What is ISO?

Photography Basics

ISO is actually a common short name for the International Organisation for Standardization.

The ISO setting on your camera is something that has carried over from film. Remember back in the ‘old days’ when you used to go and buy your rolls of film and you would buy film rated at 100, 200 or 400, maybe even 800 or 1600? Well that number refers to the film’s sensitivity to light. The higher the number, the more sensitive to light the film is. The ISO bit is from the standards for film sensitivity, and the number refers to it’s rating.

So what does sensitivity mean?

Well a low sensitivity means that the film has to be exposed to light for a longer period of time than a film with a high sensitivity in order to properly expose the image. With a lower sensitivity you also get a better quality image too which is why you should always try and use the lowest sensitivity you can get away with. Let’s not get ahead of ourselves though, a little more explanation is required.

You might remember buying film for a sunny holiday and the shop assistant would recommend using a film rated at 100 or 200. If, on the other hand, you were going to be taking pictures indoors, then you might be recommended a higher sensitivity like 400 or maybe 800. If you used ISO100 film and decided to take some pictures indoors, chances are you would need to use the flash, or your pictures would come out quite dark. This is because the film’s sensitivity is so low that the shutter would need to be open for a long time to let enough light in. Your camera may not have had the features to allow it to keep the shutter open for long enough, which is why you ended up with dark pictures.

This was one of the problems with film. Once you’d loaded it into your camera, you were pretty much stuck with that film sensitivity for 24 or 36 shots.

Bring on digital cameras and you can now change the ISO setting for each shot you take. That is one of the big advantages of digital photography.

So why do you only get choices like 100, 200, 400, 800, 1600 and maybe 3200 when it’s digital, surely you could set 154 or 958 if you wanted it? It’s only electrical currents and circuits after all, not a piece of film. Well, in theory you could choose any setting you wanted, but imagine how tricky that would be. There are three settings which combine to give you the exposure, these are Aperture, Shutter Speed and ISO. Each one can be changed individually to allow you to set then to what you think will give you the perfect exposure, or you can let the camera set them for you to what it thinks is the perfect exposure for the conditions it can detect. Already with three different options, each having several settings themselves, the combinations are numerous, so keeping ISO to set values, which people will understand makes it a little less confusing.

Image Quality

Now, I mentioned quality too, and that better quality images are achieved with a lower ISO number. If, again, you go back to film days you may remember the sort of grainy effect some images had. Well this grain effect is what is introduced with a higher sensitivity film. Digital has it’s own grain effect with higher sensitivity and is known as Noise. Digital noise can be seen a sort of speckley effect in areas of similar colour, like skies or dark shadow areas. It is a subject of much discussion and the camera is often judged on the amount of noise it produces at these higher sensitivities. This is why you should always try and keep your ISO set to the lowest number, and use aperture and shutter speed to get the right exposure. If you can’t do that with aperture or shutter speed, move up to the next ISO setting and try again. One of the big selling points about digital cameras is how they handle the digital noise at higher ISO settings. The top pro level cameras from Nikon or Canon will have better control over noise than the cheaper models and this allows the pros (or anyone that can afford a Nikon D3 or Canon 1Ds for example) to get away with using a higher ISO and still getting good enough quality for print.

Why is a high ISO setting needed?

Well for indoor work, where flash isn’t allowed, like at concerts, and the light levels are fairly low. Or you can use it deliberately to get the grainy gritty feel to the image (although I would prefer to add this later on the computer).

It’s well worth experimenting with ISO settings so you can see just how your camera performs at the various levels. Once you have got to grips with how changing Aperture, Shutter Speed and ISO effect your image, you’ve pretty much got all the technical fundamentals nailed. Almost everything else in photography is based on these three things, so getting this right is important.

Over at the Digital Photography School Blog there is a nice post on how to choose the right ISO setting which is worth checking out. I’ve also found a decent ISO explanation for beginners over at Photography Talk. Both are worth a read.

My rule of thumb is to keep the setting as low as possible until you absolutely need to increase it in order to get the shutter speed you need. Just remember to turn it back down low again when conditions get better.

63 thoughts on “What is ISO?

  1. It depends what type of digital camera you have. Some of the smaller point and shoot cameras may not have an ISO setting, others may have a limited choice, but you should get some options in the menus if it’s not on a dial on the camera body.

  2. For the question at the top, yes you can with most cameras if they are not dslr’s, go into the settings and look for ISO or light sensitivity.

    This site was a good help for my photography work, thanks

  3. Dear its very informative and understandable, if you add some pictures as an example this would give a very sure understanding. This article is quite good and i have learnt what i want to..

  4. Thank you Garry, these shared information is very usefull for DSLR beginners like me, cheers.

  5. very usefull, really, u saved my money brother, well explained, even a man with a low english knowledge can understand 🙂

  6. Giving you million of thanks for such a simple (best ever) explanation on ISO. This holds good for articles on Shutter speed & Aperture which I followed from this page. Heartfelt thanks from a budding photographer !!!

  7. Just wanted to thank you for this info and a few other articles I’ve read on your site. I just bought my first SLR (Eos 550d) and I’ve been quite confused about the different settings. I think I now understand the basics after reading your posts… Cheers!

  8. How can I use ISO in my new camera Canon EOS 1100D. I m a new user.Inform me plz

    1. I’m afraid I don’t the details of each and every camera model out there but there should be a button on the camera or a setting in your menu system to alter the ISO. It should be clear in your manual.

  9. fantastic info for beginers like me. now i can understand what iso stands for and the importance for a good result. thank you

  10. Could you tell me what is the modus operandi of ISO??
    I mean,what changes inside the camera when i change the iso from say 100 to 200??

  11. This was a fantastic article. Gave the inputs re. ISO setting. I was always under impression that the Higher ISO rating is better. I am sure this will help me a lot.

  12. Hi guys
    I have just purchase 200W umbrella and two soft boxes. They produce too much light when I take photographs. How should I set my camera (550D) to regulate the light and produce good pictures?

    1. You’ve got three choices if you are getting too much light on your subject. 1, move the lights further away. 2, try and decrease the power output of the lights. 3, reduce the size of the aperture or make the shutter speed quicker. Any one of these will help you reduce effective amount of light.

  13. As a beginner, it’s very valuble information. Please update this site with new technologies,

    Thanking You,
    Rony George

  14. this is a great website,it tells you what the options are and it helps you understand better than other sites

  15. Thank you for giving information to begineer like me. Now it will be going to easy to handle my dslr nikon D5100

  16. I’m buying a camera and I was wondering about the ISO what is it now I know thanks to you that was really helpful, great explanation. By the way what do you think about the new Sony Alpha SLT-A58 for a beginner

  17. a novice like me with a fairly good camera and dont know what is what, is the one that your explanation helps a lot.

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