Camera Histograms at a Glance...

Histograms show how accurate an exposure is. Many cameras have this feature yet many people do not know how to "read" a histogram. The left side of the histogram is the dark, or black, area while the right side is the light, or white, side. If the "mountains" crowd up against the left or right sides of the graph you are losing detail. Keep everything neat and tidy within these limits. Average tones progress across the center. The height of the "mountains" indicate how many tones are in that particular area. It is OK of they climb out of the top of the graph. It is left and right that you are concerned with. Here is a five-picture sequence of a drawbridge taken with a Nikon D 70s that was mounted on a sturdy tripod. The lens was set at f8 for all the exposures, but the shutter was changed one stop (What is a stop? See Below). There is 100% less light for each exposure as the bridge went up, then down. The histograms on the Nikon camera's rear LCD screen were photographed with another camera and reproduced here. The camera images in the center column are straight from the camera, uncorrected. The Images in the right column were corrected, dimmed or brightened (except the for the third set, 1/200 at f8) with an "exposure" control, a slider, in my copy of iPhoto (not Photoshop) on my Macintosh computer. A PC user can do the same thing, see below. Now you can "read" a histogram (if your camera has that feature) and quickly double check your exposure on the rear LCD after every shot.

Camera Histogram	Camera Image	Corrected Image	.
1/50 - f8			Badly overexposed. Histogram detail is crammed to the right. Sky has burned out from blue to green to white. Clouds gone, wiped out. Cannot be saved.
1/100 - f8			Much better. Some highlight detail is now showing but histogram shows some detail on the right side is off the graph. That indicates the lightest part of the cumulus clouds are "bloomed," have lost detail.
1/200 - f8			Best exposure. Histogram "mountains are completely inside the chart, notice the yellow ends before maximum light or dark, with most of the detail in the center. Nothing is lost. No need to correct anything. Ready to print.
1/400 - f8			Although exposure looks dark and the histogram is at the left of the chart, this image can be adjusted to make a fine print. Lesson: In digital photography there is more latitude with underexposure than with overexposure. It is just the opposite with film/chemical photos.
1/800 - f8			Badly underexposed. Histogram shows detail in the dark tones is out of the ball park. I lightened the print so can see mid and light tones but the dark shadows under the bridge and the dark portions in the water stay black. "Adjusted" print is far too gutsy. Ugh.

Looking at this layout from left to right you see the histogram, the resulting image and then the same image adjusted to look normal, if possible. Now "read" the layout form top to bottom and you can see how the "mountains" in the histogram follow the exposure changes of one stop, what that difference in exposure does to the image and how the image looks after correction.

The corrected images were not changed with Photoshop. That wonderful software was used only to sharpen the images and to reduce the images in size and resolution so you can view them on the Web. I used a brightness control that is available to anyone that uses free software. These were brightened or dimmed with the "exposure" control in Apple's Macintosh iPhoto program. Anyone on a PC can do the same thing, either with the proprietary program that came with your camera, or, better yet, download the picasa software to your PC. This is a free editing and photo organizing software program from Google.

Stop. This is the term for adjusting a lens to admit more or less light into your camera. OK, I used the term when referring to a shutter change but that is the nomenclature of the camera world if your changing the light level by exactly 100%. The same light level to the camera would have occurred if I left the shutter alone and changed the f "stop" on the lens by 100%.

Some Histogram Links you may be interested in...
dpreview.com. Everything in this massive photo/camera site is concise and complete and so is their explanation of what a histogram is and does. Also, if you enjoy reading about and comparing cameras with each other, block out a few days while you increase your photographic knowledge.

Photographer Ken Rockwell says many cameras, including some Nikons, are using only one color instead of the three primary colors to measure exposure with their histograms and that could lead to errors. My Nikon D70s' histogram show a yellow readout on the LCD screen, as shown on my layout above, but his article says my camera is reading only one primary color, green. How accurate would my automatic exposure be if I shot, say, a red car? A blue car? Hey, everybody, how many colors are in your histogram?