A 17.1 cm focal-length lens is used to project 35 mm slides onto a screen 4.77 m from the projector lens. What must be the width of the screen ( m) if the image of the slide is to just fill the screen? The width of the image on the slide is 34.3 mm.

Answer 1

Get the distance from lens to slide , do, by using the equation
1/di + 1/do = 1/f
where f = 17.1 cm and image distance di = 477 cm

I get 1/do = 0.05638
do = 17.7 cm

The magnification is di/do = 477/17.7 = 27.0

27 times the slide picture width is 924 mm or 0.924 meter

That is how wide the screen must be.