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ISO in Photography. How it works

ISO settings in menu of a DSLR

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).

ISO settings within the menu of a DSLR (Nikon D610)

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.

Film carton Fuji Provia 100
Fuji Provia 100 Film box. The 100 indicates this is 100 ISO film

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.

Fuji Provia ISO 100/21
Fuji Provia ISO 100/21. The 100 indicates the arithmetic scale (same as ASA) the 21 indicates the logarithmic scale (the same as the older DIN system)

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.

Film grain
Even though this was slow film (Fuji ISO 100) grain can still be seen in the detail of this older film image I took several decades ago of the Small Isles from the Ardnamurchan peninsula, Scottish Highlands.

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.

Noise in digital image
Noise in a modern digital image. This is a crop of an image blown up to 200%. The image was taken at ISO 4000 to illustrate noise.

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 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.

fine art wildlife and landscape prints for sale. Colin Munro Photography
A sample of my fine art prints for sale

Photography Fundamentals. A beginners guide to photography: Image Exposure, Shutter Speed and Aperture.

A beginners guide to photography, looking at the key elements of a camera; how the relationship between shutter speed and aperture size determine exposure, and the relationship between stops and exposure value (EV).

For pretty much any photographic image, there are two features it must have. Firstly it must be in focus, at least somewhere in the image, and secondly, it must be correctly exposed.  Okay, okay, I know there are lots of ‘arty’ images out there that don’t conform to either, but for most of us in the real world, those are the rules.  I’m going to leave focus aside for the moment and concentrate on exposure: what we mean by this, how we achieve it, how we control it and modify it.  Now of course, if you leave your camera on auto, it will do a pretty decent job for you.  But that’s not very satisfying, and it’s not very creative.  It’s a bit like buying a painting; it may look great on you wall but it doesn’t make you feel like a painter.  So if we want to be photographers, not just a ‘guy (or girl) with a camera’ pressing the shutter button, then we need to have some understanding of what is actually going on inside the camera in order to take the ‘autopilot’ off, and take control ourselves.  The second consideration is, if you want to produce a standard image with textbook ‘correct’ exposure then leave it on auto and your camera will do that for you, at least most of the time. However, if you want to create something more interesting, more individual, then you need more control over how the camera captures the image, and that requires you to take the wheel yourself.

One of the biggest problems nowadays, for people new to photography, is that cameras are so damn sophisticated.  There are so many controls, settings, menus and sub-menus that it can feel like learning to drive in a fighter jet.  Just where do you start to take control?  in order to help understanding, let’s pare back our camera to its simplest form.  Every camera, whether it is the latest top-of-the-range, full-frame, mirrorless, or an 1839 Daguerreotype, is fundamentally a light-tight box with a hole (the aperture) on one side to allow some light in, and some light sensitive material on the opposite side, and a means of blocking the from light reaching the light sensitive material (the shutter) .

A beginners guide to photography, Photography Fundamentals.  Camera diagram showing aperture, sensor and light path. Colin Munro Photography
A camera is basically a light-tight box with an aperture at one end and light sensitive material at the other.

The light-path between the aperture and the light sensitive material is periodically unblocked (the shutter opened) allowing light to reach reach the light sensitive material. This material is then altered in some way by the light, and this begins the process of creating an image. What exactly that material is, has changed many times.  The 19th century Daguerreotype used copper plate coated with silver; early 20th century cameras mostly used glass plates coated with silver salts; then of course film took over; and now we have solid-state sensors that convert light in to electrical signals, but the the basic design is exactly the same.

So we talk about ‘exposing’ the plate, or film, or sensor, to light in order to create an image.  I’ll stick with sensors from this point, as that is what we use now. The amount of light hitting the sensor determines the lightness of the image.  The more light hitting the sensor, the lighter the images, the less light, the darker the image.  We talk about images being correctly, under- or overexposed.  By underexposed we mean that the mid-tones are too dark and we start to lose detail in the shadows. I’ve taken an image of mine of a tokay gecko and changed the exposure of it to illustrate this.

Beginners guide to photography. Photography Fundamentals. An example of an under-exposed image. Colin Munro Photography
Underexposed image of a tokay gecko on a wall

By overexposed the mid-tones are too bright and we start to lose detail in the highlights.

Beginners guide to pphotography. Photography fundamentals. An example of an over-exposed image. Colin Munro Photography
Overexposed image of a tokay gecko on wall

So we control the degree of exposure primarily by controlling the amount of light hitting the sensor, and we do this in two ways.  We can vary the length of time that the sensor is exposed to light (i.e. the duration the shutter is open) and we can vary the size of the aperture allowing light to reach the sensor.  So let’s look at those mechanisms in turn.

Shutter speed

The shutter is a pair of metal curtains, located in front of the sensor.  They act to block light passing through the aperture from reaching the sensor, except for the period the shutter is opened.  This is the same mechanism used in film cameras. However, today most mirrorless cameras, and some DSLRs, have an electronic shutter, where the sensor is switched on and off to produce a similar effect.

