Exposure: Understanding Aperture, Shutter and ISO
The very first photograph and the most recently taken, a few picoseconds ago – in fact every photograph taken and every photograph that will be taken in every possible future – was, is and will be made by opening a hole of variable size and allowing light to fall onto a medium of variable sensitivity for a variable amount of time.
These three factors – how big a hole, how sensitive a medium, and how long the hole remains open to expose it to light – are the only controls a camera actually has: the very first camera had them, and despite the confusing plethora of stuff a digital camera appears to do, actually – deep down – today’s state-of-the-art cameras still only vary three things at the point of capture: aperture, shutter and ISO. And focus: four things. But the first camera had that, too, so nul points for 150 years progress.
These crude controls offer a limited palette of creative choice, so it won’t take us long to grasp them and move on to the really interesting business of making images worth making.
Good photographs tend to come either from extensive practice, or complete ignorance. What they have in common is being undistracted by theory (wondering how to achieve a desired outcome) or the mechanics of your camera (wondering which button to press to make it happen). The aim is to make the knowledge second nature and to become at one with your equipment – both functioning on auto-pilot, the camera an extension of your brain. Let’s start.
Aperture
In photography, apertures (a fancy word for holes) are defined with the letter ‘f’. At this point, it’s not necessary to understand “how big” f8 is. Think of it like a dress size: empirically, it means nothing – but it works well enough to find clothes that fit.
Unlike dress sizes, smaller apertures are described using larger f-numbers. You’re right: it doesn’t make sense, but go with it for now.
Photographers like (and like to talk about) ‘fast’ lenses – that is, lenses with large maximum apertures that drink in the most light. When you see an f-number written on a lens it describes its light-gathering potential: the maximum possible aperture. Lenses considered ‘fast’ typically have f-numbers of f2.0 or less.
Shutter Speed
Shutter speeds are commonly blink-and-you’ll-miss-it quick: often fractions of a second – ie, 1/60th of a second, or 1/60s. There are times when very short shutter speeds are needed: as rapid as 1/8000s in mainstream mirrorless cameras. At the other end of the scale, long shutter openings are sometimes needed: most cameras allow metered exposures of up to 30 seconds, and many have a ‘B’ (or ‘bulb’ – don’t ask) setting that props open the shutter indefinitely, or until your finger falls off.
ISO
In the film era, ISO was known as ASA. It was much the same thing, and curiously enough the definition hasn’t altered now our shutters allow light to squeeze through our apertures onto our sensors rather than celluloid.
But it is helpful at this initial stage to consider the difference: with film, greater light sensitivity is achieved by changing the chemistry of the entire film roll: you can’t just ‘switch ISO’ from one shot to the next. This was one of the great joys experienced by the first generation of digital shooters in the 1990s, and one of the great ‘huh?’ moments first experienced by a generation of digitally-weened hipsters returning to film in the 2020s. The second, on being handed a film camera, is “Where’s the ‘On’ button?” But we digress.
Adjusting the light sensitivity of a digital sensor is like rotating a dimmer switch: if you rev it up with more current, it’s brighter. Switching metaphors, each sensor has a ‘base ISO’ that’s somewhat like the idle speed of a car: usually ticking over at ISO 100 or ISO 200. However, current cameras can be revved up to ISO’s in excess of 100,000 – which isn’t as amazing as it sounds, as ISO figures – like many photographic measures – proceed by doubling.
So ISO 400 is only one notch ‘faster’ (or, better at light-gathering) than ISO 200, and ISO 1600 is only two notches better than ISO 400.
Bringing them all together: Stops
You may have noticed that each of these factors are variations on the theme of gathering enough light to form an image. In each case, we talk about ‘fast‘ in the sense of being better at gathering light. And, we have an interchangeable unit of measurement that brings them all together: The Stop.
The handy thing about stops is they have the same meaning when applied to aperture, shutter speed, or ISO. We speak about a shutter speed of 1/2s being ‘one stop faster’ than a shutter speed of 1 second (note the doubling again) just as we’d talk about ISO 800 being ‘one stop slower’ than ISO 1600.
It works for apertures too, but the scale is sneakier. For reasons we won’t discuss, the aperture scale goes like this: f1.0 / f1.4 / f2.0 / f2.8 / f4.0 / f5.6 / f8 / f11 / f16 / f22 / f32. Personally, I’d prefer to believe that f2.8 is one stop faster than f5.6, but it isn’t: it’s two.
On behalf of photography I’d like to apologise for this inconsistency (and indeed the silly nature of f-numbers in general). Unfortunately, the aperture scale is one of those things you just have to drum into your head until its senselessness seems normal. Which it will in time.
Putting Exposure Controls to Work
Exposure controls have a mutually exclusive push-pull relationship. If you’re a gamer you’ll be familiar with the idea of creating a character or vehicle to which you can allocate a fixed number of ‘strength points’: if your ‘Warrior’ scores high for ‘Strength’ and ‘Bravery’ there won’t be points left over for ‘Intelligence’. If you race car is maxed out for ‘Speed’, you’ll have to split the remainder of your allocation between lower marks for ‘Handling’ and ‘Reliability’. Such is life.
