Home Photography 101 Is there a Perfect Exposure for a Given Image?
Is there a Perfect Exposure for a Given Image? PDF Print E-mail
Written by Edward Eastman   

Simple answer - No!

However, there is an optimal Signal-to-Noise Ratio (SNR).

Perfect exposure to us humans is subjective. However, from an digital image processing point-of-view, things are different. You really want to strive for the best SNR. A high SNR gives you more image processing options.

If you are an ETTR photographer, then you already know what I'm referring to. (If you have no clue what ETTR is, click here for an excellent, detailed discussion.)

Consider a back lighted subject.

The first image was exposed for the background details so the main subject - the chair - is underexposed.

The second image was exposed for the subject - the chair - thereby over exposing, or blowing out, the background details to almost pure white.

Both Images were shot as RAW files, opened in Lightroom and no adjustments made. There were then exported as JPEG files.

The Perfect Exposure depends on your original intent. If you wanted a under exposed chair, then image one was correctly exposed. If you wanted the chair properly exposed then image two was the correct exposure. I have not indicated the ISO, Shutter Speed or Aperture for the two images because they mean nothing to this exercise. I could have used any combination of ISO, Shutter Speed and Aperture to achieve the same results for each image. I find too often, we focus on the technical aspects of exposure, i.e., ISO, Shutter Speed and Aperture, instead of intent.

Original Exposure

The Signal-to-Noise Ratio is fixed once you capture the image. It can be adjusted later in image processing software, but even then there are limits. There are some things you can do to maximize an image's SNR.

Second, stop looking at the LCD screen to judge and image's exposure. Start displaying and using the Histogram.

Learn what the Histogram tells you about your captured image. If you use a camera with Live View, learn the displayed Histogram tell you about the image you are about to capture.

Third, use the lowest ISO value the lighting conditions will allow. If you have a tripod and the conditions allow, use it.

Fourth, learn about and practice ETTR all the time.

All of the above will maximize an image's SNR. And that gives you far more options when processing the digital image file.

Here is a more detailed article on ETTR, SNR, Dynamic Range and some very practical examples.


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