There are plenty of guides out there on the internet about exposure, be it one about the exposure triangle or in-depth discussions on ISO, shutter speed and aperture. They're all good, and I've used many of them myself to learn. But it took me a while before I came to truly understand how these three interacted with confidence. The missing piece of the puzzle is what is pushing me to write this brief introduction as a way to explain what really made the lightbulb go off for me.
When looking for basic photography tutorials (say, after you've picked up a camera), there are a few key terms that you'll learn quickly enough are repeated over and over again as key settings for dialing in your exposure. For those that are absolutely beginners (like me, once upon a time!), an exposure is the picture you take. The language comes from the "exposure of your photographic medium to light." Sometimes, the word can also be shorthand for "what settings did you use?" As an example, someone might say:
Exposure is where the buck stops when it comes to making images, regardless of what you're using to make the picture. From a pocket wizard like the iPhone 7s to the beastly Canon EOS-1D X, the main function of the camera is the same: expose the photographic medium to light to capture an image. Now that I've done a quick crash course on exposure, the obvious question is what determines exposure?
Let's walk through an example and talk about elements of exposure, so we can understand it. It's a bright, sunny day and you're out and about. How do we get a good exposure in such conditions? Well, you could set your camera to auto and let the camera decide. Lots of cameras make really good decisions about what settings to use to get the right exposure. But of course, exposure doesn't just happen - at every step, the camera is making a decision according to its programming. Cameras these days are really fancy and have pretty powerful microcomputers inside of them that make thousands and thousands of calculations every second. The result of that math are the exposure settings that your camera uses to get the best exposure given the conditions it sees. But the best exposure calculator is you (a phrase I shamelessly stole from Fred Parker) and if you understand the basics of exposure, you won't need to rely heavily on your camera's computer. So, let's dive into the details of what makes exposure. As you've no doubt seen, there are three key factors that affect the exposure. They are (1) the sensitivity of the photographic medium to light, otherwise known as ISO; (2) the speed of the shutter opening; and (3) the aperture of the lens. Each of these elements impacts the amount of light that ends up being captured. Let's explore these three factors in turn. Before we dive in, I will mention that each of these factors also has a secondary impact on the image, but I'm going to set that aside for now. I will return to them later, but I wanted to mention it now.
The ISO, as I mentioned above, is simply the sensitivity of the photographic medium to light. It's fairly straightforward in this sense: the more sensitive your medium is to light, the less light you require to make a technically good exposure. Whether you're shooting with film, or with a digital sensor, the sensitivity of the medium is going to be a key factor you work with. The major difference between film and digital in this area is that the photographic medium cannot change its characteristics from shot to shot. Of course, with a digital camera, this is not the case. A good way to think about sensitivity is with an analogy. For any given point in the image, let's image that that pixel is a bucket that's catching light falling onto it. Once your bucket is "full", it can't catch any more light. If you have a high sensitivity medium, think of the bucket as requiring relatively less light to fill compared with a less sensitive medium. As you can imagine, with a higher ISO medium, even a tiny bit of light can make a big difference. The higher your ISO, the less light your recording medium inherently needs to fill those buckets. Using the analogy, higher ISO mediums have shallower buckets. There are, however, other considerations to keep in mind. Remember those secondary effects I mentioned earlier? Each factor impacts the image in a different way apart from being central to determining the overall exposure. Suffice to say for now, the more sensitive your medium the less light you'll require. How do we measure sensitivity? Well I'm glad you asked. There is a standard scale that's been internationally standardized that all camera and film manufacturers use called the ISO. The higher your ISO number, the more sensitive your medium is to light. Easy right? I'll dive into the ISO numbers a little bit more specifically in section 2.
Your shutter speed is the next element of exposure we're going to discuss. A good way of thinking about this is imaging it like your eye lids. Really! Bear with me, this will make sense, I promise! Remember that a picture is just the result of a flash of light on your photographic medium, so then shutter speed is a huge portion of that. Right?? The shutter speed controls how long your camera is open to let light hit the recording medium in the first place. You could have the most sensitive CMOS sensor ever, but if you never let any light into the camera body, it's useless! So shutter speed is simply how long you open the camera to the outside world. Whatever you're looking at through your viewfinder, that's more or less what your camera is letting in. Neat, isn't it? As with ISO, shutter speed has a secondary effect on the image that will be recorded, but let's leave that aside for now and move on to the final element of exposure. Before we move on though, I will mention that shutter speeds are generally marked in 100th's of a second. I say generally because there are usually a few longer-than-one-second options plus some mysterious option called "B." The "B" stands for bulb and is a term that dates back to the analog days when a film photographer would have a bllb cable release that would keep the shutter open as long as the bulb was compressed. Long story short, most shutter speeds are marked as 100th's of a second unless it's full seconds or bulb.
N is the f-number, and f is the focal length of the lens while D is the diameter of the opening. This is why as the diameter gets bigger, the f-number itself gets smaller.
