Exposure can be defined as the process of capturing light with your camera to produce an image on the digital sensor. Basically, this is how much light gets into your camera. Each of us has had the experience of taking pictures only to find that they are very dark or way too light. This is caused by incorrect exposure. Today, let's talk about exposure, what it means to be overexposed and underexposed, and how to achieve proper exposure.
Introduction to Exposure
The aperture is the adjustable hole in your lens that lets light in. The aperture can be open wide (small f-stop number) letting lots of light in, or tiny (large f-stop number) allowing only a small amount of light in. The shutter speed controls how long the light comes in. Your shutter opens to allow light in and slides shut again. Slow shutter speeds allow light in for a longer period of time; fast shutter speeds mean the shutter is opening and closing more quickly. ISO, light sensitivity, refers to how fast the light is coming in. If you shot in film before digital then you are familiar with the concept of film speed. A high ISO means the sensor is more sensitive to light, which is why it's useful in lower light conditions. In a digital camera high ISO does introduce “noise” into your picture, which reduces its quality.
These three components work together, and if you alter one the others may be changed too. In order to take a picture that is properly exposed - not too light and not too dark - these three components must be balanced.
Overexposing a photo will result in a bright, light, washed out look with areas of blown highlights. This means certain areas of the photo (those that were lighter to begin with) will be so light they lose all their detail. If the highlights are truly blown, there is no data there and you cannot fix the highlights in post processing. If you shoot in raw you can simulate highlight recovery and improve the look of the picture to some extent. It is really important to try and achieve the correct exposure while you are taking the picture! In the image below there are some overexposed areas in the background, and particularly in the laundry basket in the foreground. Notice there is no detail in this area. For this particular photo that works because the cat is the focal point, and it is properly exposed. If this photo were any more exposed though you could lose detail in the cat's hair, which would reduce the quality and appeal of the photo.
Underexposed photos are too dark with detail lost in shadow. If there is a lesser of two evils in the exposure department then err on the side of underexposure. This type of exposure error can be fixed in post processing to some extent, by using the levels tool, but it comes at a price! Pulling out detail in dark, shadowy areas will introduce noise into your picture. You may encounter this type of problem in more challenging lighting conditions where a scene is unevenly lit. Perhaps you are taking a backlit photo and you exposed for the brighter background. This will result in dark subjects in front of a properly exposed background. In the photo below, the photographer has created a silhouette because the camera correctly exposed the bright background. This is great for this particular composition, but because of the large brightness range, we can't fix this to also show the subject's face.
How to Achieve Proper Exposure
Achieving proper exposure is a big topic to tackle, but I will address a few basics. If you shoot in auto mode, your camera is choosing the aperture, shutter speed, and ISO for you. You do not have control over the exposure of the picture. Your camera is pretty smart, so in many situations it does a great job metering the light and choosing a combination that results in a good exposure. With that said, in other cases your camera will not expose the picture correctly. For example, if you try to photograph a white, snowy scene your camera’s light meter can easily be fooled and tend to underexpose the image.
If you are taking pictures at an outdoor event on a sunny day you will probably look for a shady area to snap some pictures. That is a great idea but if you rely on your camera to set the exposure it will meter for (or, correctly expose) the bright background light, and you will end up with dark underexposed subjects. You will be disappointed!
Your first line of defense against improper exposure is to look at your images on your LCD screen. How do they look? Too light? Too dark? What if it is so bright outside you cannot tell?
Frustrating, right? This is where the histogram comes in. On most DSLRs you can view a histogram along with the photo you took. The histogram is the graph you see on your LCD screen. It counts the number of pixels at each level between black and white and you'll see peaks where there is the most of that brightness in the photo. For most images what you do not want to see are tall vertical lines to the far left or far right. If your peaks go all the way to the top on the black side (left) you have likely underexposed your image and it will be too dark. On the flip side, tall peaks all the way to the right probably indicate an overexposed image - your highlights have been clipped, washed out, blown out- not good! Those details are lost forever.
If you determine your exposure is not correct, then you can typically adjust it using your exposure value (EV) compensation button. This button allows you to make stepwise adjustments to your exposure in either direction until it is perfect! For example, if you take a picture and it appears under exposed you can quickly adjust it using your EV button (it usually looks like a +/-). For an underexposed image (too dark) try holding the button and adjusting the EV value to +1 (normally it will be 0). Take another picture and view the it along with the histogram. The picture will now be brighter, but if it is still not quite right you can keep adjusting the EV. Similarly for a photo that appears too bright you can choose a negative EV value and try again. Keep in mind that it is okay for some areas to be over or under exposed (remember the photo examples above), so you need to know what is important in your particular composition. Sometimes you may purposely under or overexpose a photo for dramatic or artistic flair.
In order to have more control of exposure as you shoot, get away from shooting in auto mode. You will get the most control by shooting manual. This allows you to meter the light while focusing on your subject and obtain the correct exposure for the focal point of your composition. If you are not comfortable with manual or feel it is too difficult to make so many adjustments on the fly you could try aperture priority or shutter priority modes. Unless you are photographing sports or something else where movement is key, try aperture priority. Once you have chosen the aperture for your picture, your camera will adjust the shutter speed to obtain the correct exposure. The result can still be poorly exposed in various light conditions, but if you are shooting in one of these modes you can use the exposure compensation button described above to correct it.
Exposure is tricky. It is often the difference between a beautiful photo and one destined for the recycle bin. Like any aspect of photography, you need to get out and take some pictures. Learn the skills necessary to get out of auto mode, so you can have some control over exposure adjustments. Use your LCD screen and histogram to check yourself while shooting. Practice makes perfect!
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