With

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’)

The so called “lens equation” reads

## Solving for focal length

## Solving for distance to object

## Solving for distance to Image

## Interpretation of the lens equation

What can be derived from this formula?

- The Image of infinite distant objects is in the focal plane.

When the object moves to infinity (= when g gets infinitely large), then

Say,

- An object that is at double focal length distance on object side is mapped to an image that is in double focal length distance on image side.
- In other words : If we want a 1:1 mapping in distance X from the sensor you have to choose

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

- If the object distance equals the focal length then the image is at infinity.
- The closer an object coming from infinity approaches the focal length, the closer get image distance and focal length. So holds.
- An object in focal length in front of the lens is mapped to infinity on image side.
- For an object closer to the lens than the focal length holds, say, the image is generated on the
**object side**(!) of the lens.