Boring Answer (from Wikipedia): An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image. It plots the number of pixels for each tonal value. By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.
While true, and full of more cross links than I can ever muster, what is it really?
Sexy Answer: A Histogram is your secret key to well balanced exposures sure to blow the minds of duchesses and dukes alike.
Mildly Serious Answer: It shows the tone and intensity of light across your camera’s total dynamic range so you know if things are going horribly, horribly wrong.
Total dynamic range, as mentioned before, is the number of stops of light your camera sensor can handle. These days it’s about nine stops of light from the darkest to the lightest, on average. The histogram for that type of image looks like this:
And can be accessed on all DSLRs, mirrorless and a lot of Point and Shoot cameras if you hit display or info. button enough times when viewing an image. What that metering is showing you (this version is actually from Adobe Lightroom and not the back of the camera, so your histogram may show more of less information, especially the color information and the shot information) is how the light is falling and with what intensity.
On the left are the darkest blacks (RGB color of 0,0,0) and on the right are the brightest whites (RGB 255,255,255). If the line that is the histogram does not come back down to zero, the bottom basline of the graph, before it hits one side, that is called clipping.
It’s a lot like clipping in American football; it’s not nice. And that means there was light data beyond what the camera captured. Lost data. Lost quality.
The example above is fairly well balanced. It ends before getting to the extreme sides and thus, has no overexposure and no underexposure. Hooray for our team.
How can histograms help? For one thing, the screen on your camera is small and not able to show the finite detail in exposure you might need. It’s not the same as your computer screen, which has more range.
So the histogram is a way to see a statistical representation of where light falls. Let’s look at an example of a ferry boat in Washington.
This first photo was shot on Auto, so the camera surmised this is the proper exposure, with its accompanying histogram below:
You can notice there is already some clipping. The darks/shadows area of the histogram on the left is not down to the baseline bottom and ends quite high on the left side. This is evident in the photo (which is admittedly harsh, with more dynamic range than my camera can handle) by the fact that there is, to put it simply, some really dark shadows.
Most cameras these day will show you just which areas are too bright and too dark by flashing the over/under-exposed areas or coloring them red like this:
If we were to under expose or over expose the picture on purpose (see yesterday’s Exposure Compensation), the histogram shifts.
The histograms below are examples of the same image above, over exposed by +1, +2 and +3 stops:
Some things to note here. When changing exposure, it’s not like the light is always symmetrical. See how that mound in the middle at first moves right (indicating it’s going towards over exposed) and then it changes to being all bunched up?
Second, take a look where the light falls on the left side, the shadow side. You can see it creeping to the right as the exposure is biased more and more overexposed. This shows you you have room to spare on that side (left side). And that main area close to the right in the ‘proper’ exposure? It quickly went off the deep end of overexposed with major clipping.
Now let’s under expose things one stop at a time.
Things get bunched more and more to dark side and there is a lot of clipping. Below is an example of the image under exposed by 3 stops.
How Histograms Can Help You
How can this help you? It can help you by taking some of the guess work out of exposure. In tricky situations it lets you know how much latitude you will have to adjust the exposure brighter or darker without losing information.
For example, here’s a shot of the moon rising behind some trees in Western Australia, along with its histogram.
And now a “bad” moon overexposed by +2.25 stops and histogram:
See how much clipping there is and how much loss of detail in the moon? And notice that we don’t care about the black clipping in this case because that is all silhouette and the blackness of space?
It’s important to note here that the histogram is just giving you information. It’s a messenger and should not be despised nor shot. In this case, the histogram is not evil, the exposure is. Don’t go hating on histograms when they tell you things are wrong. Fix them!
For this moon, I can also move the exposure -1.25 and underexpose a bit and see what happens to the image and histogram:
While the moon is darker, it is not beyond the range of the sensor and that means we have more room to play with the image in a computer once home. The histogram shifted left but still well within range of acceptable.
Truth be told, both the slightly underexposed version and the first version will come out just fine with modern computer help.
Putting It All Together
Histograms are very helpful when shooting from a drone as there is often glare on the screen, making exposure estimates hard. Trust your Histogram, let go!
If there is clipping on one side, try moving the exposure the other way, via Exposure Compensation. If there is clipping at both ends, you may need to used a graduated neutral density filter to cut down on light in certain areas, otherwise, you simply have to accept that digital cameras do have a finite capability that can be surpassed, currently.
An option here is to use well thought out and executed High Dynamic Range techniques. These use a number of overexposed and underexposed images that are merged, typically in a special program or in-camera on a phone, to increase the overall range of the finished image. But I’m not going to get into that now. That’s for another post.
Right now, I need to find a bunny and my library card.
Next Up: The Rule Of Thirds
Questions? Pop ’em like Pez in the comments section below. or email me at firstname.lastname@example.org.
Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.
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