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Do you want to understand your camera and take great photos today?

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ISO is one of three factors which determine the exposure of a photo. The other two are aperture and shutter speed. But what is ISO exactly?

Read this post to gain a more in-depth knowledge of what ISO is, how to use it and how to start taking more professional photos.

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

Section 1 – What 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 first.

Where Did ISO Come From?

Ever since the photographic emulsion was developed, different countries tried to enforce standards. This is so they knew what photographers could do with the film. These classifications are now known as film speed.

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 – These two worked on emulsion sensitivity since 1890. It was the standard until 1928. Their system had an inverse system of classification and exposure. The higher the number, the lower the exposure the emulsion needed.
  • GOST – From 1928 the Soviet Union used this as their standard until 1951. They then replaced it with the GOST system, which was the Eastern version of ASA.
  • DIN – Deutsches Institut für Normung was introduced in 1934. They used degrees for classification. A 3° change would either double or halve the sensitivity.
  • ASA – American Standards Association started in 1943. A linear scale showed that 400 ASA is twice as fast as 200 ASA, and 100 ASA is half as fast as 200 ASA. They left these film classifications in 1988 when they adopted the ISO photography standard.
  • ISO – By combining DIN and ASA, they used an arithmetic and a logarithmic scale. Its arithmetic scale corresponds to the arithmetic ASA system. A doubling of the film sensitivity is represented by a doubling of the numerical film speed value.

A roll of Kodak professional film - ISO in photography

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.

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 list of different ISO photography settings and the conditions to use them in

What Does ISO Do?

The ISO of your photography 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 on our camera 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 exposure triangle. This triangle helps you to capture the perfect exposure.

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

A diagram showing the exposure triangle - iso, shutter speed and aperture

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.

A close up of a DSLR camera showing the ISO value - what is iso

Section 2 – How Does ISO Affect Exposure?

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 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 in camera, you’re effectively doubling the exposure taken, 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 show how camera ISO affects the quality of the image, I took another series of photos and displayed them below. For this experiment, I 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 taken with 100 ISO
A close up image of ran ban sunglasses taken with 200 ISO
A close up image of ran ban sunglasses taken with ISO 400 - what is ISO?
A close up image of ran ban sunglasses taken with ISO 800 - what is ISO?
A close up image of ran ban sunglasses taken with ISO 1600 - what is ISO?
A close up image of ran ban sunglasses taken with ISO 3200 - what is ISO?

As you can see, the higher the number, the stronger the noise becomes. There is noise reduction software that can help to correct this. But you’ll find that this only “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).

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. Take yours into low light conditions to discover the maximum ISO you should be 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.

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. It will result in the presence of easily avoidable grain.

An outdoor portrait of a female model 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.

A portrait of a couple kissing while sitting on a stairway, taken with ISO 200-400

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

A low light atmospheric portrait of a female model taken with ISO 400-800

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

A low angle shot of a performer onstage , taken with ISO 800-1600

5. ISO 1600-3200: Again, event photographers will use this range for live gigs. 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).

A night street photography shot of a red bus with artistic motion blur, taken with ISO 1600-3200

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

A grainy black and white street photography shot taken with ISO 3200+

And here’s a handy video you should watch before you go.

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

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Craig Hull

Craig is a photographer currently based in Budapest. His favourite photographic areas are street and documentary photography. Show him a darkroom and he'll be happy there for days. As long as there are music and snacks. Find him at craighullphotography.co.uk and Instagram/craighullphoto

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