Practice question · Estimate
A photometer reads 640 units from a white studio wall with the blinds open and 20 units from the same wall with them shut. Observers call the wall white in both conditions. By what factor did the reading change?
Estimate on a scale from no change (0) to a factor of 60 (60).
Hints
- Divide the larger reading by the smaller.
- 640 divided by 20.
Show the answer
32 (answers within ±4 count)
Why
By a factor of 32, with no change at all in what was seen. That gap between the measurement and the report is the whole of constancy, and it is the reason a painter can carry a white through a full range of values on one canvas and have every part of it read as the same wall.
Practise The Principles of Visual Perception
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