You've heard of the exposure triangle, which is the relationship between the three main camera settings in order to take a well-balanced exposure, but there's a fourth dimension that also has a huge impact on what your photos look like.
The exposure triangle is one of the first things we learn when starting out in photography. This is the visual cue we give to balancing three main camera settings involved with capturing a decent exposure, namely: aperture, shutter speed, and ISO. It all seems a bit daunting at first because there are lots of numbers to remember, things called f-stops, and people talk about apertures or ISO as being "fast". But soon we come to realize that it's not just these three major components that influence how we photograph and what our end results look like. There's a fourth major element that drastically changes how you shoot. You can skip to the bottom to find out the fourth element that forms this exposure square, but for now, let's recap the three main parts of the exposure triangle.
The aperture is the size of the hole in the lens. It lets light pass through and onto the film or image sensor. There's nothing particularly special about the aperture in a lens, but there are some specific characteristics to get into your head. First up, the wider a hole in a lens, the shallower the slice of focus is in front of the lens. If the hole is narrower, then the slice of focus in front of the lens will be deeper. This slice of focus is referred to as depth of field. The depth of field dictates how much of the scene in front of the lens is in focus, front-to-back.
A lens' aperture (the hole) is measured in f-stops (hey, Fstoppers, get it?). Weirdly, the lower the f-stop number, the bigger the hole. So a wide hole (or wide aperture) might have an f-stop number of f/1.8 but a narrow hole (or narrow aperture) might have an f-stop number of f/16. A bit counterintuitive, but there we go. If it helps you remember you can use this: for a big depth of field use a big number, or for a small depth of field, use a small number.
The shutter is the little curtain that sits in front of the image sensor (or film) that opens when the shutter release button is depressed. The light then passes through the lens and onto the sensor to make an image. The number in your camera literally translates to the length of time that the shutter is open.
Imagine it like the stage curtains at the theater. They open for one second and are then quickly drawn again, that's a shutter speed of one second. Or perhaps the curtain is open for ten seconds, now you can see the actors moving and breathing as they move around the stage. The same is true with the camera. The longer the shutter is open, the more any movement is captured in the final image, and it results in a blur. So if you're capturing something fast-moving, you might want to use a faster shutter speed so that the curtains are opened only for a fraction of a second and your eyes (or the camera's image sensor) don't have time to register the movement. Essentially, the faster a shutter speed, the more like your subject is frozen in time.
Let me confuse you a little, some modern cameras do not have a shutter because they use electric to scan the photosites (pixels) on the image sensor to take a photo. There are some good and bad points of using a camera like this, but ultimately all you need to know is that a camera without a shutter might still refer to the exposure length as shutter speed.
ISO is a (rather backward) acronym for the International Organization for Standardization. They develop and publish standards across a wide range of stuff ranging from health and wellbeing, to clean water and sanitization, and industry, innovation, and infrastructure. A digital camera has an image sensor that has photosites (read: pixels) that are sensitive to light (and to a lesser extent, heat).
The standardized chart for measuring this sensitivity is done in roughly equal tens, hundreds, and thousands. A lower number (ISO 50) is less sensitive to light than a higher number (ISO 1,000). This is perfect in theory because when it's bright and there's plenty of light you can keep the ISO low, but when it's darker you can turn it up. But it comes at a cost.
The higher the ISO sensitivity the more noise, or grain, appears in your photo. The extra charge running across the image sensor creates electrical interference (and sometimes heat) which is then recorded by the highly sensitive photosites on the sensor. So ideally, we keep the ISO as low as possible to make for a cleaner picture.
So why ever shoot with a high ISO? Well, it depends on what you're shooting and how much light is available. Let's say you're trying to capture an owl in flight. Owls typically come out at twilight or in darkness, so light levels are much lower than in the middle of the day. But you still need to get that owl nice and sharp so you'll use a fast shutter speed, maybe 1/1000 sec. You've opened your aperture wide to f/2.8 but it's still too dark. So you turn up the ISO to compensate. Sure, you'll get more noise, but at least the photo will be sharp. This is the third and final part of the exposure triangle, but there's a fourth important aspect to shooting that massively impacts on your ability to get a decent picture, and can dictate the settings of these three camera settings, and that's the lens' focal length.
You see, it's all well and good dialing in the "perfect" settings to capture your subject, but the focal length of the lens changes things wildly. The focal length is the distance between the optical center of a lens and the image sensor. The longer the focal length the more "zoomed in" you are to a scene. For instance, a wide-angle lens with a short focal length may have a focal length of 24mm whereas a telephoto lens with a long focal length may be 200mm.
So how does the focal length of a lens change the three settings in the exposure triangle? Well, imagine you're standing outside with a laser pointer. You shine the laser on a nearby wall about 100 meters away. A small movement with the laser pointer shows up as a huge movement of the dot on the wall. The same is true with your camera and your subject. The longer the focal length the more exaggerated this effect is, with only minute movements at the camera-end resulting in massive changes at the subject-end. Remember that longer shutter speeds capture movement as a blur more easily and we can see why this might be a problem. You can see this in the example below where a 50mm shot appears markedly sharper than the blurrier 200mm photo.
Shooting a football match from the sidelines is tricky because the players are running around the field and often quite far away, so you'll want a long lens (long focal length) to zoom in close to the action. But if you shoot at a slow shutter speed any camera movement will result in blur, so you have to choose a shutter speed that's fast enough to freeze any movement to get a sharp photo.
This has a knock-on effect. Now you have to shoot with a fast shutter speed but your exposure is too dark, so you have to either open your aperture wider to let more light in but that changes the depth of field and you may not want to change that for a multitude of reasons. So the only other option (aside from adding your own light) is to boost the ISO sensitivity to account for the darker exposure, but then you're met with more noise in your photo.
So as you can see the focal length of a lens has a direct relationship to the camera settings in the exposure triangle. There's even a rough law that photographers follow in order to monitor the issue between shutter speed and focal length, where a law of reciprocals falls into place in order to avoid camera shake blur. For example, a 200mm lens should have a shutter speed no slower than 1/200 sec. If you're on a 50mm lens then this is 1/50 sec and a 24mm lens this is 1/24 sec (usually rounded up to 1/25).
Lens Depth of Field
Due to the physical properties of how light waves propagate and converge we find that wide-angle lenses naturally create a longer depth of field than their longer focal length counterparts such as telephotos. If we shoot a portrait at f/5.6 on a 24mm lens and compare that to f/5.6 on a 200mm lens we can easily notice the difference between the two images' perceived depth of field. The longer the focal length, the shallower the depth of field appears and as such requires us to alter our aperture to account for this. And as before, the alteration of aperture then has a knock-on effect on shutter speed and ISO sensitivity and so on, and so on.
It's for the above reasons that I think the exposure triangle is actually more of an exposure square. Sure, the three camera settings of aperture, shutter speed, and ISO are the key to making a good exposure, but the focal length of a lens has a huge impact on what those three settings ultimately are and even impact on the kind of kit you take when shooting. Those that photograph with huge lenses such as 500-600mm will need a monopod to steady the beast for tracking and to keep things slightly more stable so that the shutter speed doesn't have to be ridiculously fast.
In all, you can't have an exposure triangle outright, you must take into account how the focal length of a lens interferes with the settings and this shapes your ability to capture certain styles of images. So, in my opinion at least, it should be more of an exposure square, than a triangle.