What’s the Correct Exposure?

Today’s Post by Joe Farace

In a post on my main blog about digital infrared photography I talked about the importance of obtaining the proper exposure but as significant as it is for IR, it’s crucial for every photograph that you make. That’s because proper exposure is a perfect storm of shutter speed, aperture and ISO that produces a pleasing result.

Pleasing to whom, you might ask? I strongly believe that only you are the ultimate arbiter of what is “correct” but one way to objectively evaluate a particular image’s exposure is to use your camera’s histogram function. The histogram is a graph showing a photograph’s range of light values in 256 steps. Zero is on the left size  and represents pure black; 255 is on the far right-hand side and represents pure white or the famous “Polar Bear in a Snowstorm.” In the middle are the mid-range values representing grays, browns, and greens.

How I made this shot: This classic Bentley was photographed with my old Canon EOS-1D Mark II N and 28-105mm f/3.5-4.5 II USM lens and an exposure of 1/160 sec at f/7.1 and ISO 200. The wonderfully useful EF 28-105mm f/3.5-4.5 II USM lens has been discontinued but you can pick up a used one on Amazon for approx. $120, as I write this.

On a typical photograph, all of the tones are captured when the graph rises from the bottom left corner, reaches a peak in the middle, then descends towards the bottom right creating a “bell-shaped” curve because it’s, well, shaped like a bell. If the histogram’s curves starts out too far in from either side or the slope appears cut off, then the data is missing and the image’s contrast range may exceed the camera’s capabilities or simply the exposure selected for that specific image.

While the classic histogram features a bell-shaped aka Gaussian curve, not every photograph fits this type of distribution. Dramatic images with lots of light or dark tones areas often have lopsided histograms but that doesn’t mean they aren’t correctly exposed for that subject matter.


 

 

Along with photographer Barry Staver, Joe is co-author of Better Available Light Digital Photography that’s out-of-print but new copies are available for $21.88 or used copies for giveaway prices—four bucks—from Amazon, as I write this.