Understanding the histogram
Understanding the histogram
The grandfather of all landscape photographers was, of course, Ansel Adams. Adams developed and employed a theory of exposure called the Zone System, which allowed him to precisely calculate exposure settings, which when combined with specific development techniques, afforded him an incredible degree of control. As a modern landscape shooter, you can also employ the Zone System, but fortunately, as a digital photographer, you have the advantage of the histogram. For shooting, and especially for postproduction, understanding how to read a histogram is essential to getting good results from your landscape shooting.
We're going to take a quick look at what a histogram is and how to read it. I'm here in Photoshop CS5. I've got a few different images open. First thing I need to do is view Photoshop's histogram. If you're using the standard workspace, there is a little icon for the Histogram palette. If you don't have it, you can pull it off of the Window menu, Window > Histogram. Here we go! This is a histogram. You'll see these in lots of places in Photoshop. You'll see them in lots of other image editors. You'll see one on your camera, which we'll talk about later.
A lot of people are immediately intimidated by the histogram, because it looks like it probably has something to do with math, because there is all of these numbers, and plainly just data here. Well, it does have something to do with math, but not in a scary way. A histogram is a very easy thing to understand. It is simply a bar chart. The bars are real, real skinny. That's all. It's a bar chart of the distribution of tones in your image, from black to white, with black over here on the left and white over here on the right. So let's just take a look at this actual image here.
It's lacking something. It just looks a little dull. It's almost like it's got this gray thing laying over it. This is a low contrast image. It's a textbook low contrast. Nothing in here is actually black, and nothing in here is actually white. That's what the histogram tells us, also. There is no black information, because there is no data above it. There is no bar there, nor is there any white information. All of the tones in this image are clustered in the middle. There is a whole bunch right here. There are some darker ones here - a textbook low contrast image.
Let's look at a couple of others here. Another low contrast image. I'm going to get the histogram out of the way here by clicking on its tab, and dragging it down here. Then I can close that up. It's interesting how in this case, the histogram actually kind of looks like the image. That doesn't happen very often. Notice this big chunk of data right here. That's this sky up here. Next, we've got something else that's fairly light-colored. I know that, because remember the right side of the histogram are lighter tones. That would be this big piece here.
Then I've got some middle tones and then some darker tones. These darker tones would be all of these dark bits in here. There is no right or wrong shape to the histogram. It simply reflects the tones that are in your image. If I shoot a picture of a penguin standing on a black and white checkerboard, I'm going to have a whole bunch of data down here, and a whole bunch of data up here, and non in the middle. That will be a correct histogram for that image. Here is an image that has been overexposed, and it's kind of lacking contrast. Our histogram again belies that.
There is no shadow detail, a whole bunch of detail in the middle. The thing we never ever, ever want to see, this big spike over here on the right side, this means the image has been overexposed to the point where we have lost detail, and it's very obvious, in this image, where that's happening. It's up here in the sky. All of these tones have blown out to complete white. If you're coming from the film background, this may be a new thing for you. Film rolls off the highlights. You have to work really hard to get to an area of really bright white.
Digital is not so forgiving in that regard. Fortunately, if you're working with RAW, there are some things you can do about it later. So that's what an overexposed image looks like. It's just what you would expect. The bulk of the tones are over here on the right side. Let's look at another one. This one is not quite as bad. We've still got that spike over here. That's probably coming from these blown out parts in the clouds here. A bulk of the image data, all of these middle tones, the sky, and these gray tones, that's what's right down here. This big blob right here is going to represent the lighter gray shades, which are going to be this foreground element, and all of this cloud detail.
So there's no geographic relationship between the histogram and the image. The tones over here don't mean that there is image data on the left side of the image, or anything like that. Again, it's just representation of darkest to lightest. Here is an underexposed image. As you would expect, the bulk of the tones have heaped up on the left side. So I've got a spike here, which means I have underexposed some images to the point of detail lost. Those are going to be these areas that have gone to complete black. Now we don't care as much about shadow areas that have stopped up completely, because shadow areas are supposed to be dark. That's okay.
Nevertheless, it's important to know that they're there, and that if there was detail that we wanted in there, we probably could not get it. There is no white in the image. There is no light gray. Almost all the way back down to 50%. There is very, very little data. If that sounds kind of "Well, yeah. So what? That's real easy," the histogram is real easy. The so what part is going to come later when we start editing. You're going to begin to see that the histogram is a necessary tool for assessing what edits an image needs, for understanding when you've pushed the edit too far, or far enough, and in some cases some tools are built around a histogram.
You will actually decide how to use the tool based on the histogram. Now your camera also has a histogram feature in it. You can take a shot and then view a histogram of the image on your camera's LCD screen. This is a great way of determining if you have overexposed an image, say, out in the field. This is critical for landscape shooters who very, very often face high dynamic range situations where it's very easy to overexpose your highlights. You can take a shot when you're in a situation where you think that might be happening, like a big sky, full of white puffy clouds.
You can take the shot. Look at the camera's histogram right away and know if you've got the exposure you need. One caveat when you're working with Raw: When you look at the histogram on your camera for a RAW file, what you are actually seeing is a histogram of a JPEG file that the camera has made, so that it has an image to show on the back of the camera. What that means, for technical reasons that we won't go into here, is that your histogram on your camera is going to show overexposure before it actually exists, meaning you've probably got another stop worth of exposure that you can go to before you actually get clipping.
We'll be spending a lot of time with histogram throughout the rest of this course, so if you're not comfortable with it, get some of your images out and look at their histograms, and start trying to get it understanding of how the histogram relates to the image.
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