This is the arrangement I set up to examine how the camera (Olympus OMD E-M1) deals with metering various reflective surfaces using the built-in meter. The white is a reflector, the mid tone is a card envelope (with a reflectance of 20%, I measured it with a Vipdens densitometer to assuage my inner geek) and the black is a slip-cover for a book. Finding really black things is difficult; they look black but then when you light them up you see the texture and reflections. It was the blackest thing I could find so I used it.
In these photographs the panel below each one shows the exposure details plus the exposure mode – “Normal” is automatic, “Manual” is, well, manual. The histograms don’t have any legend but black is to the left and white to the right.
Now here’s a close-up of the three surfaces, slightly defocussed on purpose to blur some detail:
The camera has decided to place the 20% card pretty much in the middle. The range of brightness – the difference between the brightest and darkest area – is small enough to fit all three within the histogram meaning it is adequately exposed, at least in digital terms, because the image can be manipulated later. Film would be a different matter, the negative values are fixed after development.
Now the white on its own:
The camera doesn’t know this is meant to be white and has treated it as grey, placing it as a mid tone. Here’s the actual grey midtone:
Right in the middle again. Now the black(ish):
In the middle once more. Now with the camera set to manual, all three tones:
I set the exposure so that the white was right up against the end of the histogram but not ‘clipping’. The other tones fall into place behind. Below I set the exposure to make the black darker:
Less than one stop difference (1/125 instead of 1/80) and the dark tones are nudging the left side of the histogram. Now for individual tones:
Using manual exposure I have placed the white in the ‘correct’ spot, towards the right of the histogram but retaining detail. Now the grey and black:
The exposure has been set to place these tones in a more appropriate place.
When editing can move the tones around pretty much on a whim it may be said that placement doesn’t really matter for digital and practically that’s often true. But the exposure range is made up (for many cameras) of 255 discrete levels and the information available for manipulation is mainly at the bright end, meaning that visible artifacts may occur when too much manipulation is attempted with the darker areas.