"When I read about these two lenses one thing I never really understood was
how they and their slower manual counterparts could be considered 20 and 38
mm lenses or how they have the f2, 2,8 rating when for the 20mm the 1-12
screen is the only screen that shows the image with any brightness. Based on
the specs shown on e-sif their true specs are more like 400mm (based on
degrees of FOV) f22 or something like that. Thanks in advance for your
enlightenment."
As Jack Benny would say... "Well!"
The focal length and f/stop of a lens are measured with the lens focused at
infinity. All lenses increase in focal length and decline in f/stop as they
are focused closer. * One of the reasons most lenses don't focus closer than
1:8 is because that's the magnification at which the effective light loss is
1/3 of a stop.
If a 20mm lens is extended 40mm from the film plane, the magnification is
1:1. You could argue that the "focal length" is 40mm. But doing so would
only complicate calculating how much extension was needed for some other
magnification.
* Please don't get started on the exceptions and variations. The general
rule is correct.
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