Basics of Exposure
When you depress the shutter release button on your camera to take a picture, the shutter opens, “exposing” the image sensor to the light coming in through the lens. The quality of this exposure will greatly affect the look of your image. In many cases you’ll allow your camera to make some (or all) of the decisions regarding exposure via its program modes and metering system. This is fine, but a skilled photographer also knows how to manage their own camera settings in order to really control the look of their photos. Understanding how exposure works is key to becoming a skilled photographer. The concepts covered here might be challenging, but learning how exposure works will be worth the effort. My advice: take things one step at a time when learning the harder concepts.
ISO, aperture, and shutter speed are not only useful creative tools, but also the most important components of exposure. They are interdependent; any change made to one of these settings requires a reciprocal change to another to maintain the same overall exposure. Here you’ll learn how ISO, aperture, and shutter speed relate to each other during exposure.
Keep in mind that just because you can get the same exposure using different combinations of ISO, aperture, and shutter speed, it doesn’t mean that all combinations will result in identical images. Combinations using slower shutter speeds and smaller apertures may result in blur, while faster shutter speeds and wider apertures will give you less blur and shallower DOF.
Measuring Light in Photography
We control exposure by controlling the quantities of light reaching the image sensor or film plane with a given ISO. In order to do this in a consistent way we need a standard, easy-to-follow system of measurement that ties into our camera settings. And while your camera may allow you to make adjustments to these settings in various increments, including 1/3 steps, we are going to discuss exposure using the traditional unit of light/exposure measurement, the basic, “stop.” A stop is a unit of light (or exposure) measurement that doubles with every increase. The following are examples of one-stop differences:
- If you start with an f-stop of f/8.0, and change it to f/5.6, you will double the amount of light coming in through the aperture (f/5.6 is 1 stop wider than f/8.0).
- If you start with an ISO of 200, and change it to ISO 400, you will double the sensitivity of the sensor to light (ISO 400 is twice as sensitive to light as ISO 200).
- If you start with a shutter speed of 1/60 second, and change it to 1/30 second, you will double the amount of time light is allowed to pass through to the sensor.
Consequently, going the other direction with any control “halves” the light, time, or sensitivity:
- If you start with an f-stop of f/8.0, and change it to f/11, you will halve the amount of light coming in through the aperture (f/11 is 1 stop narrower than f/8.0).
- If you start with an ISO of 200, and change it to ISO 100, you will halve the sensitivity of the sensor to light (ISO 100 is half as sensitive to light as ISO 200).
- If you start with a shutter speed of 1/60 second, and change it to 1/125 second, you will halve the amount of time light is allowed to pass through to the sensor.
While standard ISO setting increments are straightforward (100, 200, 400, 800, etc.), other settings are a little less so. Full-stop increments in shutter speed might not always be mathematically divisible by 2, and aperture f-stops are not labeled with simple numbers. Still, knowing that each standard incremental adjustment from one to the next results in a full-stop difference is what’s important.
In the following sections, we’ll go into more detail about how this works from the perspective of each of the three major components of exposure.
As stated previously, a camera exposure adjustment can be thought of as either halving, or doubling light (or light’s effect on the sensor). For example, given a consistent amount of light and the same amount of time, going from an f-stop of f/2.8 down a full stop to f/4.0 is effectively cutting the amount of light passing through your lens in half. Conversely, adjusting your aperture setting from f/5.6 up a full stop to f/4.0 doubles the light passing through your lens. Aperture affects the amount of light allowed through your lens in a given unit of time. You might think of it as a light “valve” that provides a hole of varying size for the light to pass through.
There are f-stops between the full-stop values we’re discussing. For example, there are 1/2-stop and 1/3-stop values. But to keep the concepts simple here, we will use examples featuring full-stop values.
Aperture also affects depth of field (DOF) which is the area of the scene that is in-focus. The numbers associated with f-stops might not be intuitive, but the important thing to remember is they are standard and allow us to measure light as described here.
It’s important to realize that your camera can only make aperture adjustments based on the limitations your lens. You can’t dial your aperture up to f/2.8 or f/1.8 if your lens’ widest aperture is only f/4.0. It’s advisable then to purchase the best and fastest lenses you can afford (“fast” referring to lenses capable of wider apertures of at least f/2.8). These will give you the best optical quality and the most versatility in terms of DOF and exposure.
Shutter speed is the time factor in your exposure. It is the unit of time your camera allows light to pass from the rear of the lens onto the image sensor.
Shutter speeds are most often fractions of a second. For example: 1/60, 1/125, 1/250, 1/500, etc. Discounting a few unusual looking increments (after 1/8 we have a 1/15 setting, and after 1/60 we have a 1/125 setting), from one of these settings to another is a full stop.
There are also speeds between these full-stop values, but to keep the concepts simple here, we will use examples featuring only full-stop values.
Again, given a consistent amount of light passing through the aperture, from one standard shutter speed to the next, we are either halving or doubling the duration of exposure, which halves or doubles the amount of light allowed to pass through to the sensor. Moving from 1/250 to the twice as fast 1/500 cuts the time light is allowed to reach the sensor by half. Thus, moving from a shutter speed of 1/250 to the twice as slow 1/125 doubles the amount of time light is allowed to reach the sensor.
Your camera’s ISO setting controls the image sensor’s sensitivity to light. In the case of film cameras, the ISO tells you how sensitive the film is to light. It’s the final part of the light pipeline we are concerned with during exposure. The higher the sensitivity of the image sensor, the less light needed in a given unit of time to record a good exposure.
As with the aperture and shutter speed settings, changing ISO settings can either halve or double the amount of light (in this case the “effect” of the amount of light reaching the sensor). However, the numbers involved tend to be more straightforward.
An ISO setting of 400 indicates twice the light sensitivity of an ISO setting of 200. If you can get a correct exposure with an ISO setting of 400, then you can get a similarly correct exposure if the ISO is set to 200 AND you adjust accordingly the aperture and/or shutter speed to make up the difference. In other words, if everything is set for a good exposure of a scene, and you change the ISO to be less sensitive to light, you’ll have to allow more time for light to build up on the sensor (via a slower shutter speed), or allow more light to go through the aperture in the same amount of time (via a wider aperture), or both.
A side effect of forcing the recording media to be more sensitive to light is that an increase in noise might be noticeable. Similarly, you’ll often get more grain with film rated at higher speeds/ISOs.
The Three Controls in Combination
Given the amount and type of light we have to work with when taking a picture, finding the balance we desire of these three components, aperture, shutter speed, and ISO is always our (or the camera’s) goal. You might adjust one, two, or all three in various ways, but all three have to add up correctly to create the exposure you want. And there is more than one way to get that exposure as shown in the following tables. For example, with regard to exposure, if we start off with the following combination for a given constant amount of illumination on our subject:
All four exposure setting combinations above will result in the same exposure. An example of this is shown below.
In the case of flash illuminated subjects, shutter speed plays less of a factor and adjustments you can make for flash exposure on the camera are basically limited to ISO and aperture. There is a little more to it than that, but the takeaway is that shutter speed shouldn’t be thought of as a way to control flash exposure (only ambient/constant light exposure). I’ll note here that ND Filters are another way to control the amount of light entering the lens, and they can be very handy as a creative tool.
So, in a nutshell, we generally measure light in “stops” and control it with aperture, shutter speed, and ISO. Within any of these components, each standard incremental change either doubles our light (or its effect), or cuts it in half. We have to balance these components and compensate for a change in one, with a change in the other(s) to get similar exposures across adjustments.