Thus if the aperture is one eighth the focal length, it is marked f/8, if a sixteenth f/16, and so on.
This would also be about the focal length of the finished lens.
The tube was thereby enabled to be shortened by twice the focal length of the mirror in question.
B is a double convex lens, which has a focal length of twenty inches.
The diameter is proportional to the focal length, but the difference of effect upon the details is very important.
If the lens is a plano-convex the focal length is twice the radius of its spherical surfaces.
Note down this focal length, and proceed to measure your diaphragms in sixteenths of an inch.
The focal length of the lens was 150 feet, and the mirror was placed about 15 feet within the principal focus.
The size of the wire and the other measurements will depend upon the size and focal length of the lens.
As the diameter of the lens was so small in comparison with its focal length, its want of achromatism was inappreciable.
focal length n.
The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.