The Easy Guide To Understanding ISO


Recently I was asked to write a tutorial explaining ISO. This is it. I try not to get too technical, but this is one digital photographic concept that is immersed 100% in techo geekdom, so bear with me.

What does ISO stand for?

We have the ISO to thank for ISO. The ISO is the acronym of the International Organisation of Standards. (Yes, I know it should be IOS, but it isn’t, ok?) The ISO settled on “ISO” for the photography world as an abbreviated word as it stems from the Greek word isos, which means, in English, “equal” (eg isometric). Cute, huh?

Some say ISO is pronounced eye so and some, including me, say it as eye ess oh. Whatever. The term ISO replaced the term ASA, which stood for the American Standards Association, or maybe the Australian Scripture Assembly. Who really knows?

What is ISO?

ISO refers to the sensitivity of film to light according to speed of exposure. The higher the ISO rating, the more sensitive the film is.

The equivalent concept applies to digital cameras, where it is all about the sensitivity of the camera’s sensor to light.

Just stop now and think about this. It is a big difference between film and digital….

Choice of film is very important to a film photographer. The choice is 99% about ISO and 1% about colour or B&W. This is because using a film with an ISO 100 rating means putting into the camera a roll which will produce noiseless (non-grainy) photographs able to be shot in nice clear light conditions at high speed, whereas using a film with an ISO 1600 rating is preferable for low light conditions as the film is way more sensitive to light.

Once you’ve made that choice of ISO rating when shooting with a roll of film, you are stuck with it for the whole roll, with 2 exceptions. First, now you know why some cameras have interchangeable “film backs”, whereby an unfinished film roll at, say ISO 100, can be swapped for another unfinished film roll at a different ISO rating. Second, and this is something I do all the time when shooting with film, you can “trick” the camera into thinking it is shooting with, say, ISO 400 instead of ISO 100 by manually changing the ISO setting. (I did try upping ISO from 100 to 1600 on a rafting trip once, but the camera was on to me and produced crap. More recently I upped an ISO 25 roll to ISO 100 as I was shooting inside.)

Digital photography, however, affords us the luxury of calibrating the sensitivity of the sensor with each shot with equivalent ISO settings to film ISO ratings. Thus, sRGB image files created by a digital camera will have a similar lightness to what is achieved with a film camera using the same ISO rating, aperture, and speed. Setting the ISO on a digital camera is actually adjusting the sensitivity of the sensor. It is a pro-active process, as distinct from film, which is a re-active process.

What ISO setting should I use?

As a general proposition, the lower the ISO setting, the better. For example, and ignoring aperture for the moment, consider these two options for taking the same evenly exposed hand-held shot (approximately) of a non-moving object:

  • Speed of 1/1000th of a second, ISO of 1000
  • Speed of 1/100th of a second, ISO of 100

Which is preferable? ISO 100. There are several reasons. First, a speed of 1/100th of a second hand-held will rarely give you blurred images. It is fast enough to capture the object with a well-focused and sharp image. Second, the higher the ISO, the greater the noise from stuck pixels and grain.

Why is this? It’s because as you increase the ISO setting and make the sensor more sensitive to light signal, you are also making the sensor more sensitive to noise coming from everything in the frame, not just your light source or subject.

The higher the light signal to noise ratio (the S/N ratio), the better the image quality. Digital cameras are calibrated to achieve the greatest S/N ratio using the lowest available ISO setting. The smaller the sensor, the greater the problem with noise at high ISO. (Attention all geekheads – this is a general proposition, ok!)

Noise in the form of stuck pixels and grain can be dealt with by noise reduction applications, either in-camera or in post-production. However, it’s a bit like hitting a nail with a sledgehammer as the apps also remove detail from your image. And, in the case of noise-reduction in-camera, the application takes just as long as your shutter speed. Therefore, if you take a 120 second exposure with Noise Reduction “On”, the camera will then take another 120 seconds to do the noise reduction before you get to see the image on the camera’s rear LCD. As I said somewhere else, I’m a patient man but not that patient! (Technology is getting better, however, with in-camera noise reduction.)

So….. the idea is to use as low an ISO as possible. A suggestion would be to not let your camera choose an ISO for any shot (Auto ISO) but set it to ISO 100 (or your camera’s minimum ISO) as your default, and only increasing it when there is a need to compensate for poor light and you don’t want to stop down your aperture any more or shutter speed.

