What's a Histogram ?

I got an email question this weekend, and since I was struggling with a topic, here's the quick answer.  You will often hear photographers talk about the perfect histogram while they're checking the back of their cameras.  If you're new to digital photography, you may think this is just another example of something you need to learn.   Just as soon as you learned the difference between an aperture and ISO, some smart-alec throws in a new term - what the heck is a histogram?

A histogram is a simple graph, displaying the levels of brightness in your photograph.  You read it as the darkest levels on the left to the brightest on the right.  The height of histogram is a reflection of the number of pixels that fall within that brightness level.  Almost every digital camera can display the histogram.  Here is a "normal" histogram.

The distribution pixel brightness in many scenes, at proper exposure, will be even, but you need to consider the image. You can, instantly, get good information about your photo's exposure values by checking it often.  But, like many so-called rules, there are exceptions.  With no knowledge of the corresponding image, you'd think that these histograms are bad.  The first one is too dark and the secpmd is obviously too bright, right?

Not necessarily true.  If your scene is dark - and it's supposed to be - the majority of your pixels will fall to the left side of the scale.  Think of a black cat, lying on a piece of black velvet.  Not too many bright pixels in that scene.  The top histogram is from my day my grandkids took me to Luray Caverns.  I've told you (quite proudly) that the oldest occasionally has an interest in cameras.  She borrowed the point-and-shoot and went crazy down there in the caverns.  I caught this one as she paused to admire her photos.  There was very little light down there, and her face was dimly lit by the LCD on the back of the camera.  It is a dark photo, resulting in a histogram that is skewed to the left.  I wouldn't want to brighten this just to make a "normal" histogram.

Luray Caverns, LCD glow, dark

Likewise, a bright scene will have pixels that fill the right side of the scale.  In this photograph, there is still quite a bit of dark pixels, but the majority are clearly on the right. 

New Schwanstein, Germany, castle

There are other ways to use the histogram as you work on your photography.  There is plenty of information  on the web if you want to go to greater depths.  We'll get into some of them on another day.  I hope that answered the question.  You can always ask questions here or on our Facebook page, www.facebook.com/efcubed.