Exercise 4.1 – the problem of reflected-light metering

This exercise demonstrates a well-known issue with reflected-light metering, the problem of calibrating to medium-grey if the scene does not average out. The exercise is useful because it caused me to demonstrate the effect, rather than just read about it and accept it.

All images were taken with a 100mm f/2.8 macro lens on a Pentax K-1. Sensitivity is set to ISO3200. All images are JPEGs, imported into Lightroom and extracted as screen-grabs complete with the histogram. The panel below the histogram is irrelevant except that if confirms that no post-processing adjustments have been made.

The subject in each case is a calibration target with white, mid-grey and black panels.

The first set of images are taken in programme automatic mode.

Auto control

In the first image we see all three tones correctly and the histogram has three corresponding spikes.

In the black panel, the camera has tried harder to resolve the weave detail, which explains the broader ‘spike’ and may explain why it is displaced slightly to the left. For the mid-grey and white panels, the spikes are almost identical. Subjectively viewing the image itself shows that each panel has been rendered as the same overall shade of mid-grey.

The second set of images were taken in manual mode. I set the exposure to render the mid-grey panel as ‘correct’ with a zero meter bias. The identical aperture and shutter speed were used for the black and white panels.

The tones are rendered accurately and the histogram spikes correspond tolerably well with those of the control image at the top of this posting.

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