Photography Terms Explained: ISO

Welcome to the second post of my ‘Photography Terms Explained’ series, where I try to explain some of the technical-sounding photography terms so that anyone can understand them – well, that’s the plan, anyway! By the way, the first one I did was all about demystifying focal length, which you may find useful too.

So what is ISO, then?

Basically, when we talk about ISO, we’re talking about just how sensitive to light your camera is. It really is as simple as that, and it’s easy to remember too: the higher the ISO number, the more sensitive your camera is.

So, if you dial in a low ISO number, such as 100 or 200, your camera will be less-sensitive to light than if you dial in a high ISO, such as 3200 or 6400.

So simple even this horse could understand it (shot at 100 ISO, by the way)

ISO and Image Quality

So far, so simple. However, it’s not really enough to just know that a higher number means more sensitivity, as we need to know what the trade-offs are when we make our cameras more sensitive to light, and why we would choose to do so in the first place, too.

The first thing to know is that when we raise the ISO, the image quality of our photos degrade somewhat – and they degrade more the higher the ISO we choose. This degradation appears as ‘digital noise’ – dirty pixels that we don’t want to see. However, before you run away and say ‘Well, I’m always going to shoot at a low ISO, then’, take into accout that modern DSLRs handle high ISOs incredibly well, with a lot of cameras being able to shoot virtually noise-free at such high ISOs as 1600 or 3200.

Taken at a high 3200 ISO - but image quality is fine! (Shot with a Canon 7D)

Also, the other really important thing to bear in mind is that noise is only ever really apparant when you’re viewing your photos at 100%, or are printing them out at prints of around A4 size or larger. For photos that you’ll be sharing on the web, or printing out at 6 x 4, then you just won’t see any noise anyway. So, my advice is to not worry about high ISO and image-quality: it’s always better to actually ‘get the shot’ than not taking the photo at all because you were scared about a little noise…

Which brings me on nicely to…

Why Do I Need To Think About ISO?

And the simple answer to this is because when we make our cameras more sensitive to light by raising the ISO, then we can start to take photos in lowlight without using a flash (click that link for a guide I did earlier on just how to do that), and we can also get faster shutter speeds, which in turn means sharper photos and action-freezing photography.


The logic for this is easy: if we’re in a lowlight situation, such as indoors, then we need to make our cameras more sensitive to the light around us. By dialing in a high ISO, such as ISO 3200, we’re telling our camera that it needs to up its sensitivity, because there isn’t much available light. If we don’t do this – if we just keep our ISO at 100 or 200 – then your photo may end up being completely dark, or, depending on what camera-mode you’re shooting in, your camera may decide to hold the shutter open for a whole second or even longer – resulting in a total blur of a photograph.

But if we have a high ISO, then your camera is nice and sensitive, so it can capture lowlight scenes, and it will also be able to capture these scenes using a fast shutter speed – resulting in sharp photos, even inside, without any flash.

Take, for example, the below photo I took of a band – the high ISO 3200 meant that, even in such dark surroundings, the resulting photo was well-exposed and sharp:

Shot at ISO 3200

By the way, I did a post of gig photography tips a while ago, which you may be interested in.

Takeaways and Tips

My personal advice is to really start to use ISO to your advantage – turn off the auto-ISO on your camera, and start to adjust it yourself. Auto-ISO is OK, but it can unnecesarily raise the ISO of a shot when you don’t need it to, and it can also not raise the ISO when you really need it too! So the best bet is to try it out for yourself – it really is simple.

Keep your ISO low when your outside on a nice sunny day – such as ISO 100 or 200 – for the best possible image quality, and when you’re inside or in realy dark conditions, up the ISO to 800, 1600 or 3200. Don’t worry about the image quality degrading, as you just won’t notice it if you view your images at web-size, or print  6″ x 7″ photos – it’s always better to take the photo rather than missing the moment forever.


  1. Cyrus /

    Great website! thanks for the pointers

  2. …and thanks for the kind comment!

  3. i’ve always had an idea about what you’ve been writing about already but by explaining them in a detailed way, or by relating it to your own personal experience, i learn things i’ve not known yet. you explain it in a way that it so easy to understand and it makes me learn easier. i love your website. hope you could post some more i would surely read them. thanks! i’m just a beginner and i have a 550D. your website is so informative! Mervin from the Philippines

  4. Thanks for your lovely comment, Mervin!

  5. Nitin /

    Being a newbie to photgraphy and my T2i I have been looking for something like your website which is so informative.Thanks a ton .This is on my fav list now .

  6. Thanks a lot, Nitin! :)

  7. Mayhul /

    I’ve been glued to this site since i got my 550d two days ago!!!!!!!!!

  8. Hi Mayhul, thanks for your kind comment! :)

  9. Yass Man /

    Thanks for the detailed explanation. especially for newbies like me..

  10. Jessica /

    I bought the T2i today, and have been researching the information regarding all of the technical terms and settings that I do not understand. I have to say, ALL of your information on this sight has been the most informative yet simplified explanations that I have found…Thank you soooooo much for this information. When I become a famous photographer I will give you a shout out! Thanks again!

  11. That’s so ice of you to say, thanks a lot Jessica! :)

  12. Hi

    Thanks for such a clear explanation on ISO settings


  13. No problem, Jez – thanks for commenting!

  14. Hai! Very useful to me as a beginner, thank you very much!

  15. Glad you found it useful, XinNing! Thanks for commenting.

  16. Balaji /

    hi thanks for making the terms simple in your website . I am a beginner and have a canon 600 D.

  17. No problem, thanks for your comment, and I hope you’re enjoying your 600D, it’s a fab camera.

  18. Thank you so much for this. I know it’s been said a lot in this comment stream but I really found it so much easier to understand! You teach it so well it’s easy to understand… I’m now going to read more of your blogs :)

  19. Thanks a lot, Ben, that’s really nice of you to say! :)

  20. Vicki /

    I have officially bookmarked your page. I have bought books on dslr and on the Canon Rebel t21 specifically, and had a hard time wrapping my mind around iso. You really made it that simple!!! I am new to photography, and feel like I need to start at a Kindergarten level, and so many other sites and books assume we all know what certain things are. Thank you for not making that assumption!

  21. Ah, thanks Vicki – lovely of you to say! :)

  22. It is one great website here for newbies like us! Thank you so much for useful pieces of advice! I had this problem coming across tons of websites with too many technical words, which oftentimes discouraged me from learning through their experience. Your website is absolutely great in terms of simplicity, yet, essential knowledge for beginners. I’ve just read a few articles here and already experimented at home:) It works!!! I only wished I had found your site before I went on my trip of a lifetime to Iceland and screwed so many great photo opportunities (of course, many still did happen). Keep up the great work!

  23. Thanks so much, Elena, that’s really lovely of you to say – kind words like yours really make me love doing this site :)

  24. Lona /

    I also have bookmarked your website. Great information for me since I’m new to this type of photography. I recently purchased a Canon T2i that included the usual kit lenses and also a Canon 100mm macro lens which I’m struggling with. You have alot of great advice the even I can understand. Thanks

  25. Thanks a lot Lona! :)

  26. really helpful site! please dont ever stop! 😀

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