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
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 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