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Theory n Technique

ISO Sensitivity: What’s it all about?


You hear a lot about ISO, ISO speed, ISO sensitivity, in both film and digital photography…you see “ISO” numbers on film boxes or canisters, you see ISO controls feature very prominently on all digital cameras…but do you know what it means? What it really means? Aperture and shutter speed, the other two parts of the exposure triangle, are pretty easy to comprehend once you know what they do…but ISO? I found it a bit confusing when I didn’t know what “ISO sensitivity” really meant, back when I was starting out…and all it took was me taking the time to read a couple of pages to understand it better…and that’s the couple of pages I’m trying to write here for you! Understanding ISO  might not seem as important as the other stuff I have written about earlier, but of course it’s something you should know if you’re going to be a photographer! So let’s get into it

ISO speed or ISO sensitivity basically refers to how sensitive your camera is to light. This can be the sensitivity of your camera’s digital image sensor, or the film inside your camera. Pretty simple, really. A low/slow ISO speed (smaller number) means the medium is less sensitive to available light, so naturally a higher/faster ISO speed means the medium is more sensitive to light – and yes, a less sensitive medium requires MORE light to expose the shot than a more sensitive medium would

Film ISO Vs. Digital ISO

A film camera’s ISO sensitivity cannot be changed, as it is dependent on the sensitivity of the film you’re using – if you pop in a roll of ISO 100 film, that’s the sensitivity you’re stuck with until you change the film roll!

A digital camera, however, has an option to increase the sensitivity of the sensor. This is why you see the ISO control on digital cameras, the control that sets values ranging from values like ISO 100 to 3200 etc.  However, increasing sensitivity comes at a cost – usually degrading image quality, more on this later – but on digital cameras, it is possible to adjust the ISO speed, which is a huge benefit. More sensitivity means less light is necessary to expose the image, meaning smaller apertures, and more importantly, faster shutter speeds

Base ISO

Digital cameras have what is known as a ‘base ISO’ or ‘native ISO’. This is how sensitive the camera’s sensor really is, without any amplifying, without increasing the sensitivity. This generally means that images taken at this ISO speed are of the highest quality that the camera can produce – increase the sensitivity from here and the quality will begin to degrade (I’ve designated a separate space below to talk about this, so I will avoid talking about these issues yet!)

ISO Increments

ISO sensitivity increases in ‘stops’ – a full stop increment means the sensitivity doubles, just like a full stop increase of aperture or shutter speed means double the exposure – I explained this when I talked about Exposure some time back. Common ISO speeds are ISO 100, 200, 400, 800, 1600, and so on – the number doubles and the sensitivity doubles – for example, ISO 200 is twice as sensitive as ISO 100. Easy

There are advantages and disadvantages of both high and low ISO sensitivities, of course – this is why the ISO speed is an important part of exposure, this is why it is a part of the aperture/shutter/ISO exposure triangle…and this is why I’m writing about it today! Let’s have a look

Pros of high ISO:

  • In low light, extra sensitivity means the shutter speed can be kept fast – this reduces the chance of getting blurry images due to camera shake. That’s the only real positive for high ISO I’m afraid

Cons of high ISO:

  • Quality of images degrade as the ISO sensitivity is amplified
  • The biggest factor that degrades image quality at high sensitivities is the digital noise that forms on the image – chrominance, or color noise, which appears as colored specks on the image, as well as luminance noise which looks more like grain
  • The other factor that lowers the image quality is the noise reduction that is applied to high ISO images. This reduces noise, but along with the noise goes some of the detail in the images – and you end up with a fairly clean picture that has a soft texture to it. It’s a sort of compromise, losing noise as well as detail
  • High ISO film tends to look grainy

 Pros of low ISO:

  • Images taken at base ISO, or low ISO speeds are of the highest quality your camera can produce
  • Without amplifying the sensor’s sensitivity, there is hardly any digital noise found on the images
  • As there’s hardly any noise, it is not necessary to apply noise reduction to these images – therefore, little to no detail is lost from the original image
  • In film, slow ISO speeds means finer grain, resulting in a smoother image

Cons of low ISO:

  • Low sensitivity means less the sensor needs more light to expose properly – meaning slower shutter speeds in low light, which could result in blurry images due to camera shake. That’s the only con

Look at the following four examples that show the difference that ISO can make, and how it looks. Each shot is heavily cropped to help you understand exactly what I mean. Make sure to click on each shot to view in full size!

The first one was shot at ISO 100: see how much detail is visible, even when cropped in this close,  and how there’s no noise or distortion. This is the highest quality this particular camera can produce

The second one was shot at ISO 3200: now see the bit of noise starting to show, messing with the finer details in the shot. This probably would not be this visible when viewed at full size (remember this is very heavily cropped) but it’ll definitely show in large prints

The third, shot at ISO 12800: ok, here’s a bit of an extreme example – shot at 12800, with no noise reduction applied! See the horrible specks of red and green? That’s the color noise I was talking about. Completely ruins the image and distorts so much detail, don’t you think? That’s how noise, the biggest negative of high ISO sensitivity, can ruin your shot!

