In photography, your exposure determines what gets recorded on your camera’s image sensor. The science behind exposure in photography is called Sensitometry.
Getting the right exposure is fundamental in photography. It’s like getting your balance in riding a bike. You’re never going to win a competition unless you have an awareness of your balance from the get-go.
Three camera settings will factor into your photography exposure: Aperture, Shutter Speed, and ISO. So the question then becomes, “How do we achieve correct exposure?“.
How to Control Exposure in Photography?
Between the lens and the camera’s sensor are three settings we use for image exposure control, to control the exact amount of light hitting the camera sensor. The three basic functions to control exposure settings are:
- Shutter Speed
This article will teach you the basics of these three settings that create the exposure triangle, as well as different techniques you can use to measure the exposure value. With this knowledge in mind, you’ll be able to expose your photos perfectly every time!
First, let’s look at ISO. ISO is a value that represents how sensitive the sensor in your camera is to the light passing through the camera lens and hitting the sensor. Each value of the ISO rating represents what is known as a “stop” of light.
Each ISO number represents a doubling (going up) or a halving (going down) of the sensor’s sensitivity to light.
The ISO can easily be changed in your camera in the menu settings or on a dial. Keep in mind that the higher the ISO setting, the more grainy your photos will appear. Depending on your lighting situation, you’ll want to adjust the ISO accordingly.
Your shutter speed is the speed at which your camera shutter (sometimes called the curtain) opens and closes. This duration determines how much light passes through the lens to reach the camera sensor.
Think of a shade pulled down on a window, and then quickly open it and close it. For an instant, the room was filled with light, and the length of time that burst of light filled the room was shutter speed! That is basically how a shutter controls the amount of light getting to the sensor. The time the shade was open determined – to some extent – how much light came into the room – but so did the size of the window! That window opening acted as the aperture explained below.
Shutter speeds are measured in seconds and fractions of seconds. Each shutter speed value also represents a “stop” of light.
If you’re hand-holding your camera, remember to set your shutter speed to at least 1/60th sec. Anything slower than that will cause your images to be blurry.
The aperture is built inside each lens and controls how much light enters the lens. Now for some clarification on shutter speeds. Looking at the photo below, you will see the changing numbers are the shutter speeds in fractions of a second (i.e., 30 = 1/30, 60 = 1/60). This is the time taken from when the shutter opens to when the shutter closes after you press the shutter release.
Moving from one speed to the next halves the amount of light that enters the camera. Moving the other way to a slow shutter speed doubles the amount of light that enters the camera. This change from one speed to another is called moving a stop. For instance, moving from a speed of 1/30th to 1/60th of a second is going 1 stop faster, and from 1/60th of a second to 1/250th of a second is moving 2 stops.
The aperture controls the volume of light that passes through the lens to hit the sensor. Aperture settings are measured in numbers, called f-numbers, or f-stops. Each f-number represents a stop of light.
An easy way to think of aperture is to compare it to the pupils in your eyes. The bigger the opening, the more light will come through, just like how your pupils function. It expands to allow more light in darker settings.
Check out the camera settings in the photo above. The display shows you various camera settings that let you control the exposure. As you can see, the ISO is 200, the aperture is F7.1, and the shutter speed is 1/200 (one two-hundredth of a second).
The key thing to remember about these three camera settings is that they are interdependent. They depend on each other. This means that if you change one setting, you must change one or both of the other settings in order to maintain your exposure.
For example, if you increase your aperture f-stop from F7.1 to F5.6, you make your aperture larger, which increases the amount of light passing through the lens and hitting your camera’s image sensor. Increasing the size of your aperture affects your depth of field by making it shallower.
If you reduce your shutter speed, you change how movement is captured in your image. Fast shutter speed capture fast movements.
Slow shutter speeds do not capture movement, but instead render movement as a blur.
Long shutter speed have the advantage of letting more light hit the sensor, something that is often necessary during night photography.
Then there’s your ISO. If you decrease your ISO, your images will be sharper, but you will require a wider aperture or a slower shutter speed setting to balance out the exposure in your photo.
ISO, Aperture, and Shutter Speed are Interdependent
Again, the key thing to remember about these three settings is that they are interdependent. Change one, and you change your exposure.
If you have a perfect exposure, for example, and you increase the size of your aperture, you suddenly make your image overexposed. You will need to increase your shutter speed or lower your ISO to compensate for that wider aperture.
Increasing the ISO allows for shooting in lower light situations, but you increase the amount of digital noise (also called grain) in the image. It is impossible to change one element and not obtain an opposite effect in how the other elements affect the image.
The image below is the result of a high ISO setting, causing image grain. The photo lacks clarity and sharpness.
