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  • Writer's pictureGary Holpin

A beginners' guide to ISO in photography

If like me you're old enough to have used a film camera, you might vaguely remember the term ISO from the markings on the film (usually ISO 100 or 400). If you've got a modern digital camera you've probably seen ISO on one of the buttons, but do you know what it is and how it should be used in your photography? If not, then this beginners' guide should help.

What is ISO?

ISO stands for 'International Standards Organisation', however, knowing what it means is completely unhelpful in understanding what it is! In the old days of film cameras, ISO was used to specify how chemically sensitive the film was to light, with a low number (e.g. 100) meaning low sensitivity to light, and a high number (e.g. 400) meaning a higher sensitivity to light. So, if you were taking photos on a bright sunny day with plenty of light, then you would use an ISO 100 film, and if you were taking photos later in the evening when light was lower, you might switch to an ISO 400 film.

In these days of digital cameras, we no longer have to worry about only having 24 exposures and changing films when the light changes (if you're too young to remember then lucky you!) but the term ISO has been retained.

On digital cameras, ISO still refers to sensitivity to light, but now it is the sensitivity of the camera sensor rather than a chemical roll of film. The other major change is that instead of being limited to a single ISO sensitivity per roll of film, we can now change the ISO for every picture, and depending on your camera. can choose between an ISO of 50 to up to 32,000, 500,000 or even more!

What does changing ISO do?

On most cameras, common ISO values go something like, 100 (low ISO), 200, 400, 800, 1600, 3200, 6400 (higher ISO) etc. Each of these values is essentially doubling the amount of light that the sensor will capture (assuming other settings such as Aperture and Shutter Speed stay the same). So, setting your camera to ISO 200 will capture twice the amount of light as ISO 100.

Is there a down side to increasing ISO?

Quite, simply, yes! Although in theory you can keep increasing ISO so that you can take photos in less and less light, increasing ISO eventually has an impact on the quality of the photos, through something called 'electronic noise'. As you increase ISO, this increases the sensitivity of your sensor to capture more light (so you can take photos in darker conditions) but in doing so, the sensor also produces more and more random electronic noise. This can be seen in photos taken with high ISO by an unsightly speckled pattern, especially in uniform areas such as the sky. The noise gets more and more apparent as the ISO increases, and eventually can completely ruin the visual look of the photo. I was once on an evening tour of the beavers which now live on the River Otter in Devon. As it was dusk and they were moving about in the deep shadow of the far river bank, although I could hear them, I could not see them with my eyes. However, using my camera on ISO 64,000 (very high ISO!) I managed to take a photo of them where they could be clearly seen as if it was daylight! However, the photo was so affected by the dappled appearance of electronic noise, caused by the high ISO, that it was totally unusable as an image. Below are two photos taken on my Sony A7RIV at ISO 100 and ISO 8,000 (my camera is very good at not producing much noise so I had to use the extremes to prove the point!). Hopefully you will see the difference between the 'clean' ISO 100 image and the 'noisy' (grainy) ISO 8,000 image; it's particularly apparent in the brick in the centre of the image.

An example of the effects of high ISO in photography

Why and how to change ISO

If you only ever shoot on automatic modes on your camera, then you need not ever worry about ISO, since your camera will change it for you when light levels drop. However, if you aspire to moving off auto and taking full control of the power of your camera, then knowing how and why to change the ISO is important. If you've not yet taken this leap yet, have a look at my blog explaining how to get off auto mode. On some cameras, you are able to change the ISO on Aperture Priority or Shutter Priority (semi automatic) modes, but the real creativity starts when you take full control on Manual mode. ISO then becomes one of the trio of settings for controlling the light entering your camera, the others being the Aperture (size of the hole in the lens shutter) and Shutter Speed (how long the shutter is held open for). In some styles of photography such as street photography, the dappled appearance caused by high ISO can actually work well and add to the creative appeal of a photo. However, in landscape photography, the dappled appearance caused by high ISO is very undesirable, and as a result I always keep my ISO as low as possible. During the daytime when there is plenty of light, this means that my ISO stays on 100 and I can forget about it, instead concentrating on Aperture and Shutter speed to define the amount of light I want the sensor to capture. Even in low light, I usually tend to use a longer shutter speed rather than increasing the ISO, and this generally works well as there are usually no moving subjects (people, pets etc) in the photo and so no concerns about motion blur from longer shutter speeds. So when might I increase the ISO? below are a couple of the main examples of when controlling ISO becomes critical:

Hand held in low light.

If you're hand holding the camera, this means that you need to keep the shutter speed relatively fast in order to prevent motion blue caused by camera movement, or to make sure that anything moving through the frame (such as people) are not blurred. In low light conditions (indoors or at dusk for examples), especially if you want to use a relatively small aperture to create a deep depth of field (see my beginners' guide to depth of field) then you may be forced to increase the ISO in order to capture sufficient light to achieve a well exposed image.

Moving subjects in low light

If there are moving subjects in the photo (people or pets for example) then it's important to keep shutter speeds relatively fast (greater than around 1/220s) in order that their movement is not blurred. When light levels are low, achieving this often requires increasing the ISO.


Obviously there isn't much light around when you're taking photos of the night sky, so lack of light becomes a real issue! Although astrophotography is usually done with the aperture wide open to let in as much light as possible, this is not sufficient to produce a well exposed image. One option would be to use a long exposure to let in more light, but since the Earth is rotating, using a long exposure would mean that you would end up with star trails, not points of star light. The only way to therefore capture point stars is to use a high ISO (usually around ISO 6.400) and accept that the sky will contain some dappling from electronic noise.

Final words

ISO is one of the three ways to control light in photography, and used correctly it can help to create well exposed images in the most difficult of lighting conditions. However, it's important to realise that high ISO can also produce unsightly noise which can ruin photos. As a general rule, you should therefore always use high ISO as a last resort when light is low; opening aperture or increasing shutter speed are preferable, but may not always be appropriate to the situation.

Want a learn more about photography?

If you're a beginner photographer who is local to East Devon and want to learn the art of landscape photography, check out my range of local photography courses. Alternatively why not make a weekend break of it and attend one of my residential photography workshops - either my beginners landscape photography masterclass for those starting out in photography, or my intermediate photography masterclass for those who already understand the basics and want more practical help progressing their landscape photography.

A student learning photography on a Devon beach, with Devon Photographer Gary Holpin Photography

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