, ISO is an exposure index. Use the base ISO for the cleanest image.
Cham taskFind the base ISO, or dual base ISO values, of a cinema camera in its manual.
Check yourself: You set EI below the base ISO. Does the sensor now record with a different sensitivity?
No. The recording sensitivity stays at the base ISO. EI changes the monitored brightness and the metadata.
Which ISO suits each kind of photography?
Short answer. Use the lowest ISO that still gives the shutter speed and aperture the subject needs. Fast action and dim light need a higher ISO. Treat every line below as a starting point, and test your own camera.
- Birds. ISO 400 to 1600 in good light. Use Auto ISO with a minimum shutter speed of 1/1000 s.
- Animals and pets. ISO 400 to 800 outdoors. Raise it indoors when the animal moves.
- Reptiles. ISO 100 to 400. They often stay still, so a tripod lets you keep the ISO low.
- Insects and macro. ISO 100 to 400 with flash. Without flash, raise ISO only as much as the shutter speed needs.
- Portraits. ISO 100 in daylight and in the studio. ISO 400 to 1600 indoors.
- Fashion. Base ISO in the studio with flash. ISO 200 to 400 outdoors.
- Wedding and events. Auto ISO with a limit of 3200 to 6400 in dim halls.
- Product. Base ISO on a tripod. It gives the cleanest result.
- Automotive. ISO 100 to 400 for parked cars and panning. Raise it when you need 1/1000 s to freeze a moving car.
- Photojournalism. Auto ISO with a maximum you can accept, so the shutter speed never drops too low.
- Sports and action. ISO 400 to 1600 outdoors and 1600 to 6400 indoors, to keep 1/500 s or faster.
- Landscape. Base ISO on a tripod.
- Street and documentary. ISO 400 in daylight. Auto ISO with a limit in the evening.
- Night and astro. ISO 1600 to 6400 with a fast, wide lens. Noise differs between cameras, so test yours.
- Cinema. Fixed at the base ISO of the camera. A dual base camera switches to its high base in low light.
Questions beginners ask
Does ISO change the exposure?
ISO does not change the amount of light that reaches the sensor. It changes how strongly the signal is amplified, so the picture looks brighter or darker.
What is the best ISO for photography?
The lowest one that gives the shutter speed and brightness you need. That is usually the base ISO, such as 100 or 200, in good light.
Is a high ISO always bad?
No. A noisy picture of a moment is better than a blurred picture. Raise ISO when you need a faster shutter, and accept the noise.
Why is my picture grainy at a low ISO?
The picture may be underexposed and then brightened, or the sensor may be hot after a long session. Expose properly and check the shadows.
Is ISO the same as film speed?
Yes. Digital cameras use the same ISO numbers that film used, so ISO 400 means the same idea in both.
References
- Imatest. (n.d.) ISO 12232: Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index. imatest.com/imaging/iso-12232
- Nikon USA. (n.d.) Understanding ISO Sensitivity. nikonusa.com/learn-and-explore/c/tips-and-techniques/understanding-iso-sensitivity
- Sony Cinema Line. (n.d.) Dual Base ISO explained. sony-cinematography.com/articles/dual-base-iso-explained
- Sony. (n.d.) Using the Exposure Index in shooting. Cine EI mode help guide. helpguide.sony.net/di/pp/v1/en/contents/TP1000756720.html
Further reading
- Ray, S.F. (2002) Applied Photographic Optics, 3rd edn. Oxford: Focal Press. See the sections on film speed and exposure.
- Langford, M., Fox, A. and Sawdon Smith, R. (2015) Langford’s Basic Photography, 10th edn. Abingdon: Routledge. See the sections on film speed and exposure.
How to cite this guide. Alex, A., & Joseph, A. (2026). ISO: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/what-is-iso/
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