When you’re out there taking pictures, how can you tell if you have a well-exposed shot? One might look at their pictures on their digital camera’s display, but that’s not going to be very accurate because it’s not calibrated to display the picture as it truly appears. If you’re out in bright sunlight, and you have the brightness on the screen turned up, there’s no telling.
But there is a solution. The wonderful folks at the camera companies make it possible (on a lot of models, but not all) to view a histogram of your photo right after taking it. But what good is a histogram if you don’t know how to read it? Let’s have a look at one and figure it out. Here is the histogram that appears when you adjust curves in Gimp. The histogram on a digital camera should look very similar.
Correct exposure
A histogram is basically just a bar graph. The horizontal axis of a histogram represents the lightness of pixels. On the left is pure black, represented by the value 0. On the right is pure white, with a value of 255. Neutral gray is smack dab in the middle. The vertical axis of a histogram represents the number of pixels of a particular lightness in the photo. In this example, we see that on the left side of the histogram, there are no pixels in the photo that are absolutely black. But just a little bit to the right of it, you see the bar graph go up, meaning there are several pixels that have a lightness value that is very close to black. Then the graph dips down just a little bit and goes way up when you get into the darker part of the middle ranges. It dips back down in the higher part of the middle ranges and then swoops way back up again to the top as you get close to white, and then drops off completely right before you get to absolute white. The exposure on this photo is pretty close to correct. There is a wide range of lightness values, but they don’t go off the chart on either side. Also with this photo, because of the U-shaped dip in the middle, it makes for a higher contrast. Because we have many pixels in the darker range, and many pixels in the lighter range, and fewer pixels in the middle range, the photo has more contrast.
Overexposure
If the bar graph were to go all the way to the right edge of the histogram, it would mean that the photo is overexposed, and there would be areas that are blown out completely white, and there’s not much you can do to fix an overexposed digital photo. Here is what an overexposed photo’s histogram looks like:
You can see there are almost no pixels showing up on the left side of the histogram, meaning there are almost no pixels in the photo that are black or close to black. Over on the right hand side, you see that the graph starts curving up at the end and goes all the way to the right side of the histogram. That means it’s overexposed and you can see in the photo itself that the grass is largely blown out, along with the white birds.
Underexposure
If the bar graph is crowded over to the left side of the histogram, it generally means your photo is underexposed. Here is an example:
You can see there are almost no pixels showing up on the lighter end of the histogram, and everything in the picture is very dark. But there is still hope if you have an underexposed digital photo. Often it can be saved (using tools like curves in Gimp), especially if you’re shooting in RAW format, but we’ll go over that another time.
What to do
The best practice is to correctly expose your photo when you take it. After taking a photo, look at the histogram and figure out if you’re overexposed or underexposed. If you are, make necessary adjustments on your camera and try again. Try to get a photo where the histogram stretches from one side to the other, but comes down to the bottom on each end (like the first example above). Don’t worry about running out of film.
There also are other situations where getting correct exposure for the entire photograph is just about impossible, and I’ll touch on those later, too. What are your thoughts? Leave a comment.
