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An introduction to ISO

August 1, 2017

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This week, Ian from Chatham asks about ISO  

 

I took a picture and it’s all grainy and covered in little coloured dots… is my camera dying?

No, probably not. If it looks like the photo above, it sounds like digital noise to me, most likely from your camera using a high ISO setting.

 

Err… yes… I see…
No, not ISEE… ISO! It’s all about how sensitive you camera is to light. A throwback to the days of film, ISO film ratings were a set of agreed international standards for how sensitive a given roll of film was to light based on its surface chemistry, so a photographer could reliably set his shutter speed and aperture confident it would give the desired exposure independent of batch number of manufacturer. Typical, everyday film ‘ISO’s were 100, 200, 400 with ISO 100 being less sensitive to light than ISO 200 or 400. With the move to digital, the idea of a light sensitivity scale for different films didn’t really fit but the camera makers kept the same scale for their electronic light sensitivity ‘electronic gain’ system as it helped photographers make the transition from analogue film to digital cameras.

 

…So what is an ISO setting then? Why isn’t it just 1, 2, 3 etc?
A typical ISO scale starts at 100 (although some start at 50 or 200) and goes something like this, although some cameras also allow you to set intermediate half or third stops as well.

 

 

The key thing to spot here is that each point on the above scale is double that of the last – ISO 200 is twice as sensitive to light as ISO 100, ISO 400 is twice as sensitive to light as ISO 200 and 4 times as sensitive as ISO 100 etc. How you set the ISO on a camera varies; some cameras have dedicated ISO buttons or dials whilst others let you set the ISO via the shooting MENU or Fn (Function) button. It’s also possible your camera has an AUTO ISO setting. Here the camera sets the ISO for you. This can be useful if your chosen shutter and aperture settings are critical and can’t be changed but you want the exposure to stay constant but use with caution – the camera often goes for a higher ISO setting than you might like, particularly if you forget to turn it off when you don’t really need it!


Err… why would I want to change how sensitive my camera is to light?
Adjusting the ISO means that as it gets darker, you can adjust the ISO to compensate for the loss of light without changing your shutter speed or lens aperture. Problem is this also increases the noise in your image…

 

Wot? It’s a photo, there’s no sound on it! You’re thinking of a video!
No, by noise I mean visual colour or contrast / brightness noise… the little coloured or grey speckles that shouldn’t be there in the photo you shot. It’s also known as Chrominance (colour) and luminance (contrast and brightness noise).

OK, but most of the JPEG images straight out of my camera don’t have little speckles on them… but some of them are all cartoony / plastic looking or blurry!
That’s because the camera’s built in software has applied ‘noise reduction’ or ‘NR’ routines to remove the speckles before generating the JPEG image... but often this is at the expense of detail. The camera tries to remove the dots by looking at the colour of the pixels around them – its smooths things out but doesn’t always get it quite right. Large areas with a single colour look fine but when it comes to fine details it has to guess and it’s often wrong. Basically you have to make a compromise between removing the noise and retaining some detail. The image below shows a close up of a high ISO image with noise reduction disabled, a medium setting and basically TOO much noise reduction.

 

 So what setting should I use?
You should use the lowest ISO setting that lets you achieve the exposure and look you want for your photo. Read that sentence carefully as many people just see the ‘lowest ISO setting’ and ignore the rest! Yes, low ISO settings will give you a cleaner, more detailed image but you still need the right exposure and the right shutter speed and aperture setting – don’t be afraid to set a higher a higher ISO value if you need to set a fast shutter speed or specific aperture value! If an image is blurry because you used too long a shutter speed and your subject moved, there’s nothing you can do about it but if your image is noisy, you can at least apply a little noise reduction in software afterwards. Depending on what software you are using to edit your images, this might be a simple apply noise reduction (or NR) tick box or slider through to adjusting the colour (chroma) and brightness (luma) noise sliders separately along with settings for things like detail or contrast. This is because it’s easier to remove colour noise with very little loss of detail whilst brightness / contrast noise needs to be carefully adjusted or you end up with that blurry / plastic effect. As a general rule of thumb, zoom in to an image to 100% size and slowly adjust the sliders one at a time from 0 to 100% to see how it affects the details and find a compromise that gives you a balance between noise and loss of detail.

 

 

Alternatively, if you are shooting using in camera JPEGs, your camera might have settings to allow you to control the amount of noise reduction applied, which can range from no noise reduction through low, medium and high. 


OK… so all I read from that paragraph was ‘use the lowest ISO’…
Err, no. Don’t get too hung up on the fact you had to increase your ISO and the image is a little noisier. Some people (we call these people ‘pixel peepers’) like to zoom into a photo on their computer monitor and get stressed about how grainy it looks without noise reduction being applied. However, think about how you intend to use the image once you’ve applied a little noise reduction… if you have a 20 megapixel image displayed on a full HD monitor, its actually been shrunk down to around 2 megapixels and at that size a lot of the noise just disappears! The same goes for uploading to a website or projector. Printing an image up to A4 and beyond is also surprisingly forgiving when it comes to hiding the noise. If you have a colour image that really is rather noisy but a nice composition, why not see how it looks in black and white? It doesn’t work for all images but sometimes a slightly ‘grainy’ black and white photo can be quite atmospheric (and make you nostalgic for the ‘good old days’ of high ISO black and white films). Finally, most modern cameras handle noise and noise reduction pretty well up to ISO 800 and some higher end cameras can create fantastic images at ISO 12,000 and beyond.

Right… now I hear you! Just set it to a high ISO and forget about it!
NO!! If you do that, you will always get noisy images / lose detail after applying noise reduction and overexpose your images. What’s more there’s another drawback to using really high ISOs - you lose some colour accuracy and the camera’s ability to record both very dark and very bright areas in the same image is also reduced (we call this ‘dynamic range’). Just think about ISO when you take an image – don’t leave it stuck at 100 all the time but don’t jump up to say 1600 unless you think you need to. Why not try testing out how you camera behaves as the ISO increases by taking several shots of the same scene at different ISOs to see what you camera can and can’t handle in terms of noise? As with most things in photography, knowing what ISO to use is all about practice and experience.

OK… is that it then?
Well, it’s actually quite important to understand how to juggle ISO, shutter speeds and apertures – we often refer to this balance between the three as the EXPOSURE TRIANGLE – but that’s a blog in itself! Maybe we’ll get to that one day.


DISCLAIMER

The opinions in this blog are solely those of the Photo Doctor and do not necessarily reflect those of Medway DSLR camera club or any member of the committee. In fact the Photo Doctor constantly argues about technical stuff with them. They normally give in and admit he's right in the end...

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