For example, my camera (EOS 5D Mark II) allows me to set the ISO to 50 as a Custom Function. This is my default setting. There are times however, when I want to take a shot without stopping down further than f8 (because I want a wide DOF) and without slowing down more than 1/30th second (particularly if I am shooting hand-held), but the light won’t let me get a good exposure. Changing the ISO is always my third and last option. Upping the ISO to 100 instantly means I can shoot faster, and thus avoid a blurred image, and I will routinely go up to 640 without hesitation. And to tell you the truth, there is bugger all difference that I can see between ISO 50 and 100. (Except for long exposures, where ISO 100 is probably preferable. But that’s another story.)

As a general proposition, the smaller the sensor, the bigger your problem with ISO noise and grain is going to be, sometimes even kicking in at ISO 400 (and rendering images unusable at 800 or 1000). This can be important when choosing a digital camera to buy.

Buying a camera and ISO

If you are in that position, check the ISO range of the potential purchase. The standard for a lot of digital cameras, even now, seems to be Auto, 100, 200, 400, 800, 1600, 3200, which, for example is the range applying to the Sony Alpha DSLR-A390. That’s not exactly a great range for a brand new model, to be honest. Apart from severely limiting your shooting options, I would want to make sure the camera can actually produce usable images at ISO 1600 and especially ISO 3200. The Sony Alpha DSLR-A390 has only one more ISO setting than the much older Olympus E-520 (3200) so you have to wonder where Sony’s ISO heads are at. I mean, look at the Olympus E-620, which was released a year ago. It’s ISO range is 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200. Now you’re talkin’!

So, if you are looking at which camera to buy, take the time to check out its ISO range as they can vary grealty. Also check out the camera yourself shooting at high ISO or take the time to research on-line for an in-depth review which covers ISO performance.

Still on the Nikon D700, it’s default ISO settings are in the range 200 to 6400, with a low boost to 100 and a high boost to 25600 – the same as the Nikon D3. The Nikon D300 has a default range of 200 – 3200, with a low boost to 100 and a high boost to 6400. Canon is much the same. The EOS 5D Mark II has a default range of 100 to 6400, expandable to 50 and 25600.

The reason why the “extreme” ISO settings are available separately, on any digital camera, is to do with calibration. The default range is the calibrated to international standard range, whereas the expandable settings are not.

I could go on and on about ISO, but I reckon I’ve said enough to get you started.



  1. Tutorial first published 30 June 2010 on
  2. Republished 11 December 2010 on Peter Hill’s Weblog.
This entry was posted in iso, photography, photography tutorial, tutorial and tagged , , , , . Bookmark the permalink.

18 Responses to The Easy Guide To Understanding ISO

  1. Kevin Bergen says:

    Terrific to see your tutorials here! I wish you ALL the best in this endeavor!

  2. Lois Bryan says:

    Hi Peter!!!!! Best of luck with your new endeavor!! Rest assured your new address will be added to my bookmarks!!!!

  3. Lorraine says:

    Good tutorial Peter. If I can ask…. I was hoping to get a 5d2 next year and the info I look at in regards to this model , shows ISO at 100. I always thought it went to 50! You’ve mentioned it here also. Are you able to shed light on this for me?

    • Peter Hill says:

      Hi Lorraine. ISO 50 on the 5D Mark II is a Custom Setting (accessible from the back LCD menu). When you select it, it shows on the top LCD menu as L when you scroll down past 100. The other 2 expandable ISO options are 12500 (H1) and 25000 (H2).

  4. newfiedish says:

    I’m going to be learning lots from you! I am all about learning new things and you like teaching. I am and will be your #1 student.

    Thank you,
    Gail Bridger from Red Bubble!

  5. Jan Timmons says:

    Excellent. Thank you, Peter.

  6. Don says:

    An Excellent Tutorial Peter.
    Keep them coming mate.
    I’ve put you in my Blog Watch List.
    All the best,

  7. Very professional look to this Peter. This is going to be a really popular blog and I just wanted to wish you every success.
    All my best

  8. Linda Bianic says:

    I am with you all the way Peter,, I am confident you will have great success with this blog!! Great job!!

  9. Patrick Monnier says:

    Great presentation. I bookmark your site in my fav. Good luck Peter, and all my wishes for a great success.
    And a merry Christmas, and very happy new year, full of joy, for you and your beloved ones.

  10. susankimball says:

    I bookmarked this page too and hope to see your wonderful work and tutorials for a long time to come.

  11. Oh Peter. You are a saviour!

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