The “Gauss lens equation,” as it is known, goes like this:

where

f = focal length,

g = distance to object (**measured from object side principal plane H**).

b = distance to image (**measured from image side principal plane H’**).

It can be solved for focal length, object distance and image distance.

## We may find the focal length by this equation:

## We may find the object distance by this equation:

## We may find the image distance by this equation:

## Interpretation of the lens equation

What can be derived from this formula?

And therefore

(but subtract the distance of the principal points from X)

**object side**(!) of the lens.

The object and image distances are calculated from the primary planes on the object and image sides, respectively.

As a result, the object distance differs from the working distance in most cases.

As a result, the object distance differs from the working distance in most cases.

The focal length is a paraxial concept. Therefore the Gauss lens equation is only valid in the paraxial region of the lens, the region where holds. Most calculators don’t care (including ours), As a result, anytime larger viewing angles are involved, the results are essentially a “informed guess.”