Yesterday’s Topic: How To Take A Shot
What’s a Histogram?
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.
Totally dynamic range, as mentioned before (I hope) is the number of stops of light your camera sensor can handle. These days it’s about 7 stops of light from the darkest to the lightest. It looks like this for a ‘properly’ exposed image:
And can be accessed on all DSLRs and a lot of Point and Shoot cameras if you hit display or info. 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 is the darkest tone and on the right are the brightest tones. 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 football, it’s not nice. And that means that there was light data beyond what the camera could represent at that end. The example above is fairly well balanced. It ends before getting to the extremes and thus, has no overexposure and no underexposure. Hooray for our team.
How can the histogram help? For one thing, the screen on your camera is small and not able to show the finite detail in tone you might need when making critical decisions about exposure when viewing after a shot. It’s not your screen at home. Duh. So the histogram is a way to see a statistical representation of where light falls. Let’s take a look at a past example of the ferry boat images from the Exposure Compensation/Bias post.
First, the first shot which the camera believed to be ‘proper’ and its accompanying histogram:
You can notice there is already some clipping. The darks/shadows don’t end before they get to the left edge and the lights don’t end before they get to the right. This is evident in the photo (which is admittedly harsh and over a range larger than the sensor can handle) by the fact that there is, to put it simply, some really dark shadows and some overblown highlights. 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:
To make this post sane, I’m not going to post every image from the Exposure Compensation/Bias post here (you should open that post in another tab to follow along), but I am going to post their histograms so you can see what happens as we change the exposure.
These will go in order, starting with the overexposed images first. These are +1, +2 and +3:
Some things to note here. When changing exposure, it’s not like the light is just 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? Don’t expect life to be all simple. Second, take a look where the light falls on the left side, the shadow side. You can see it creeping right as the exposure is biased more and more overexposed. This shows you you have room to spare on that 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 and there is a lot of clipping. Take a look at the photos and you’ll see this to be true.
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.
Reviewing the Moon Photography post, here’s the proper moon exposure and histogram.
And now the “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.
If there is clipping on one side, try moving the exposure the other way. 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 which use a number of overexposed and underexposed images that are merged, typically in a special program, to increase the overall range of the finished image. But I’m not going to get into that now.
Right now, I need to find a bunny and my library card.
Next Up: The Rule Of Thirds
31 Days To Better Photography is a series written by professional photographer Peter West Carey on The Carey Adventures.Com. The series is designed to unravel the mysteries of photography so you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.