How to read/use histogram on a digital SLR camera

General Questions
#1

Howdy, I’ve got a Canon Rebel XT as my first “serious” camera. I like it a lot, but I’ll spare you the product review. I’m still a photographic newbie, but I have a pretty good understanding of ISO, shutter speed, and aperture size. But I don’t get how to “use” the histogram that’s created for each image.

I know that depending (on the white balance? exposure?) the histogram shows distinct “peaks” in certain areas, and also shows other areas perhaps completely unrepresented. The whole thing may be shifted to the right or left, etc.

Any advice on how to make use of the histogram when taking a picture? If I snap a picture, then check the histogram when reviewing it, what am I looking for? What settings do I change for the next one?

I know that different situations will yield different results, but I’m looking for some examples.

Thanks!

#2

There’s some good information here: http://www.shortcourses.com/how/histograms/histograms.htm

I don’t use them much in the camera myself, but do use them in Photoshop to help me determine how much to lighten or darken a photo.

#3

A histogram is a graph showing the frequency of each value. Horizontal axis is brighness of each pixel, and vertical axis is the number of pixels that have that brigntness.

If you take a picture of a perfectly uniform gray surface, the histogram would be a single sharp spike, because all pixels have more or less the same brightness. The position of the spike tells you whether the exposure was correct: if it’s near the left end the image is underexposed, and if it’s near the right end it’s over-exposed (or close to it).

If you take an image of a smooth gradient, the histogram would be a flat line. That means for each given brightness, there are several pixels with that brightness.

A real-life picture would produce histograms with more complex features, and the shape depends on the image. If you’re taking a picture of a person in front of a white wall, it may have a very broad peak or plateau at the left end (corresponding to the person, whose skin/clothing isn’t very uniform), and a sharp peak towards the right (corresponding to the white wall). So at a glance, you can see which parts of the photo is properly exposed. If the sharp peak is at the right edge, that means the white wall is completely saturated; all the features of the wall are washed out. This might be what you want, maybe not, that’s your decision. (The only time I purposely do it is when taking photos for eBay). On the other hand, if the sharp peak is near the middle (or further left) and the broader feature is bunched up on the left edge, that means the person is underexposed.

#4

I look at the histogram everytime I take a picture. Generally a properly exposed picture will have a peak in the middle of the graph, and taper off to nothing at both sides of the graph. If the graph is shifted to the left of the right you have an under or over exposed picture. That means you have lost information that was in the picture to shadow or overexposure. Unless this is the effect you are looking for you should force the camera to change its exposure settings to get a more centered histogram.

#5

I always look at the LCD after taking a shot, to check the histogram, rather than the actual composition. It’s a way of checking that I haven’t over- or under-exposed the photograph, or, if I have, to make sure it’s what I want. It’s very easy, especially while taking landscapes, for the camera to meter for the land, leading to the sky being washed out and overexposed. Checking the histogram allows me to make sure I haven’t done that. Although there’s no “ideal” histogram for any particular photograph, in general you’re looking for the best possible exposure short of “blowing the highlights” (overexposing parts of the picture, so they come out a uniform white wash, without any details). That tends to be a histogram that is slightly bunched up towards the right hand side. Although some exposure correction can be done later, in Photoshop or the like, it’s always easier when you have a correctly exposed photograph to begin with.

I don’t know how I’d manage without a histogram now - it’s become a part of my shooting habits that I have no desire to change. Shoot, check histogram, reshoot if necessary.

This is a good resource for getting to grips with histograms, and this is another.