Photography Basics: Exposure and Aperture


This tutorial introduces two of the most basic and important concepts in photography: exposure and aperture. Although one can find explanations of these all over the web, many are not written well. These concepts are related, and by understanding the relation, the photographer is able to have much better control and is able to obtain better results for the photographs he desires.

The goal of this tutorial is to give the reader that understanding in as simple as possible way. We don't want to over-burden the reader with the technicalities, but instead give the minimal information the photographer needs to understand and control his results.

How the Camera Works

You take pictures by pushing the shutter release button. When this happens, the camera opens a shutter which is in front of its light sensors, and this allows light to come in and reach the sensors. These sensors record the light that comes in. When enough light is recorded, the camera closes the shutter and converts the result into the image you will eventually see. Digital cameras and film cameras record the light differently: we don't get into the science of that here.

When you leave your camera on auto mode, then all of this is done for you by the camera in the way it thinks is best. But the camera attempts to deal with the average case which gives the photographer no control over the result.

How much Light to Record?

How much light to record is the topic of metering. For now, we let the camera decide the metering (later we will take control of this too). But conceptually, there are two ways to control the amount of light coming to the sensor: (i) how wide the shutter is open, and (ii) how long the shutter is open. The former is called the aperture, the latter is called the shutter speed or exposure.

The camera's shutter is circular in shape, and that circle can increase or descrease in size. The wider it is open, the bigger the aperture is said to be. The standard measurement that tells how wide the shutter is open is called the f-stop. Rather confusingly, higher f-stops correspond to smaller circles = smaller aperture, and vice versa. If you leave your camera on auto mode, you're likely to get apertures like f5.6 or f6.3, which are rather large openings. However, the typical DSLR can have much smaller openings like f32. Why is this important? We'll get to that soon!

A good analogy for the aperture is the pupil of your eye. When you are in low light, your pupil dilates (gets bigger) so that it can let more light in, allowing you to see things as it gets darker. In this case the aperture is large. The same thing happens with the camera: it opens its shutter wider in low light. In bright light, your pupil constricts (becomes smaller) so that the bright light doesn't damage your eyes. Similarly, your camera uses a much smaller aperture in bright light so that it does not get too much light in.

As mentioned above, the camera also can control light by choosing the amount of time the shutter remains open. Typically, your camera on auto mode might have exposure times between 1/60th of a second to 1/500th of a second, though it could be slower or faster depending upon the strength of the light. If the exposure time is too slow (slower than 1/60th of a second), then the picture may show blur due to the photographer moving while the shutter was open and the sensors were recording light. One way to prevent blur from the photographer moving is to use a tripod, but blur can still happen if the subject is moving.

Getting the Exposure Right

This section gives examples of how important it is to get the amount of light correct.

If the camera lets too much light in, then the photograph is said to be over-exposed. Look at the photograph below which is over-exposed. It's not too appealing, is it? Notice especially that bright areas like the wall above the apple have almost no detail in them. When a picture is over-exposed, detail is lost in bright areas.

If the camera does not let enough light in, then the photograph is said to be under-exposed. The photograph below is under-exposed. When a picture is under-exposed, detail is lost in dark areas. The black surface has lost a lot of details.

Somewhere in between is the right exposure that gives just the right amount of light. This does not guarantee that no detail is lost, but it usually minimises the amount of detail lost. The photograph below is properly exposed.

Trading Off Aperture and Exposure Time

As we explained above, there are two ways to allow light in to get the perfect exposure: either by choosing how wide the shutter opens (how big the aperture is) or by how long the shutter is open (how long the exposure happens). The photographer can trade off between these two: bigger aperture can be balanced off by shorter exposure, and vice versa. In other words, there is more than one way to get the perfect exposure. For every chosen aperture (up to some limit), there is an exposure time that will give you the right amount of light for a properly exposed image. Put your camera on aperture-priority mode to take photographs this way. Similarly, for every chosen exposure time (up to some limit), there is an aperture that will give you the right amount of light for a properly exposed image. Put your camera on exposure-priority mode to take photographs this way.

So what's the big deal here? Who cares if there is more than one way to get the right exposure? Although many different settings/tradeoffs allow the same amount of light, they produce different results! Higher f-stops (lower apertures, smaller shutter opening) give more depth of field, meaning that more is in focus. This is important for landscapes where you want a lot in focus including the foreground and far into the background. Lower f-stops (higher apertures, wider opening) result in shallow dept of fields, which ends up blurring the background. This can produce beautiful results ("bokeh") when done properly, and is especially useful for nature photography.


Let's look at some examples.

The picture below is a landscape taken soon after the sun came over the horizon. I wanted to catch the beautiful light on the rocks, but also have detail of the coastline far in the background. I set my camera on aperture-priority and went up to the highest f-stop I could, which was f/22 (a very tiny shutter opening). Since this allows little light in, the camera balances it off by a long exposure: in this case it was 1.3 seconds. That would be very blurry if I held the camera since I can't be still that long, so I needed a tripod to prevent this blur.

Clovelly Rocks

The photo below was taken in strong light. This could have been done on aperture-priority mode but since I know my camera tends to choose big apertures, so I just kept it in P-mode. Indeed, the aperture turned out to be f/5.6 for this shot which is the biggest that lens can go. Notice the beautiful, simple green background. How did I get this? In fact, that green was grass in the distance. Because it was far away and because I had the shallow depth of field with the big aperture, it all blurred together into a simple looking bokeh background. This makes the bird stand out beautifully and adds to the artistic quality of the shot.

Singing Female Wren

The above examples were mainly concerned with depth of field, so aperture was the priority. There are other examples where exposure is the priority. Perhaps surprisingly, photographers often want to create blur in their photograph to show movement. This technique is called motion blur. It is often used to produce beautiful results of moving water. The shot below was taken soon after sun rise on a cloudy day. The water was rushing towards the shore, so I put my camera on exposure priority mode with the exposure set at half a second. Typically anywhere from 1/4th to 1 second can produce nice results depending upon the amount of movement you want captured. A tripod was a necessity for this shot: I only wanted the subject's movement captured, not my shakey hand movement. However in some photographs, the movement of the photographer also contributes to the image: it all depends what you're after.

Dee Why Sunrise #2


When you leave your camera on auto mode, the camera decides for you the aperture and exposure time, giving you no control over the depth of field. The camera doesn't know what you want: it can only take its best guess and hope you like the result. You have no control over the outcome.

To have more control, you can switch to aperture priority or exposure priority. In aperture priority mode, you are explicitly telling the camera what depth of field you want by choosing the f-stop, and the camera is then determining the right exposure time for your image. In exposure priority mode, you are explicitly telling the camera how long the shutter is to be open, and the camera is determining the correspond aperture (and hence depth of field) for you.

The most important concept is that aperture and exposure time can be traded off. Larger aperture (wider opening of the shutter) is balanced by shorter exposure time. Smaller aperture (smaller opening of the shutter) is balance by long exposure time. All such tradeoffs allow the same amount of light in: the only difference is the depth of field of the result.

Bringing ISO into the Equation

Although understanding the concepts above give you a lot more control of your photography, there are still limitations. Consider the following scenarios:

All of these scenarios have one thing in common: low light which prevents you from getting the exposure / aperture combinations you desire. The good news is there is one more variable that can be added to the equation to help you deal with these situations -- to some extent. That is ISO, which determines how sensitive the camera's sensors are to light.


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