English: Measuring focal length of a negative lens.
D - Circular diaphragm (± 0.1 mm) = Entrance pupil d - Virtual exit pupil diameter R - Micrometer reticle Link ->File:MicrometerReticle.png f1 - Lens with known focal length fx - Negative lens with unknown focal length f2 - f3 - f4 - measuring microscope lenses Example:
f1 = 150 mm ; D = 20 mm (± 0.1 mm) ; d = 6.666 mm ; fx = ? fx = 150 ( 6.666 / 20 ) = 50 mm
Idea measurements is to make a Galilean telescope[1]File:GalileanTelescope.png with a objective lens which is known focal length (f1).
Measuring virtual exit pupil diameter with a measuring microscope - Link ->File:MeasuringMicroscope.jpgFile:MeasuringMicroscope 2.jpg can find focal length (fx) of a negative lens according to the equation. Another method for measuring focal length of a negative lens:
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