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Yes Please

Why You Should Know What ISO Does

ISO is one of three factors which determine the exposure of a photo, along with aperture and shutter speed. To really get the most out of your photos, you need to know what all 3 do and how you can use them.

Read this post to gain a more in-depth knowledge of how to use your camera properly and start taking expert photos.


Section 1 – What Exactly Is ISO?

The name stands for  International Organization for Standardization. I know this doesn’t offer much help. We need to look back at photographic history a little bit to understand Where it came from, What it means, and What it does?

Where Did ISO Come From?

As soon as photographic emulsion came around, each area or country tried to create classifications. People needed the classification to know what they could do with the film.

There were many different groups of people who tried to come up with a method that classified their film in the best way. This is the early process of trying to work out what we now call film speed.

Hurter and Driffield were one of these people. They were working on emulsion (silver halides dispersed in gelatin) sensitivity since 1890. Their system had an inverse system of classification and exposure. The higher the number, the lower the exposure the emulsion needed.

This was used until 1928.  Changes in light sources and chemicals they developed the glass plates with led to a need for a different system.

From 1928, the Soviet Union used this as their standard until 1951, when they replaced it with GOST. The GOST system was similar to ASA in classification and was overpowered by ISO in 1987.

A Canon A1 35mm film camera showing ASA instead of ISO


During this period, DIN (Deutsches Institut für Normung) was introduced in 1934. This system was based on an earlier version called Scheiner. The name came from a former German astronomer who tried to photograph the sky at night.

The Scheiner system used degrees to classify their emulsion. A 3° change would either double or halve the sensitivity. He based his findings on the least exposure to produce a visible darkening upon development.

After the 80s, emulsion and film ratings were shown as ISO 100/21° for a while.

A roll of Kodak film


ASA (American Standards Association) came around in 1943. Kodak and General Electric’s own film classification deeply influence this. They created a linear scale, where 200 ASA is twice as fast as a film with 100 ASA.

After some revisions, ASA changed its name to ANSI in 1988 and withdrew its name from film classifications. They then also adopted the ISO standard.


Since 1974, ISO has combined DIN and ASA. This system uses both an arithmetic and a logarithmic scale. Its arithmetic scale corresponds to the arithmetic ASA system, where a doubling of film sensitivity is represented by a doubling of the numerical film speed value.

ISO 400 is double 200, and 800 is double 400.

To show the equivalents of each of the three most important film speed classifications, see the table below:

ASA   –   DIN   –      ISO


50   –     18     –     50/18°

100   –     21     –    100/21°

200  –     24     –   200/24°

400  –     27     –   400/27°

800  –     30    –   800/30°

1600 –     33     –  1600/33°

We all now use ISO for our photographic film. It is also what we work with in digital photography. But other systems are still out there.

While travelling across the Balkans, I even came across film that still showed GOST (or ГОСТ in Cyrillic). It didn’t work, as the film was over 30 years old.

Some cameras I still use today still show DIN ratings, such as the Smena-8M and Smena-Symbol.

A Smena Symbol showing DIN, ASA and GOST/ГОСТ and film speeds

What Does It Mean?

So, as we saw, ISO is just a way to classify your film speed and how sensitive to light the film is. They are usually in the range of 50, 100, 200, 400, 800, 1600 and 3200. There are other films that are outside this range, but they are more specialised.

These increments seem a little strange, but what you will notice is that the numbers either double or halve. This is because moving from ISO 100 to ISO 200 halves the sensitivity of the film. Moving from ISO 200 to ISO 100 doubles it.

It is linked directly to the quality of the film. When photographic film was used, the ISO rating of the film indicated how big the silver halides were in the emulsion.

Photographic emulsion is what we covered (first glass plates) a roll of plastic with to make it sensitive to light. The emulsion contained silver, which is what ‘recorded’ the light information.

Film with a low ISO number held very small silver halides. Films rated as ISO 3200, for example, had large pieces of silver. These helped capture more light in low light conditions.

The bigger the pieces of silver, the least amount of light you’d, but the halides were visible. This is where we get film grain or noise.

Effectively, the silver pieces were twice as big in ISO 200 than in ISO 100, but it could capture twice as much light.

In the digital world, the same numbers continue to be used. The only differences are that we use digital sensors, not photographic film. And ISOs on DSLRs tend to go way above ISO 3200.

There are also added increments in the above range. 1/3 stops would be found between ISO 400 and 800, for example.

A close up of a DSLR camera showing the ISO value

What Does ISO Do?

The ISO of your photograph depends on many factors. It comes down to how much light is available, and what you want to capture.

We tend to try and shoot with a low ISO as that gives us the best quality and a smaller amount of noise/grain.

What you are capturing determines the value you should use. If you’re photographing outside in the middle of summer, you should use ISO 100 due to the abundance of light. But if you’re photographing inside, this value might jump to ISO 800, or even higher.

One of the best ways to explain the use of ISO in photography is by using the photographic exposure triangle. This triangle helps you to capture the perfect exposure.

Using the three interchangeable areas that control light, you can photograph anything, anywhere.

Showing the exposure triangle to understand correct exposure

For example, photographing a scene with ISO 400 at f/16 and a shutter speed of 1/400 and then wanting to create a smaller depth of field. You would first lower your aperture to f/8 (two stops).

To keep the same correct exposure, you would have to decrease either the ISO 2-stops or increase the shutter speed by 2-stops. Decreasing its value from 400 to 100 would give you better quality.

If you choose Av (Aperture Priority) or Tv (Shutter Priority) shooting modes, the ISO will change automatically.