And the last one, also shot at ISO 12800: another extreme example, this is the same shot as above, but with a heavy dose of noise reduction. Notice how all the noise is gone, but along with it, a whole load of detail has been wiped off too. See the picture of the face there, the text – the soft, blurry look it has got – this is due to lack of detail after noise reduction – the other major negative of high ISO sensitivity. If you’re not seeing the soft appearance, compare it immediately with the first image (click the LEFT arrow) – and notice the face, as well as the wood of the bookcase just behind it. See now?

When to use a low ISO speed?

Always. If possible. Keep your ISO as low as you can, especially in digital photography. In film, the grain found on high speed black and white film can be desirable in some situations, but the noise found on high ISO digital images (and the lack of detail in some cases) is always horrible. So only increase ISO when the light is insufficient and you can’t use a tripod

When to use a high ISO speed?

When you’re shooting handheld in low light and using a tripod is not possible. In this case, as I always say, I’d take a noisy but sharp shot over a blurry but noise-free shot any day. Yeah, so bump up the ISO until the shutter speed is just fast enough for you to safely take the shot without a tripod

So…it’s pretty simple. If you’re shooting digital, keep your ISO as low as possible – in fact, keep it at your base ISO as often as you can. Increase the ISO only if you need to i.e. only if the necessary shutter speed for the available light is too slow, and if you can’t use a tripod. I can’t think of any other reason I’d want to increase the ISO of a digital image

In film, again I’d try to keep the ISO low. That’s just how I like it. However, like I said, the film grain that is found on high speed films sometimes looks good, and it’s gritty, almost harsh look can sometimes suit the image – if you feel this to be the case, it’s a good idea to go with a faster film. Also, remember that with film, once you load a roll, that’s the only speed you have until you finish it – so unless you’re sure you’re shooting in daylight, you might want to buy a faster film to be prepared for all lighting conditions – and if you feel like you’d like the grain, definitely go for a faster film. I prefer cleaner shots even with film so I generally go for ISO 100, and I rarely use my film cameras for night/low light work


And that’s ISO! Pretty simple really, but I know many people who don’t really understand even the basics that I just explained above. Knowing to set your ISO wouldn’t really ‘make or break’ a shot as they say – but it’s still a very useful thing to know about as a photographer, and can definitely help you out in tough situations…so just experiment with different settings, practice, and you’ll be able to set the best sensitivity for your shots every time. Comments are always welcome here, as you know! Leave all your thoughts, questions, or anything else, in the comment box below, and make my day! Until next time

By Heshan Jayakody
All content in this post is my own

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Discussion

20 Responses to “ISO Sensitivity: What’s it all about?”

  1. yet another very useful post – ISO drastically affects a photographer’s results, they’d best learn to use it to full potential!

    Posted by Chris | October 15, 2012, 15:11
  2. very well explained. thanks for sharing this info. it’s extremely useful to know how to control your high ISO settings

    Posted by Bruce | March 28, 2013, 08:38
    • Thanks Bruce! Yeah, ISO control is something that you really need to understand. It’s really easy to grasp, though, so it’s no big deal, but you still need to read up a short post like this one to get it, right?!

      Posted by pixelogist | March 28, 2013, 20:29
  3. superb post! how far would you push ISO for low light conditions? a limit of sorts

    Posted by Jason | March 31, 2013, 19:28
    • Thanks Jason. To answer your question, well…it really depends on the camera i’m using. If it’s a new, mid-to-high-end DSLR, which are known to have great high ISO performance, i wouldn’t hesitate to go as high as 12,800, as they really produce very clean images even at such high settings. If it’s a decent compact, i’d avoid going past 1600. But in any case, regardless of the camera i’m using, i’d increase ISO as much as the situation calls for. If i’m shooting in really dark conditions, and i’m shooting handheld, and i need to go up to ISO 6400 to get a sharp shot (3200 is still giving me blur), i’d even set my compact to 6400 (if it can) to get the sharp, steady shot. Like I always say, a clean but blurry shot is pointless…but if its sharp, even with a lot of noise, it’s going to be more useable

      Posted by pixelogist | March 31, 2013, 22:42
  4. Yes, ISO is so important to know – how to balance it. Push it too high than necessary and you ruin quality, push it too low and you make shutter too slow and also ruin quality in a way! It’s one of the more important aspects of exposure.

    Posted by Kevin | June 30, 2013, 07:01
    • Exactly. It’s a fine balance, and getting it just right is a huge part of how good your image quality is. Sure, just because you use the right ISO speed doesn’t mean you’re automatically going to get beautiful pictures, but the quality will be the best this way

      Posted by pixelogist | June 30, 2013, 07:17

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