The duration the shutter is open is known as the shutter speed.  There are a range standard shutter speeds we see on most modern cameras, e.g. 1 second, 1/2 second, 1/4 second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, 125 second, 1/250 second, and so on.  You will notice that each is (with minor exceptions) half the duration of the previous one.  Half a second allows in half as much light as a one second shutter speed; 1/125 allows in twice as much light as 1/250 of second. So changing from one ‘standard’ shutter speed to the next nearest either halves or doubles the amount of light hitting the sensor, depending on the direction of change.  Having or doubling the amount of light reaching the sensor is known as changing it by one stop.  Understanding the concept of stops (and exposure equivalent values, which I will explain further on) is fundamental to photography.

Aperture size

We also have a second means of regulating the amount of light reaching the sensor; we can change this by changing the size of the aperture. If we make the aperture twice as large, then twice as much light will hit the sensor (if the shutter is open for the same duration) if we make it half as large it will allow half as much light to reach the sensor.  This is preety self evident, I’m sure.  And this modifying the aperture by halving or doubling is known as changing it by …. one stop. Whilst the shutter speed values are pretty intuitive, aperture values are not.  They are known as f stops, and commonly values range between f2.8 and f22 (sometimes f1.4 – f32).  These f stops also change in standard increments, but at first glance they are quite meaningless: f2.8, f4, f5.6, f8, f11, f16, f22.  Even more puzzling, the largest number denotes the smallest aperture.  The simplified answer for this is that the f value is not a physical measure of the diameter of the aperture, it is a ratio between the focal length of the lens and the aperture: f value = focal length/aperture diameter.  From a practical stance, none of this theory matters to the photographer.  What does matter is knowing that each f stop allows in half or twice as much light the next full stop to the left or right, and that the smaller the number, the greater the aperture diameter, the greater the amount of light reaching the sensor.  Most modern cameras work on a click-stop principle.

A modern lens showing the aperture wide open
A lens showing the aperture stopped down to smallest size

So that the size of the aperture, rather than smoothly variable from smallest to largest, changes in a series of steps.  So when we change aperture on our camera, either by rotating a ring on the lens or, more commonly nowadays, a dial on the camera body, we feel a series or positive clicks.  These steps correspond to the f stop values listed above, plus (on modern cameras) 1/3 of a stop intervals.  So, for example, on our camera we might find f values of F2.8, f3.2, f3.5, F4, f4.5, f5, f5.6. The figures in bold are full stops, the figures in lighter text are 1/3 and 2/3s of a stop up or down.  The beautiful symmetry of the stop system is this.  Say for example, our image is correctly exposed at a combination of shutter speed 1/250th and aperture size f8.  If nothing else changes, then we know that if we change the aperture to f11 (one stop smaller) then the image will be underexposed by one stop. Equally, if we change the shutter speed to 1/500th, the image will be underexposed by one stop.  If we change the shutter speed to 1/500, and simultaneously change the aperture to f5.6, then the exposure stays exactly the same, because the former acts to half the amount of light reaching the sensor, and the latter acts to double the amount of light, thus the combined effect is that the total amount of light reaching the sensor is exactly the same.  The obvious questions are: why have two separate mechanisms to achieve the same control over the amount of light, and why would one alter two controls in order to produce exactly the same exposure.  There are several considerations here, and to detail them all would distract from the main purpose of this blog, so I’ll most of those for a future blog.  However, a key consideration is motion blur. If we have an aperture of fixed size, we can still achieve correct exposure by changing the shutter duration; this is in fact, exactly how a pinhole camera works.  The problem comes with photographing moving objects. The faster a subject is moving, the faster the shutter speed needs to be in order to freeze the motion and stop the subject appearing blurred.  This explains how, in some pinhole camera photographs, streets can appear empty of people or vehicles. because the aperture is so tiny, the shutter speeds often need to be very long (many seconds).  Fast moving objects move across the aperture too fast to register on the image. But even modern DSLRs, if we set the shutter speed too slow, then a fast moving object (e.g. a bird in flight or a sports player) becomes blurred in the image produced.

Exposure Values

The final concept I want to talk about here is Exposure Value (EV or sometimes Ev). As explained above, different combinations of shutter speeds and aperture sizes will still produce the same image exposure. Exposure values assign one value to all the combinations that produce the same exposure.  In practical terms, exposure values are mostly interchangeable with stops.  Underexposing an image by one EV is the same as underexposing by one stop.  We tend to refer to stops when taking about shutter speed of aperture.  When we want to vary exposure in automatic or semi-automatic modes (not something I’m covering here) then we start to refer to EV and vary it using the Exposure Compensation button; but that’s all for another blog.

What I’ve missed out

In order to keep things fairly simple, I haven’t talked at all about the third factor in what is often called the exposure triangle, namely ISO.  That will be the subject of another blog soon.

Photography workshops, online classes, one-to-one tuition

This blog is an excerpt from my beginners photography teaching. You can learn more about my photography lessons, online and one-to-one tuition and photography workshops at my main website During the current restrictions due to covid-19, I am mostly running online one to one teaching. The good news is that these can be accessed anywhere in the World. You can find out more about them here.