What is ISO in Photography?
What exactly is ISO? How does the ISO control on your camera work? How does changing ISO affect your photography? When learning the basics of photography, we learn about focus and we learn about exposure; and we also learn that exposure is controlled, at its most basic level, by three variables: shutter speed, aperture size and ISO. We also learn of the relationship between the three variables, the concepts of STOPS, and how to change the exposure by changing any one of these variable. (If these concepts are unfamiliar to you, I will be producing a blog these basic principles soon, and will add a link here). Now shutter speed and aperture size are fairly intuitive concepts to grasp. If we slow down the shutter speed from 1/100th of a second to 1/50th of a second, so that the image sensor is exposed to light for twice as long, then we can understand how this will double the amount of light hitting the sensor, increasing the exposure by ONE STOP. Similarly, if we open up the aperture from f16 to f8, again doubling the amount of light hitting the sensor, this again will increase the exposure by one stop. (I know, it’s a little counter-intuitive that the higher f value is the smaller aperture; there is a reason for this I’ll explain in a future blog). But ISO? What physically is changing? How does changing ISO change exposure? Is the amount of light hitting the sensor changing somehow? (Spoiler, no it’s not).
What does ISO mean?
Okay, so what exactly is ISO? ISO is the title for the International Organisation for Standardisation (ISO was decided on as the initials would be different depending on the language used). But that doesn’t really tell us anything about what it is, or does, in relation to cameras. The name dates back to film cameras, and was originally called ASA, an acronym for American Standards Association, if the film was from the USA (e.g. Kodak film) or DIN, the Deutsches Institut fur Normung, if the film was from Germany (e.g. Agfa film). These were simply national standards associations that standardised a great many things, from computer programming languages to thread sizes on machine bolts. DIN and ASA film speed standards became shortened to DIN and ASA on the labels on camera film. There was also a Soviet standards system known as GOST (Gosudarstvennyy Standart) which was also used for film in Soviet and some asian countries. When these standards were amalgamated into the ISO system so the film speed standards were changed to ISO. For example ASA 100, and DIN 21, became ISO 100/21; ASA 200 and DIN 24 became ISO 200/24 (often the logarithmic DIN part was omitted e.g. the 21 or 24 in these examples). Similarly GOST 90 became ISO 100, GOST 180 became ISO 200. So that explains where the name came from, but not what ISO actually is.
ISO speed in Film
As explained above, the term ISO (or ISO speed) originated in camera film, so lets start there. Unlike shutter speed and aperture size, ISO works rather differently in film and in digital photography. Those of you old enough to have used film cameras may remember we used to talk about ‘fast film’ and ‘slow film’. This referred to how much light was required to correctly expose the film. Fast film required relatively little light (and so could be exposed at faster shutter speeds, all other things being equal) while slow film required more light.
Film has light-reactive silver halide grains embedded in one or more layers of gelatin. Fast film has larger grains, slow film has smaller. As photons of light hit each grain it changes state, producing what is known as a latent image, across the film frame. But, and this is the crucial part, it takes roughly the same number of photons to change the state on a larger grain as it does on a smaller one. As the larger grains present more surface area, and there are fewer of them packed across the film frame, so fewer photons of light need to hit the exposed frame of larger grained film than fine grained film to produce the same exposure level. The downside of this was that fast film produced a ‘grainy’ look in the final image. This could look pretty cool in black and white photographs, not so much in colour (at least that’s my personal opinion). Now the thing about ISO in the days of film, it was fixed – more or less. By that I mean you bought a roll of film of a set ISO value, determined by the average size of the silver halide grains embedded in the film. So that could be Kodak 64, or maybe Fiju Velvia 100, both slow, fine grained films of set ISO values 64 and 100 respectively. And once loaded into your camera that was it. You set the ISO on the camera (normally a calibrated wheel on the top of the camera body) to the ISO value of the film, and exposed each shot based on that value. Well almost set. You could, if really necessary, ‘push’ the film. That is you could expose the film as if the ISO was one or two stops faster, then adjust the film processing times (extending the times in chemical baths) to account for this. I say ‘if really necessary’ because it did tend to shift the colours and contrast somewhat.
ISO speed in Digital Cameras
Okay, so that’s how ISO works in film cameras, but there is no film in digital cameras, and no silver halide grains, so what happens there? In digital cameras things work rather differently. The amount light hitting the sensor (as opposed to the film frame) is still governed by by the size of the aperture and the duration the shutter is open. However, the sensor itself does not become more or less sensitive to light as the ISO changes. Instead the process is one of changing the output voltage or charge on amplifier circuits, rather like changing the volume on an audio amplifier.
Although ISO speed changes are achieved in fundamentally different ways in film cameras and digital cameras the effect on exposure is almost the same. For example, changing the ISO from 100 to 200 changes the exposure, for the same amount of light reaching the film or sensor, by the same amount in either system. So changing from 100 to 200, or 200 to 400, changes the exposure by one full stop (i.e. the equivalent of doubling of the amount of light lighting the sensor or film). There is of course one crucial difference. As explained earlier, in film the ISO is set, it is a physical feature of the composition of the film. You want to change the ISO you need to load new film, which of course means using all the film currently in the camera first. But with digital cameras the ISO can be varied simply by pressing a button or turning a dial.
High ISO. Image grain and image noise
There is another difference also nowadays. With film, slow, fine detailed, ISO is between 50 and about 200. Fast, grainy ISO is between 800 and 3200, 3200 being pretty specialist film. In the early days of widely available digital cameras (early 2000s) ISO 800 was considered pretty fast for digital cameras also. With the Nikon D200, for example (introduced in 2005) the ISO could be ramped up all the way to 1600. But it came at a price. Where analogue film produced grainy images from high ISO film, digital cameras produced ‘noisy’ images at high ISOs. What exactly are ‘noisy’ images? Noise in images is similar to the distortion in an audio system when the amplifier is turned up too high. How this amplification is achieved in in digital cameras depends on the type of sensor used (CCD or CMOS) and on the manufacturer and model.
But fundamentally the light hitting the sensor is converted in to a voltage or charge. This voltage of charge is then amplified. At this stage it is still analogue, so any unwanted artifacts present are also amplified. This can manifest itself as random flecks of colour or brightness in the final image. For this reason the rule used to be to take a photograph at the lowest practical ISO in order to have as little noise as possible in the final image. However, the processing of images within digital cameras has advanced so much in recent years that (at the time of writing) ISOs of 6400 or even 12800 are possible with only minimal amounts of noise in the final image. This allows sharp, detailed images of fast moving objects to be taken in low light conditions that would not have been possible only a few years ago. In point of fact, I would argue that improvements in high ISO image quality have been the more meaningful developments in digital cameras over that past 15 years. I still own a very old DSLR from around 2006 (a Nikon D200 as it happens). It is an excellent camera that still serves my purposes perfectly well – most of the time. The one area it falls down on is in low light conditions where I need to use high ISO.
Fine Art Landscape and Wildlife Prints
If you enjoyed this story why not check out my other blogs. I write about travel, the environment, marine biology, diving and wildlife. You may also want to check out my photographic prints. These can be viewed on my main site www.colinmunrophotography.com and include landscapes from around the World, people of the World, marine life and other wildlife. The prints are only available through my website, each one individually processed and made to order. This includes fine art giclee prints, limited edition prints and canvas wraps. I use only the best, carefully selected print houses employing the finest papers and printing processes to ensure image fidelity and longevity.
