Projection Lens Requirements

What size mask do I need?

Some projectors let you change the object size by masking the stage of the projector. The formula for calculating the size of the mask needed, used in this calculator, is:

Object Size=(Image Size in Feet*12)/(((Lens to Screen Distance in Feet*12)-Focal Length in Inches)/Focal Length in Inches

Suppose you are using an overhead projector with a 10" x 10" stage. The projector is going to be 25 feet from a 10 foot diameter projection screen. By measuring the projected image at ten feet and using the other calculators on this site you know that the lens on your overhead has a focal length of 12.77 inches. What size circular mask should you cut for the overhead stage? In order to calculate the mask size you would enter 12.77, 25, and 10.00 into the calculator and then press "Solve". The result will be 5.33 inches.

Enter the focal length, in inches.
Enter the distance from the lens to the screen, in feet.
Enter the projected image size, in feet
The required object size or mask size.
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