Exposure is everything in photography. If your images are too bright or too dark, you lose a lot of the information and contrast that makes a good image worth viewing. You may have heard me talking about exposure before, but now it’s time to go into more detail. There is a way to ensure that every picture you take is correctly exposed. In this tutorial, I’ll show you how.
An exposure is a single image on your camera’s sensor. We call it an “exposure” because the sensor is being exposed to the light outside of the camera. One image is equal to one exposure. But there is also an art to the process of exposure, which is the balance you strike between lightness and darkness in any single exposure.
To get the correct balance of light and dark, you need to allow the right amount of light to hit the sensor. This can be accomplished in many ways, but it is typically done by pairing an aperture with a shutter speed. Both the aperture and shutter speed control the amount of light that gets into the camera. When they are combined in the correct proportions for the amount of light available, you will get a correctly exposed image (i.e. an image that isn’t too bright or too dark).
How apertures and shutter speeds control light exposure
The aperture is the hole in your lens through which light travels to hit the sensor. The shutter speed is the duration of time your shutter is open to allow the light in. If the size of the hole increases, and the hole is open to more light for a longer period of time (decreased shutter speed), the resulting image on the sensor will be brighter.
Large aperture + Slow shutter speed = Overexposure
When an image is too bright, it creates a condition we call “overexposure.” In other words, either the shutter was open too long, or the aperture we picked was too wide. Too much light hit the sensor, and all of our colors were “blown out” by a supernova-like white haze in the image.
An example of an overexposed image is the photo at the top of this article.
Notice how colors that should be darker and more full are “washed out” like an old pair of jeans. You’ve probably heard these terms before but never put two and two together. Some photographers will talk about “blown highlights” as well. This means so much light hit the sensor that the tiny and subtle details are no longer present. They’ve been blown away by a powerful white haze. The background, in this case, has been completely blown out. It is pure white. You can detect these buy turning on Highlight Mode if your camera supports it.
Small aperture + Super fast shutter speed = Underexposure
Underexposure is the exact opposite of overexposure. It’s what happens when too little light hits the sensor and the resulting image is too dark. This usually happens when your aperture is too small or when your shutter speed is too fast. In most cases, underexposure is tied to an overly fast shutter speed. If you take a picture at 1/2000 sec, for example, the image sensor doesn’t get much time to receive the light. The image left behind is so dark that you can’t see many details.
Before we go any further, there is a commonly held confusion with apertures, and I need to clear that up. As the F-number for an aperture increases, the aperture itself actually gets smaller (meaning less light will get in). But as the F-number decreases, the aperture gets bigger. So, whenever you hear someone say “increase your aperture,” that person could mean one of two things. She could either mean “increase the f-number” or “increase the actual size of your aperture”. It’s important to know which is which before you try out a new technique.
In general, as you increase the f-number, you’ll get less light but more detail. As you decrease the f-number, you’ll get more light but less detail. Keep this thought in the back of your head whenever you read any advice on picking an appropriate aperture.
Here’s an example of an underexposed image:
In this case, the underexposure was done intentionally to add drama to the scene. The photographer used a studio light pointed to the right side of the face to show the viewer one side of the man's face. The rest, however, is black. You can’t see any of the details present.
How to get a correctly exposed image
The easy way us to use one of the automatic modes of your camera, as it does all the work for you, and this will work in most situations. However, it's worthwhile knowing how to create a correctly exposed image yourself for the times when the auto mode doesn't work (or when you use Manual Mode).
Now that you know how shutter speed and aperture affect the way light enters your camera, you can correctly expose every picture you take. How do you do it? All of today’s digital SLR cameras feature a built-in light meter that reads the light in the scene in front of you and tells you if the aperture/shutter speed combination you’ve picked will lead to an evenly exposed image.
You can see what the light meter says when you look through your camera’s viewfinder.
On a Nikon camera, it looks like this:
On a Canon camera, it looks like this:
If the camera thinks your photo will turn out underexposed (based on the light meter reading and the combination of aperture and shutter speed you’ve picked), the meter will start to fill in towards the negative side. If your camera thinks the photo will turn out overexposed, the bars will fill in toward the positive side. And finally, if you have an evenly exposed image, none of the bars on the left or right will get filled in. You’ll be at zero.
To get to the correct exposure, just increase or decrease the shutter speed until the meter goes to zero. If you don’t want to change the shutter speed, change the aperture to achieve the same effect. As you increase your aperture’s f-number, the meter will move towards the negative. As you decrease your f-number, it will move towards the positive. Stop at zero.
But beware! Your camera isn’t always right. Certain kinds of scenes tend to “fool” the light meter. These are usually where some of the image are a lot brighter or darker than the rest of the image, like in the shot to the right. Your camera will give you the all clear, but when you take the picture, it will turn out overexposed or underexposed. There are options available for if your image has both dark and bright areas, including recomposing and using an image editing program. Both are explained in my article on 6 Ways To Fix Too Bright and Too Dark Photos
If you’re using a point-and-shoot camera, you might not get an exposure meter like the two featured above. In that case, use the picture you just took as your “light reading.” If it’s too bright, increase the shutter speed. If it’s too dark, decrease the shutter speed. Keep making adjustments until you like what you see.
And if exposure is still giving you trouble, let me know in the comments below. I’m happy to help out.
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