Similarly, in each photograph, there are many ways to combine Aperture, Shutter Speed and ISO to achieve a correct exposure – and each makes possible a different kind of image.
Choosing & Using Aperture
We come to aperture first because it has priority. In fact, in most situations, you should set your camera to Aperture Priority (AP) mode – because your choice of aperture has a specific impact on the look of the image. In AP mode you select the aperture and the camera decides the shutter speed and/or ISO. As with anything, there are always trade-offs, and no free lunches.
Larger apertures (smaller f-numbers) gather more light: increasing shutter speed, reducing camera shake and helping to freeze moving objects.
However, larger apertures (smaller f-numbers) reduce the ‘thickness’ of the plane of focus – the amount (depth) of the image that will be sharp.
The larger the aperture (the smaller the f-number) therefore, the better you can isolate your subject by blurring the foreground and background. Some (not me) have called this ‘pop’.
These are unavoidable physical properties of light passing through a hole. But there’s another factor to consider: lenses are rarely at peak sharpness at maximum aperture (wide open). We need to unpack that a bit before it’s helpful. Up until c.2010 a handful of elite and specialised lenses achieved maximum resolution wide open, in the centre of the image. Many more modest modern lenses now pull off the same trick. However, none achieve maximum resolution and minimal distortion at maximum aperture in the outer part of its image circle (typically the corners).
Mostly, wide apertures are a benefit – which is why photographers pay so much for ‘faster’ lenses. But there are penalties to beware: first, the wider the aperture, the thinner the ‘depth of field’ – great for subject isolation; not so good if you want to see all your subject’s face, not just the tip of one eyelash. Second, lenses don’t perform best at their widest aperture; wide open lenses produce chromatic aberration, field curvature, coma, increased flare, no sunstars, optical vignetting, mechanical vignetting and compromised sharpness – particularly away from the frame centre.
Enough theory: how should apertures be used appropriately in different situations?
In low light, we usually want the widest possible aperture to avoid either very long shutter speeds or very high ISO’s – which have penalties we’ll come to shortly.
Photographing fast-moving objects like birds, athletes or children hopped-up on Sprite, wider apertures allow action-freezing fast shutter speeds.
For portraiture we often want to blur surroundings to focus attention on the subject, but very wide apertures (small f-numbers) may render parts of the face blurry. This may or may not be the look you want, but the aperture dial sets the depth of field: how much of your image is in focus.
For videographers, it’s desirable to keep the aperture at a fixed relationship to the shutter.
Sometimes we want to force a slow shutter – for instance, to create motion blur, or to turn moving water into an ethereal blur. Here we would use a small aperture (large f-number).
Sometimes we need all of a scene (such as a landscape or interior) in focus. Again, we would select a small aperture (large f-number).
Here we touch on another aperture gotcha: stopping down too far (typically f16 and above) renders the entire image increasingly soft. We don’t see the catalogue of problems mentioned above at small apertures, but contrast and resolution take a distinct hit.
With modern lenses sharpness tends not be a major issue, so we need not stress too hard the effect of aperture alone, but it’s worth understanding that all lenses have a Goldilocks Zone in which they offer peak performance: not wide open, and not too far stopped down.
Aperture & Bokeh
Bokeh was the word chosen by the global photographic community in the 1990s to describe the out of focus areas of an image that previously didn’t have a name. It’s not immediately obvious why different ‘flavours’ of blurriness would exist, and it wasn’t greatly discussed until the 1950s (and then only very meaningfully in Japan). But since the internet age, bokeh connoisseurs seem to be everywhere, and lenses are more conspicuously designed to produced attractive (and/or ‘lots of’) bokeh.
Photography students and beginners naturally first focus on getting what they want in focus. Focusing on their focus, they may not notice what happens in the rest of the frame: that’s the junk they weren’t interested in – and indeed that’s quite close to the original sense of the word ‘boke’ in Japanese. Incidentally, it’s pronounced bo-ke, with two, short, flat, choppy syllables: think ka-ra-te. It shouldn’t sound like bouquet. And for those ryhming it with ‘poker’, quartering would be a kindness.
But we digress . . . at some point you will inevitably notice (and then won’t be able to unsee) that different lenses, at different apertures, create different-looking bokeh. Defocused areas can look smooth and blended, or rather edgy and overlapping. Some lenses have a swirling character to the bokeh, as though a rotational motion-blur has been applied to out of focus areas.
Point light sources are a special case: they balloon into circles, or ‘bokeh balls’. Sometimes – you may have noticed at the cinema – the circles aren’t circles: they are vertically-stretched ovals (anamorphic bokeh). Sometimes they start out as circles in the middle of the frame, then centrifugally mutate into cat-eye shapes (mechanical vignetting). Sometimes the circles are polygons with any number of sides (depending on the number of aperture blades). Sometimes bokeh balls are rendered with internal onion rings, or distinct outlining (‘bubble bokeh’).
