Description

This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.

01. What is the meaning of the symbols in the rest of this FAQ?

f focal length

So distance from front principal point to object (subject)

Sfar distance from front principal point to farthest point in focus

Sclose distance from front principal point to closest point in focus

Si distance from rear principal point to film (image) plane

M magnification

N f-number or f-stop

Ne effective f-number (corrected for bellows factor)

c diameter of largest acceptable circle of confusion, or the diameter of the circle of confusion

h hyperfocal distance

Here we use the more technical term "object" for the thing being focused on. Informally it is equivalent to the subject. See the technical notes at the end for more information on object distances, more information on the meaning of f-number and limitations to be observed when applying these formulas to lenses in which the aperture does not appear the same size front and rear.

Continue to:

• Index
• next: 02. What is the meaning of focal length? In other words, what about a 50mm lens is 50mm?