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Photographic Photometry - New tutorial article

Doug Kerr

Active member
Many of the technical issues we are interested in involve photometry, the discipline of describing the "strength" of light. Our discussions are often hampered by incomplete or inaccurate understanding of the different concepts of the "strength" of light and the terms, quantities and units that are involved.

This can lead us to hear, for example, "What fraction of the scene luminance does an f/2.0 lens pass through to the sensor?", which unfortunately has no meaning.

I have just posted to my technical information site, The Pumpkin, a new tutorial article, "Photographic Photometry 101", available here:


It describes the different photometric quantities (luminance, illuminance, etc.) that are of importance in photographic technology and explains the differences between them and what situations they apply to. The article then briefly discusses, in terms of these quantities, what it is that a lens does (photometrically) and how do we describe what film or a digital sensor responds to.


Active member
I agree with Bart fully. Not only it is highly recommended reading, it is also an excellent source of information which is nowhere else explained so clearly and throughly.

Thanks a million Doug for doing all that hard work and sharing it with us, it is greatly appreciated :).