ISO - the base of the triangle
I have previously talked about shutter speed and aperture and how they relate to each other. The threads can be found here: Shutter Speed - Does it really matter? and here: Aperture, what is it and what does it do for us? It is important that these basic principles are understood as they are the foundation of photography.
In those threads I make passing references to ISO so let's have a look now at your cameras ISO settings and what they mean in practice...
It's actually very simple, but wasn’t always quite as clear cut. Simply put the ISO setting adjusts the cameras sensors sensitivity to light, or in the case of film the sensitivity of the emulsion used to light.
A brief look back to the good old days of film (and film is still going strong too) will help us understand how we've arrived at the system we use today. And if you ever decide to pick up a film camera it will also help there too!
When we bought film prior to digital cameras it was marked with two separate sensitivity ratings. On all film cameras and hand held exposure meters you would set the sensitivity value to match that of the film. Depending on where the camera or meter was made and where you lived you would have used one of the following settings:
ASA (American Standards Association).
A simple system where doubling the ASA number doubled the sensitivity of the film. As the ASA number increased, the film became 'faster' – in other words more sensitive to light. As it decreased it became 'slower' - less light sensitive. Simple and easy to understand.
DIN ( Deutsches Institut für Normung)
This is the German system. A setting of 21 DIN was equal to a setting of 100 ASA. Unlike the ASA system this system went up (or down) in values of 3. So an increase of 3 in the DIN scale doubled the film speed, so that 24 DIN was equal to 200 ASA. Likewise a decrease of 3 halved the film speed so 18 DIN was equal to 50 ASA. This system is pretty poor really and is at best confusing.
In the mid 80s common sense finally prevailed and both ASA and DIN ratings were replaced with a new universal sensitivity rating which was assigned the new designation 'ISO'.
ISO (International Standards Organisation)
What actually happened in arriving at this new standard was to simply dump the DIN system and adopt the ASA system under a different name. The ISO scale is identical to the old ASA scale in all respects - only the name has changed. Crafty eh?
So now we have completed our history lesson let’s have a look at what altering the ISO on our cameras actually does for us.
Most digital cameras have a base ISO. On modern cameras this is usually ISO 100, some go lower and some higher. I will assume our camera here has a base ISO of 100.
The Relationship between Shutter Speed, Aperture and ISO - The Exposure Triangle
We have already discussed the relationship between shutter speed and aperture in the two links above. A brief recap of that relationship is that for every increase in shutter speed (reduced the time it is open) there must be a corresponding increase in the aperture (open it wider) to maintain the same exposure value.
So if we need to use 1/200th at f8 to get a good exposure but wish to use a larger aperture, say f5.6, we need to increase the shutter speed and in this example that would be to 1/400th. We have opened the aperture wider so letting more light in, s we need to reduce the time the opening is open for, i.e. the shutter speed.
So let’s assume we are looking to take an image of one of our fish and it is giving us 1/60th @ f2.8 at ISO100. But we have some problems here, the fish is swimming and we want a greater depth of field (DoF), in other words we want to use a faster shutter speed and a narrower aperture.
But hang on a minute, didn’t we just agree that if we increase the shutter speed we have to open the aperture which makes the DoF shallower? So then maestro, how do we achieve this apparently impossible task?
The answer is we adjust the ISO to suit. Remember the ISO is the measure of sensitivity to light of the sensor (or film).
Remember the relationship between aperture and shutter speed where a doubling of one means a halving of the other? Well guess what? If we double the ISO we also halve the need for light. So let’s now look at what this means in how we solve our problem from above. Ready for some photography maths? …
We have metered at 1/60th and f2.8 at ISO100. We need to shoot at 1/100th and f5.6 to avoid camera shake and achieve a desirable DoF. We have stated that if we double the ISO we halve the need for light. So if we use ISO200 we could double the shutter speed, i.e. to 1/100th – but this still only gives us f2.8, and there are two stops between f2.8 and f5.6 (remember the progression f2, f2.8, f4, f5.6, f8 …). So in order to achieve the same exposure value we need to double the ISO TWICE more, i.e. once for each stop. Which gives us an ISO of 800.
So changing the ISO changes the exposure value. If we halve the ISO we have to either halve the shutter speed or double the aperture to maintain the same exposure values. That last bit trips up many people, i.e. doubling the aperture. If we go from f4 to f2.8 we have doubled the size of the opening in the lens – remember the aperture is a ratio and although we often (lazily) express it as f4, we should be expressing it as f1:4 or f ¼.
So ISO is our get out of jail free card then isn’t it? The short answer is yes and no :evilgrin: – the longer answer follows …
Grain/ Noise Considerations.
Increasing the ISO on digital means that the signal from the sensor is amplified by the cameras electronics (digital gain). This amplification is important and without getting too much more technical it also amplifies all of the “stuff” we don’t want too by the same amount. We refer to this “stuff” as noise.
On a film emulsion they use larger sized light sensitive crystals. The problem is these grains start to show up in the finished print and that is why a lot of older photos appear grainy.
For the purpose of our discussion here I’ll refer to this as noise as most will be using digital. The explanation of the following statements is really rather technical and is beyond the scope of this article.
There are some costs to increasing ISO. These are:
- A loss of dynamic range (DR)
- A loss of colour or tonal depth
- An increase in image noise
- An increase in file size (digital only)
- An increase in heat produced by the camera (digital only).
So it is clear this is not a free lunch anymore.
Broadly speaking for each stop of increase in ISO you lose a stop of DR and compress the tonal range or colour depth – basically colours become less vibrant and ultimately less distinguished or muddy if you prefer.
Image noise shows itself in digital as two types, chroma noise (colour noise) and luminance noise (for want of a better explanation white noise). However they can be considered as a whole and it becomes more visible at higher ISO numbers. It appears much the same as grain appears on film, i.e. as grains in the image.
The file size is increased with higher ISOs as the image data is amplified. This is the same reason why it generates more heat in the camera too.
The amount of noise that is acceptable is very subjective. But I will leave you with a few thoughts about noise.
- The only people bothered by noise in a an image are photographers.
- No client has ever complained about noise, but they have complained about blurry images.
- A noisy, sharp image is better than a clean blurred image.
- If the difference between getting the shot and missing it is noise, get the shot!
As always if you have any questions about this please feel free to ask and we'll do our best to answer them.
