On 13 Feb 2006 at 18:58, Yefei He wrote: > 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:-)
As the lenses FL increases the width of the projected image is reduced, it works exactly the same was as if the lens was mounted on a camera for image capture. > 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? No, because the projected image is spread over a larger surface area in the case of the shorter FL lens. > 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??? No. > 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? You really need to determine the required magnification first, there is no point getting the fastest lens if the magnification is too large or small. Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
