2. What is exposure in photography?
Exposure refers to the brightness or darkness of an image. When an image looks too bright, it is overexposed. When it looks too dark, it is underexposed.
We can control the exposure of an image using the two main mechanical functions of a camera: shutter speed and aperture. Using these two functions, either individually or in combination, allows us to control the amount of light that reaches the recording medium, as you can see in the video above.
Shutter speed (sometimes called ‘exposure time’) determines how long the shutter on your camera stays open when you take a photo. The slower the shutter speed, the longer the shutter stays open, the more light reaches the recording medium. Faster shutter speeds mean the shutter stays open for a shorter duration, allowing less light to be recorded.
Aperture refers to the opening in a lens through which light passes before reaching the recording medium. The greater the aperture, the more light passes through. Aperture is recorded in 'f-stops' and shown as f1.2, f5.6, f16, etc.
To expose an image correctly, you need to find the right balance between shutter speed and aperture. You can do this using your camera in Manual mode and referring to the built-in light meter (as shown in the video above).
As you search for the right exposure, you can add or subtract ‘stops’ of light by changing the aperture or shutter speed.
A change in any setting that doubles or halves the amount of recorded light is known as a one-stop change. For example, a change in shutter speed from 1/125 to 1/250 will decrease the exposure by one stop. Another example would be opening the aperture from f16 to f8, which will result in a two-stop increase in exposure.
The exposure seesaw
You may find it helpful to think about exposure as a seesaw. If you change either shutter speed or aperture, the other will have to be adjusted accordingly to maintain the balance you need to acheive the correct exposure.
For example, if you decreased the exposure by two stops by increasing the shutter speed, you would have to open your aperture accordingly.
In the same conditions, the combination of 1/250, f11 would result in an underexposed image. This is because the shutter speed has increased by one stop and the aperture has closed by one stop. Both changes result in a reduction of one stop of light each, darkening the image by two stops in total.
To maintain the correct exposure, because the shutter speed increased by one stop, the aperture should have opened by one stop.
Also in the same conditions, the combination of 1/60 and f5.6 would result in an overexposed image. This is because the shutter speed has slowed by one stop and the aperture has opened by one stop. Both of these changes allow for more light and result in an overall two-stop increase in exposure.
To maintain the correct exposure, because the shutter speed decreased by one stop, the aperture should have closed by one stop.
Determining the correct exposure
Once you’ve understood aperture, shutter speed and one-stop changes, the next step is to learn how how to find the right exposure. This is something many photographers struggle with.
Of course, photography is a creative art, and you are free to expose your images however you choose. But if you are looking for objectively ‘correct’ exposure, there are tools you can use.
One is your camera’s built-in light meter, which you can see through your viewfinder. Most cameras have a meter from -3 to +3 stops, with 0 indicating the ‘correct’ exposure (according to your camera). Anything to the left of 0 (in the minus) is considered to be underexposed and anything to the right (in the plus) is considered to be overexposed.
Light meters can look slighty different from one brand to the next, but essentially they all tell you the same thing. For more examples of in-camera light meters, please download the FREE An Introduction to Photography eBook.
Another tool for judging exposure is the histogram, which you can refer to after you’ve taken your picture. A correctly exposed image will have a histogram that is evenly spread from edge to edge without any gaps.
The left side of the histogram represents the blacks and shadows in an image, while the right side represents the whites and highlights.
You can view the histogram for an image on your camera or in photo editing software.
Here you can see a histogram on the back of a camera.
Don’t get too caught up in what your light meter or histogram are telling you. Why not? Because the scene or subject you’re photographing can often confuse your camera.
For example, your camera might suggest that a shot of a white cat in the snow is overexposed and overcompensate until you have a grey cat on grey snow. Similarly, it might also struggle with something like a black dog on a black blanket and try to increase the exposure until you have a grey dog and grey blanket.
Remember that it’s your image and you should make the creative decisions – not your camera. Over time, with practice, you’ll be able to judge what exposure you want and how to achieve it.
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