The last element of exposure, aperture! You made it! Are you still with me? A good way to think about aperture is like the iris of the eye. Beyond how long you open the body of your camera to outside light for your sensor or film to record, we can also control the size of the opening that is letting the light in. This is aperture in a nutshell. How big is that hole? Aperture is exactly the size of the opening allowing light into the camera. Now, aperture is a bit funny because of the way it's measured. Aperture is measured using the f-number which is a mathematical formula for the relationship between the lens focal length and the diameter of the opening. I mention this because it's the reason f-numbers, which describe the aperture, are so weird. The bigger the opening of the lens, the lower the f-number. For example, a lens set to f/2.8 has a bigger opening letting light in than a lens set to f/16. See the sidebar for more info on why that is, but just remember that the lower the f-number, the bigger the opening. Now, like the other two elements of exposure, the aperture also has a secondary effect on the image, but as with the others, we will set it aside for the meantime so we can focus, first, on the way these settings impact the amount of light that makes it into the camera body and ends up hitting the photographic medium.
A lot of guides out there talk about ISO, shutter speed and aperture while only making passing reference to the reason these controls and settings matter: each setting ultimately impacts the amount of light that makes up the exposure. With ISO, you are changing the amount of light required to make a technically good exposure. WIth shutter speed and aperture, you are controlling the amount of light that makes it into the camera body. So in this way, it's helpful to think about the way settings for exposure impact the amount of light required and to start thinking in so called "stops" of light. This way of thinking about exposure changed the game for me, and I hope it will help you too.
So, what the heck is a "stop" of light? Well, a stop of light is a relative measure, which can make it confusing for beginners, but once we put it all together, it's pretty intuitive. A stop of light is the doubling OR a halving of the available light for an exposure. So, let's say you are shooting a picture and it's coming out too dark or too bright. If you adjust your settings, you are adjusting your light by some number of stops, ether it's a full or partial stop. If the image is too dark, that means not enough light is being let in for capture and your image is underexposed. In this case, you'd want to go up some number of stops (as in, you want to increase the light entering the camera) to get a good exposure. Likewise, if your image is too bright, it's overexposed so you'll want to go down some number of stops (as in, decrease the light) to get a good exposure. Now, there's a reason I wasn't specific about the number of stops, because we haven't really talked about the standard stop numbers that are more or less standard across all sorts of cameras, both film and digital. So let's talk about that now and we can wrap it all up. You may wonder why, after talking about the elements of exposure, I took a detour to talk about stops of light. Well... it turns out that each of the factors of exposure we explored above has a standard "full stop" scale. That means for each element of exposure, an adjustment on this scale DOUBLES or HALVES the amount of light. Simple right? It's really that easy! So let's explore that.
Here are the common FULL STOP values for ISO:
Some cameras go even higher than this, but suffice to say, 25 - 12800 will be about as extreme as you get on most current cameras and films. As you probably notice, each ISO is related to the ones surrounding it by a factor of 2. 50 is half of 100, but double of 25. Likewise, 400 is half of 800 and double of 200. Remember what we said above about ISO? It is a measure of the sensitivity of your recording medium. At ISO 800, you require HALF as much light to make the same exposure as you would at ISO 400. Recall that the higher the ISO, the more sensitive to light and therefore, the less light you require. So, when you double your ISO, you require half as much light. Makes sense, right? If not, leave a comment and yell at me. Now, you can also get really fancy and segregate ISO into smaller segments too, but if you're going to be calculating exposure in your head, full stops are easier to work with. In reality, full stops will cover you in most situations, especially as you're learning.
Just as the concept of stops can be used to make sense of ISO and the relationship between different ISO ratings, you won't be surprised to learn that there is also a common full stop scale for shutter speed as well. The common full stop values are:
Some cameras in existence push this (my Google-fu turned up a claim that a CERN camera that has achieved a shutter speed of 1/10000000000000000000000000 - yikes!), but for our purposes most cameras top out somewhere around 1/2000 using a mechanical shutter and 1/8000 using an electronic shutter. You'll notice, just like with the ISO, there is a pretty familiar relationship between shutter speeds. 1/4s is half of 1/2s, but twice as long as 1/8s. Likewise, 1/250s is twice as long as 1/500s, but half as long as 1/125s. So, the higher your shutter speed, the less light you're letting in, while the slower your shutter speed, the more light you're letting in.
As you can probably guess, it turns out that there is a standard full stop scale for aperture as well. Big surprise, eh? The numbers here are not as intuitive as ISO and shutter speed, but you can Google-fu and verify the scale I provide here if you want. Here is the full stop scale that you'll likely deal with all the time:
As with before, there are other apertures as well, but most lenses generally available to people will be capable of these apertures. Again, the relation is that each full stop movement of aperture either halves or doubles the amount of light making it into the camera. f/4 lets in twice as much light as f/5.6, while f/8 lets in half as much light. Remember that as the f-number gets smaller, the opening of your lens gets bigger (I know, math right?)!
Now, before we wrap up and talk about how we can use our knowledge of stops to help us make great pictures, it will be helpful to go back to a topic I set aside while explaining each of the elements of exposure. Recall that I mentioned before I started, that each element (ISO, shutter speed and aperture) have secondary impacts on the image? Here we go!
