Confused by a paragraph in the classic photography book Understanding Exposure explaining the limits of today's camera meters

Question

In the classic photography book Understanding Exposure by Bryan Peterson, on page 113 of the fourth edition of the book, the author writes that:

[...] in addition to being confused by white and black, the meter [on a camera] can be confused by backlight and contrast.

My question is, what does the author mean here by the meter being confused by contrast? I'm only asking because I know of only two types of contrast (well, three if you count compositional contrast): tonal contrast and color contrast.

In the next paragraph, on the same page, the author suggests taking a meter reading of the blue sky, on sunny days, when dealing with winter landscapes (shooting whites), black Labrador portraits (shooting blacks), bright yellow flower close-ups (color contrast?), and fields of deep purple lavender (color contrast?).

By mentioning yellow flower close-ups and fields of deep purple lavender, the author seems to be suggesting color contrast can certainly confuse the meter on a camera, but I'd like to double-check with this website, just to be sure.

And, what about tonal contrast? Can a meter also be confused by tonal contrast?

Answer 1

The phrase "backlight and contrast" basically means the classic silhouette situation which would be tonal contrast. But it's more that the meter doesn't know which tonality matters more to you and may choose wrong "because it is confused."

In regards to color contrast it may depend on which version of the book (publication date) to some extent. Earlier meters (and even some today) only sensed luminance with no sense of color. But even modern camera meters are R/G/B (long/medium/short wavelength is a better description). And depending on what stage the metering is being done, it may not be aware of exact colors (demosaiced/processed image).

In all cases the meter really only cares about tonality and expects an average of around **12% reflectivity. By metering a (deep) blue sky you are providing the camera a source that should be about middle gray in a B&W version of the image. Whether the camera meter sees colors or not, bright yellows are very close to white, and deep purples are very close to blacks; i.e. likely to confuse the meter based on tonality.

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** Note that the 18% Gray Card is supposed to be angled 45 degrees between the light source and camera when used for metering; and that reduces it's reflectivity to 12.7% as seen by the camera/meter.

Also note that the camera is not expecting approximately 12% reflectivity from "an average scene." It is expecting approximately 12% from whatever is being metered, and it is your job to provide that by selecting the appropriate metering method (spot/weighted/average/etc) and an appropriate source.

And that is the point of metering blue sky. It is also sometimes recommended to meter green grass or the palm of your hand for the same reason... of those three I would say your palm is probably the most consistent; it may not be 12%, but you can learn/apply the appropriate offset.

And a "correct exposure," might not be what you actually want...

Answer 2

The meter isn't "confused"; it measures what it measures. What's confusing is that the meter might not (and often doesn't) measure what we think is important in the photograph. The key to getting good exposures is recognizing when the meter is measuring something other than what we think matters.

Backlighting, for example, means that the background is brighter than the foreground. An averaging meter will expose for overall neutral gray, and the result will be that the foreground looks dark. That's usually not what you want: the detail is in the foreground, and that's what you generally want to expose correctly, so you dial in more exposure.

Answer 3

It is called bracketing and experience. As already stated, a film camera averages everything out to about 18% gray scale. Pure whites can come out a little gray due to underexposure. Pure blacks can come out gray due to over-exposure.

Read up on black and white photography and darkroom printing of B & W. It comes with experience. In the good old days of black and white photography, we used a spot meter and learned how it worked.

This is why Ansel Adams was such a great photographer.