Hi, Folks,
I'm doing some research on projector brightness. Not slide projectors,
rather projectors for vitual reality applications, but the questions I have
are relevant to both types. They probably sound pretty basic but anyway
please bear with me:
1. Does a projector lens with longer focal length give a larger image
or smaller, if the projector to screen distance remains the same and assume
both are able to focus on the screen at that distance? My guess is wide
angle
lens gives a larger image since a wide angle lens on a camera has a wider
angle of view. But I've learned not to use deduction too liberally:-)
2. If two lenses of different focus length have the same working
aperture,
do the images they generate have the same brightness on an identical screen
at
the same distance? If so, then the lens that generates the larger image
passes more light since brightness is amount of light on unit area, right?
Now this gets me confused assuming my guess to the first question is
correct,
as for example, a 20mm/2 lens has a theoretical aperture size of 10mm, and
a 40mm/2 lens has a theoretical aperture size of 20mm, and the 20mm lens
actually passes more light???
3. What I really should ask is that, there is a projector with a lens
that's f=20.0mm/2.0 to 40.0mm/2.8, and I'd like to know, does the image
brightness stay the same when I change the focal length from 20mm to 40mm
and keep the screen at the same place? Obviously the image size will change
so light spreads out more in one case but does the different aperture
cancel that out?
Thanks in advance!
Yefei