As I'm sure you probably wondered when I talked about full stops of ISO, why not just use a high ISO all the time? Less light for the a good exposure is good, right? Well, the secondary impact of adjusting your ISO is grain or noise in your image. The relationship is fundamentally the same for both film and digital in the sense that the more sensitive your medium is, the less sharpness you're going to get because of physics. With more sensitive film, the trade off is that the photosensitive elements of the emulsion have to be larger because you're using less light to make an exposure which can result in more visible grain. With digital sensors, the problem isn't grain so much as it is digital noise. As you increase the sensitivity on a CMOS, there is more gain in the signal, which can amplify both wanted and unwanted signals. The practical impact of this is that, for any given exposure, you want to try to capture your image at the lowest sensitivity that you can manage if you want to avoid grain or noise. Makes sense, right? For film shooters, I've written a short post on pushing and pulling film that might be helpful.
With shutter speed, the secondary effect is fairly intuitive but important to discuss. Beyond the amount of light you're letting in to the camera, the shutter speed can also determine how much motion blur you end up seeing in the final image. For example, if you're taking pictures in a sports environment, you are likely to be shooting at a high shutter speed in order to minimize motion blur. In some cases, maybe you want motion blur as part of your image, and so you'd shoot at a slower speed. A good example of this is the classic light trail image, where the photographer has set up the camera somewhere and captured the movement of a light in the image. This principle also applies to things in your picture, so if you have people moving around while your camera's body is open, you'll see them blur too. This is ultimately an artistic choice and comes down to what you're after in the final image. As you can imagine, the secondary impact of shutter speed relates heavily to the artistic intent of the photographer beyond just getting the right exposure.
Finally, the secondary effect of aperture also has artistic consequences for the final image that comes out of the camera. Adjusting your aperture, beyond controlling exposure, also impacts the depth of field of the resulting image. Now, I will note, before we dive into depth of field, that aperture is not the sole element that impacts depth of field, but is one of the major components of it (the other two elements that can impact depth of field are the subject's distance from the capture plane and the camera lens' focus setting). I'll explain briefly here what depth of field is and how aperture impacts it, but there are many other great depth of field tutorials that you can dig up if you want something juicier. Depth of field is the way of describing the slice of the image that will be in focus where the slice is oriented parallel to the capture plane. A good way to think about this is to imagine that a camera can only focus on one distance away from where it is, but from that particular point, things can still be sharp even if they aren't exactly at that point. For example, let's say I've focused my camera at 1.5 meters at f/16. It may be the case that objects up to .4 meters away from 1.5m meters remain in focus (that is, things from 1.1m to 1.9 meters from my camera are in focus). If we adjust the aperture, the secondary impact is that it will change the .4 meters to another number. For clarity, the numbers are for illustrative purposes only. Let's stick with this scenario so we can really get into the nitty gritty. In the example given, let's say I keep my focus distance at 1.5 meters, but I adjust my aperture so that I'm opening the lens up, which means I'm switching it to a lower aperture number and I'm shooting my picture at f/4. Not only am I letting in more light into the camera, but I've also changed the depth of field. In the case of f/4, I have decreased the depth of field so that less things around 1.5 meters are in focus. In fact, the new setting of f/4 means that only things .2 meters on either side will be in focus (that is. 1.3 to 1.7 meters from the camera) as opposed to the larger depth I had when I was at f/16. I hope that example makes sense, and of course it's easier to explain in a picture so I've included a crudely drawn example below for the scenario I gave. As you can see, increasing the f-number (and therefore closing the aperture to make a smaller hole) increases the depth of field, while lowering the f-number (and therefore opening the aperture to make a larger hole) decreases the depth of field. The drawings are not to scale, and are meant to be illustrative only, okay? But, I hope you get an idea of what I'm trying to say. The lower your aperture number, the wider your aperture, the more light, and the more shallow your depth of field.
Easy right? I hope my crude drawing is helpful. If not, you can probably google up a better explanation of depth of field. Suffice to say, aperture is one of the key ways people control depth of field, but keep in mind that aperture also plays a key role in exposure! I should also mention that the aperture also has some impacts on lens sharpness, particularly as the lens is opened up to lower f-stop numbers. This has to do with optics and an explanation is beyond this article, but suffice to say, it is often recommend, to get the best lens performance, to shoot at higher aperture numbers
There you have it, a tour de force of the key elements of exposure: ISO, shutter speed and aperture and how they also have secondary impacts on the image. So, why does that matter? Well, once you know how images can be affected by your settings, you can start to see that there are certain circumstances where you might want to adjust one setting for an artistic reason and thus you are potentially throwing your exposure out of whack. However, because we understand stops, we can actually change our other settings to achieve an equivalent exposure while adjusting for our artistic choice.
Equivalent exposure, putting stops to work
Armed with the knowledge of stops and how they apply for each particular factor of exposure, it becomes easy to make adjustments to our settings to account for artistic elements while maintaining good exposure. (Good exposure, by the way, is something I'll probably write about another time.) So, when you read a light meter or estimate an exposure using the Sunny 16 rule of thumb, think of that as a basis for adjusting your settings to achieve the results you want.