The important takeaway at this stage is this: the wider the aperture, the more smoothly and strongly blurred the background and foreground will be. Photographers often aim for the best bokeh by opening the aperture as wide as possible. A wide open aperture is usually a perfect circle, with the diaphragm blades tucked into the lens housing. Perfectly circular diaphragms are said to create the smoothest bokeh. A wide-open aperture setting certainly creates the strongest blur effect, but not necessarily the most attractive, for reasons we’ll explore later. As mentioned above, stopping the lens down even one stop (ie, from f2 to f2.8) will often turn bokeh-balls into bokeh-polygons – which may or may not suit the picture.
To recap: the versatile aperture dial is also a bokeh-generator: the wider the aperture, the stronger the blur.
Choosing & Using Shutter Speed
It’s usually preferable to set your aperture in a way that steers the shutter speed in your preferred direction. However, Shutter Priority Mode (sometimes confusingly marked ‘Tv’) can be manually chosen, and the camera will make its own decision about how best to juggle ISO and aperture. In most cases this approach has no advantage.
If you’ve made it through the ‘Aperture’ section, you’ll realise that a faster shutter speed is usually desirable because it minimises camera and subject movement. However, there are times when blur is sought. For instance:
- Combining a slow shutter with slow-sync flash creates an attractive effect, particularly when shooting moving subjects – such as dancers – in low light.
- Panning with a moving object. If correctly timed, it’s possible to freeze it and retain a record of its motion (see above).
- Shooting waterfalls
- Light painting
- Star trails
Historically, slow shutter speeds (in excess of 6-8 seconds) came with the hidden penalties of increased image noise and unpredictable colour shifts. However, shooting RAW with a modern camera effectively makes these issues irrelevant.
It used to be said that handholding was likely to cause camera shake if the shutter speed was 1/Focal Length: ie, you couldn’t expect a clean result shooting a 300mm lens at a slower shutter than 1/300s. It’s still a useful starting point, but most camera bodies and/or lenses have image stabilisation – in some cases effective up to 7 stops, which in theory enables a 300mm lens to be handheld at 1/3s.
Choosing & Using ISO
ISO is the final component in the three-piece puzzle of exposure, and although it’s the least important creatively, it’s the most fundamental: it sets the base sensitivity.
Many cameras allow you retain aperture priority control, but choose automatic ISO selection – in which the camera selects a high ISO in low light, and in strong light aim to shoot at the base ISO or a setting close to it. Professional photographers are sniffy about such things, but in an unpredictable, fast-moving environment a good Auto-ISO function is like wearing expensive Reactolites – preventing much time-consuming donning and removal of sunglasses.
You may wonder why the camera isn’t permanently set to its highest ISO setting, given the typical desirability of fast shutter speeds? TANSTAAFL. The higher the ISO setting, the greater the sensitivity of the sensor to light, but the process of amplification creates noise. The base ISO setting is usually relatively low compared to a camera’s highest setting, but that’s where the signal-to-noise ratio peaks. Any deviation results in the sensor retrieving less information.
Today’s cameras tend to perform best at ISO 100-200, depending on the model. One of the key benefits of fast lenses is that it lets you shoot at lower ISOs and/or in lower light. For this reason there’s no such thing as a lens that’s too fast. However, modern sensors perform extremely well relative to every imaging system prior to 2010, and that is a game-changer. Even small-sensor systems like current Micro Four/Thirds bodies can shoot great stills at ISO 3200 with suitable noise-reducing post-production of RAW files – a good moment to note one of the fundamental advantages of RAW over JPEG.
High ISO settings don’t just introduce noise: they blur detail. Photographers working in low light constantly wrangle the trade-offs between the softer rendition of a lens at maximum aperture verses the softer, noisier rendition of high ISO, and the possibility of camera and subject movement caused by a slow shutter speed. However, writing in 2023, high ISO is the least of three evils – don’t be afraid to crank it. If you’re in that dilemma, don’t forget that above ISO 1600 or so, your camera loses the ability to discriminate between the most and least sharp apertures of your lens. Remember too that image stabilisation steadies the camera but doesn’t freeze the subject. It’s usually best to put the subject first: in extremis, don’t be afraid to ramp up the ISO and open the lens to the max to obtain the right shutter speed for your subject.
The importance of ISO and shutter speed depends largely on whether you’re shooting handheld or on a tripod; with or without image stabilisation; and whether your subject is slow- or fast-moving.
Typical Setups
Event and Wedding Photography: Mainly Low light
Auto ISO (ranging from ISO 800-6400) / Aperture: f1.2-2.8 / Shutter speed: 1/15-1/100s
Architectural Photography: Mixed Light
Manual ISO set to base (typically ISO 100) / Aperture: f5.6-f16 / Shutter speed: 1/6s-15s
Birds: Daylight, Fast-Moving
Manually set to ISO 400-3200 / Aperture: f2.8-f4 / Shutter speed: 1/100-1/500s
Astrophotography
Manually set to base (typically ISO 100) / Aperture: f1.2-2.8 / Shutter speed: 4s-3 hours with tracker