Section 2 – How Does ISO Affect Exposure?

ISO is one of three determining factors of the exposure of a photo, along with aperture and shutter speed. These two affect the lens and exposure time respectively, with the ISO affecting the sensor (or film). To be more specific, the ISO determines how well exposed a photo will be by changing the sensitivity.

The ISO scale is similar to shutter speed in the sense that, when doubled, the exposure is also doubled. They are proportional to one another. A low ISO number will give a low exposure and a high ISO will give a high exposure. It’s much simpler then aperture.

This is much easier to demonstrate using actual photos as you can see in the slideshow below…

The photos are displayed in the following order: 100, 200, 400, 800, 1600, 3200. The aperture and shutter speed remain constant throughout these photos. Only the ISO is changing, so that you can clearly see its effect on a photo.

The ideal exposure is shown in the fourth photo which was taken at ISO 800.

Photo of a pink flower in low light - demonstrating ideal exposure and ISO
Photo of a pink flower in low light - demonstrating ideal exposure taken at ISO 200.
Photo of a pink flower in low light - demonstrating ideal exposure taken at ISO 400.
Photo of a pink flower in low light - demonstrating ideal exposure taken at ISO 800.
Photo of a pink flower in low light - demonstrating ideal exposure taken at ISO 1600.
Photo of a pink flower in low light - demonstrating ideal exposure taken at ISO 3200.

As you may have noticed by now, nothing affects the exposure in one single way. There are consequences to using different ISOs.

Section 3 – How Does ISO Affect the Quality of Photos?

As a general rule, the lower the number, the better the quality of the photo. By doubling the ISO, you’re effectively doubling the exposure taken by the camera and, in turn, doubling the digital noise. This noise reduces the detail of a photo by making the image appear grainy and uneven.

Lower number = Lower sensitivity = Finer quality photos

To best demonstrate how ISO affects the quality of the image, I’ve taken another series of photos and displayed them below. For the purpose of this experiment, I have changed the shutter speed and aperture of each photo rather than simply changing the ISO.

This is to give even exposure making it easier to compare the difference in quality.

The photos have ISO in the following order: 100, 200, 400, 800, 1600, 3200.

A close up image of ran ban sunglasses
A close up image of ran ban sunglasses taken with 200 ISO
A close up image of ran ban sunglasses taken with ISO 400
A close up image of ran ban sunglasses taken with 800 ISO
A close up image of ran ban sunglasses taken with 1600 ISO
A close up image of ran ban sunglasses taken with 3200 ISO

As you can see, the higher the number, the stronger the unsightly noise becomes. There is noise reduction software that can help to correct this but you’ll find that this only really “smooths out” the noise.

This can result in an airbrushed effect on faces. It reduces the detail in a photo as shown below (cropped to 1% of actual image).

Clearly it has its uses but in moderation and with consideration as to what you’re using the photo for.

diptych close up portrait of a man with glasses - ISO Noise reduction comparison 600

Cameras with larger sensors handle noise better as they allow more light into the photo. As technology improves, however, the difference between sensor size and noise is reducing. What used to be a big problem is much less of a one now in high quality cameras.

All cameras are different so I suggest taking yours into low light conditions in order to discover the maximum ISO that you’d be comfortable using.

Enhancing the exposure in post production has the same effect as increasing your ISO. Make sure you get the exposure right in the camera first time round in order to avoid this.

Now that you know what the ISO does, let’s take a look at situations where you might use it.

Section 4 – Which ISO and When?

1. ISO 100-200: Your photos will have the most detail and the best quality. This is great for shooting in daylight because there is no need to boost the ISO any higher. Shooting at 1600 in bright conditions would be a waste as this will result in the presence of easily avoidable grain.

a rusty barbecue taken with ISO 100-200

2. ISO 200-400: For slightly darker conditions, such as in the shade or indoors where it is brightly lit.

portrait of a man with woman in background

3. ISO 400-800: I like to use this range when shooting with a flash indoors since it helps to produce a more even exposure with a detailed background.

ISO 400-800 potrait of a man indoors with a detailed background.

4. ISO 800-1600: Event photographers frequently have no choice but to use this range as live events often happen in low light conditions where you’re not allowed to use flash.

a girl onstage with acoustic guitar in low light

5. ISO 1600-3200: Again, event photographers will use this range for live gigs, but you can also use it in extreme low light conditions where using a tripod is not an option. ISO 3200 is the highest I tend to push my camera to because I’m not a fan of digital noise (grain).

fireworks at night about a concert performance

6. ISO 3200+: This range is reserved for extra low light conditions and artistic effect as, with most cameras, it’s impossible to avoid a grainy result in this range.

a blurry artistic night portrait of a dark haired woman

Your Free Quick-Start Photography Cheatsheet

In order to simplify the process of learning photography, I’ve created a free download called The Quick Start Photography Cheatsheet and you can download it below.

Here’s what you’ll get:

  • A downloadable cheatsheet to carry with you as you shoot
  • Detailed summaries of each section of this post
  • External links to relevant articles and blog posts
  • At-A-Glace Images that will explain how each exposure works
  • And much, much more…

Your Free Quick-Start

Photography Cheatsheet

This downloadable cheatsheet gives you detailed summaries of every section of this post, as well as links to relevant articles, and at-a-glace images that will explain how exposure works.

A note from Josh, ExpertPhotography's Photographer-In-Chief:

Thank you for reading...

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Hey I'm Josh, I'm Photographer in Chief here at ExpertPhotography, and I'm in charge of making sure that we provide you with the best content from the most knowledgeable photographers in the world. Enjoy the site :)

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