Rule of Thumb for Photography Exposure Outdoors in Natural Light
First, take the ISO as your shutter speed (100 ISO = 1/125th of a second or, if the ISO is 400, then the shutter speed would be 1/500th of a second for instance), your aperture setting is:
- For bright sunny days and the sun is on the subject (f16)
- For overcast, cloudy (f8)
- Sunsets and sunrises, low light, wide open @ 1/30th
These are basic starting points that usually work. Your camera’s user manual may also have some excellent, basic exposure suggestions.
So, you have two methods of controlling exactly how many light rays get on the sensor, and if you understand the above, you then understand how to control exposure for different ISO settings. Re-read it until you understand it because this is the crux of exposure for daylight photographs.
To become more sophisticated with photography exposure control, you need to learn how to use a light meter. This can get very complicated because there are so many light metering systems out there and many ways of using those meters.
After years of shooting, I submit that the only true metering system you need to master is the light metering system offered in any good DSLR camera.
The camera’s light meter reads the light coming off the subject matter through the lens you are using and is controlled by the ISO you have already set that meter to. It simply is the most sensible, accurate way to meter those light rays.
DSLR meters are getting more advanced all the time, offering “spot” metering (you can zero in on one particular spot on the subject, get the right exposure, and lock in that setting and make your photo) … overall metering, reflected metering, incident metering, and on and on.
It is no longer necessary to “bracket” your exposure (shoot one frame over by one stop, one frame at the indicated exposure, and one stop under the recommended exposure). I quit bracketing twenty years ago and have not exposed a frame improperly. Therefore, I will not get much into other methods of camera metering.
Underexposure, Overexposure, and Correct Exposure
To say an image is underexposed means the image does not contain enough details in the shadows or dark parts of the image. Of course, it also simply means your photo looks too dark.
Overexposure means the details in the brighter or whiter parts of the image are missing. These areas are known as the highlights. They are, as is said in the trade, “blown out” in an overexposed image.
Correct exposure means the image has the correct balance of details in the shadows and details in the highlights. Again, let’s make sense of this with examples!
First, look at the correctly exposed image above. Notice that you can see details in both the shadows and highlights.
From the floor, and people, to the countertops and walls, everything is sharp and properly lit. This is a well-exposed shot with a good balance of highlights, shadows, and contrast.
Now look at the underexposed image above. In this instance, this photo is slightly underexposed, so it gives off a moody tone. This is most likely intentional and can be a great way to get more creative with your photography. However, a completely underexposed image will lose all detail and clarity.
Luckily, there are several ways you can fix an underexposed image either in the camera or in post-editing which we will go over later in this article.
The photo above is quite overexposed. Notice how the details are lost because the highlights are so blown out.
You can barely see the texture of the cheetah’s fur as it blends in with the snowy background.
Here is a comparison of a two images with different exposure values:
It is harder to fix an overexposed image compared to an underexposed image, so do your best to correct an overexposed photo while you’re out shooting!
As I mentioned earlier, play with the aperture, shutter speed, and ISO to get the perfect exposure.
Get your Exposure Right with Camera Exposure Meter
On the LCD display of most DSLR cameras, there is an exposure meter that indicates if your image is going to be underexposed, overexposed, or just right.
At the left end of the meter, you see a + sign. At the right end of the meter, you see a – sign. And in the middle of the meter, you see a 0 and a vertical bar beneath. As you adjust your ISO, aperture, and shutter speed, this vertical bar travels to the left or to the right to indicate that you are either overexposing your image (the bar is to the left of the 0) or underexposing your image (the bar is to the right of the 0).
To adjust your exposure, change your ISO, aperture, or shutter speed until the vertical bar is centered on the meter scale, under the 0.
Get the Exposure Right with your Histogram
The other way to tell if your image is correctly exposed is to take a picture and then examine the histogram for that image. Most of today’s DSLR cameras have at least one histogram that shows the overall brightness of a scene.
The histogram is a rectangular box with vertical lines in it that form an image that looks like a wave. This shape tells you if your image is underexposed, overexposed, or correctly exposed. If the wave is bunched up at the left side of the histogram, your image is underexposed. If the shape is bunched up at the right end, your image is overexposed. And if the shape is divided evenly between the left and right ends of the histogram, your image is correctly exposed.
The big advantage of DSLR cameras is that you can experiment until you master photography exposure. Even if you make a mistake, digital files can be very forgiving and allow you to correct the exposure, even after you take the shot.
This gives DSLR users endless freedom to play around with varying levels of exposure. Play around with your ISO, aperture, and shutter speeds enough, and you will eventually capture the perfect exposure every time.