Understanding the Exposure Triangle
Welcome to my guide to aperture, shutter speed and ISO, also known as the exposure triangle. If you read my 3 principles to learning photography, you'll see that when learning photography, I emphasize field practice over technical study. So you may be asking, why am I taking the time to put this guide together on the technical aspects of photography if it doesn't matter? Well, it does matter! The truth is just doesn't matter as much as practice. Reading this article alone won't make you a better photographer. Rather, what I would suggest to new photographers is to take a portion of what you learn from this article and see how it applies in the real world. Learning all of these settings at once can be a bit overwhelming, but as you learn photography you'll see that there is nothing complicated about any of these 3 settings individually. Take some photos making adjustments to the various camera settings and see how the pictures change as you make these changes. Learning photography is iterative, all it takes is a willingness to practice, make mistakes, learn, and adapt.
Photography is all about light, and these three components all contribute to the exposure, or amount of light that produces a photograph. The more light that the camera sensor is exposed to, the brighter the photograph. Let's take a look at the ways each of these settings can be changed to allow more light into our photographs.
Shutter Speed: When the shutter button on the camera is pressed, you are physically exposing your camera sensor to light for a length of time called shutter speed. The longer that the sensor is exposed, the brighter the image.
ISO: ISO is the sensitivity of the camera's sensor to light, the more sensitive that we make it, the brighter the image.
Aperture: Each camera lens comes equipped with a configurable opening that can be made smaller or larger, this setting is called Aperture or F-stop. The larger the opening (lower F-stop), the more light that we let in.
Simple enough, right? Ok so if you take a picture and it's too dark, which of these settings do you need to change to allow more light? There is no right answer here, but in order to make a decision, we need to understand the "side effects" that changing each of these settings has.
1. Shutter Speed - When we consider the shutter speed of a photo, we need to think about anything moving in our picture. Some movement can be obvious, for example when shooting a sporting event. But other forms of movement, that can have a huge impact to the quality of our photographs, and is not as obvious, could be the movement of a person standing "still", the slight movement of your hand when hand-holding a camera, or even the movement of the stars when your camera is on a tripod shooting a long exposure. Learning about shutter speed is crucial in all of these situations. Generally speaking, we want to keep our shutter speed as low as possible to eliminate any movement. This is especially important when shooting handheld (without a tripod). A general rule of thumb for handheld shots is that you want your exposure to be at or below 1/FL*2, where "FL" is the focal length of your lens. So for example, if I was shooting handheld with a 50mm prime lens, I need to make sure my exposure is shooting at 1/100th of a second or faster.
Sometimes though, we actually want a longer shutter speed. For example, when shooting a flowing river where we want to achieve a silky look to the water. Also, when shooting a cloudy landscape, the moving clouds can add a neat affect to our photos.
In fact, sometimes we really want to push the limits of the Shutter Speed up. We can use an ND filter which is just a piece of dark glass that is attached to the end of the lens and requires more light be exposed to the sensor in order to compensate for the darkness of the glass. Think of it like wearing sunglasses. This is how you can achieve a long exposure in the middle of the day.
2. ISO - While increasing the ISO on our camera increases the brightness of our photo, it also has the unfortunate side effect of adding grain, and lowering the quality of the image. The amount of grain that is produced as the ISO increases will vary by camera, generally speaking newer and more expensive cameras will produce higher quality results at higher ISO sensitivities.
3. Aperture - Aperture is definitely the most difficult of the three to grasp. While increasing the opening of the lens (lowering the f-stop) adds more brightness to our photo, it also decreases the depth of field of our photos. Depth of field is the range of distance from the camera that objects are in focus.
Consider the below picture where the statue is in focus, but the background appears more and more out of focus the further away objects are from the camera. We would describe this picture as having a narrow depth of field, and was shot with a wide opening of our camera lens (low f-stop), F 1.8.
While I made the statements earlier that we generally want to keep our shutter speed and ISO low, the same can not be said about aperture, it just depends. The blurring out of objects in the background of our images, also called "bokeh", can create a proffessional look to our photo by isolating the subject matter of the photo and bringing the viewer's attention to the subject. However, when shooting landscapes, we often want a wider depth of field where everything in the foreground and background appear to be in focus. This can be achieved by by decreasing the lens opening (increasing the f-stop). Turning the f-stop too high though can degrade the quality of the image and also cause dust in the camera or on the lens to become more apparent.
So now that we have a general idea of how each of the 3 components of the exposure triangles work, let's take a look at the different shooting modes on our cameras, and see how they relate to the exposure triangle. Cameras have the ability to detect what it thinks is the proper amount of light for a scene, this process is called light metering. The light meter can be configured to choose the proper exposure based on the entire frame (center-weighted metering), or a particular area of the frame (partial metering), or a specific spot of the frame (spot metering). The type of metering that you select largely depends on the scene, but center-weighted is a good option when starting out, and for shooting landscapes. You've probably noticed the P/A/S/M settings on the top camera dial on your camera, let's talk about what these mean.
(P)Program Mode Program mode will select the appropriate aperture and shutter speed based on your ISO setting, and what the camera is actively metering for the given scene. In Program Mode, you can change either the aperture or the shutter speed and the aperture or shutter speed (whichever you didn't select) will automatically be adjusted for you.
(A)Aperture Priority Mode In Aperture Priority mode, you select the aperture that you want, and the camera will determine the appropriate shutter speed based on your ISO setting and what the camera is actively metering for the given scene. As described earlier in this article, the Aperture setting has the most distinctive affect on the way your photograph appears, so a good photographer will always want control of the aperture. As a result, this is my most often used shooting mode, and a good choice for shooting landscapes on a tripod when shutter speed is less significant.
(S)Shutter Priority Mode In Shutter Priority Mode, you select the shutter speed that you want, and the camera will determine the appropriate aperture based on your ISO setting and what the camera is active metering for the given scene.
(M)Manual Mode Big boy time. In Manual mode you are in control of aperture and shutter speed. It can be a great learning experience to practice shooting in manual, but to be honest, even some of the most talented photographers that I know almost never shoot in Manaul Mode. The truth is that the camera is pretty good at metering to begin with, and while we always want to get the shot right in camera, a RAW file usually has enough light information to compensate for a slightly missed exposure.
Ok, so now we know all about the exposure triangle, how our cameras can be used to detect the amount of light in a scene, and the various shooting modes that our camera offers, how do we know when we have the proper exposure for a given photograph? Generally speaking, the answer is subjective. That being said, when a photo is exposed to too much light it will record pure white pixels in that area, this is known as highlights clipping. Similarly, if a camera is not exposed to enough light, it will record a pure black pixel, this is known as shadows clipping. Most dSLR cameras have a histogram which simply a graph where the x-axis is the brightness of pixels with the leftmost pixels representing pure black, and the rightmost pixels representing pure white, and the y-axis represents the number of pixels in the image at that level of brightness.
The range of our camera sensors is limited though. Sometimes we need to make a decision to either clip some shadows or some highlights. It is possible for a good photo to have some clipping occuring.
The last point i'll make in this article is regarding stops. A "Stop" is the measurement we use when talking about the amount of light in our photographs. Increasing the exposure by a stop means doubling the light. This can be done by doubling the shutter speed, doubling the ISO, or dividing the aperture by 1.41 (confusing, I know). Decreasing the exposure by a stop means halving the light that the sensor is exposed to. This can be done by halving the shutter speed, halving the ISO, or multiplying the aperture by... you guessed it, 1.41!