# Object distance

The object distance or object distance describes the distance between the object to be imaged (object) and the object-side main plane of an imaging optical system composed of optical lenses and / or mirrors .

The image distance is the distance between the image and the main plane on the image side.

## The thin lens

The two main levels of the optical system are identical for thin lenses. Then very simple relationships apply. Let be the focal length of the system, the object size and the image size and the lens equation applies ${\ displaystyle f}$ ${\ displaystyle G}$ ${\ displaystyle B}$ ${\ displaystyle {\ frac {1} {f}} = {\ frac {1} {b}} + {\ frac {1} {g}}}$ Sign convention: if the object is to the left of the main plane on the object side, as in the sketch, then the object distance is positive, and if the image is to the left of the main plane on the object side, the image distance is negative. If the picture is upside down (rotated 180 °), as in the sketch, then the picture size is negative. is positive. ${\ displaystyle g}$ ${\ displaystyle b}$ ${\ displaystyle B}$ ${\ displaystyle G}$ ## Overview for collecting systems (f> 0)

The information for the type of object and image in the table only applies to the illustration on a single thin lens, as in the sketch. The object is real if it lies to the left of the first surface ( refractive or mirror surface) of the system, and the image is real if it lies to the right of the last surface.

No. Property type Object width
g
Image distance
b
Image size
B
Image type
1. real ${\ displaystyle g <-2f}$ ${\ displaystyle f ${\ displaystyle -G inverted real image
2. real ${\ displaystyle g = -2f}$ ${\ displaystyle b = 2f}$ ${\ displaystyle B = -G}$ inverted real image
3. real ${\ displaystyle -2f ${\ displaystyle b> 2f}$ ${\ displaystyle B <-G}$ inverted real image
4th real ${\ displaystyle g = -f}$ ${\ displaystyle b = \ infty}$ ${\ displaystyle B = - \ infty}$ not inverted virtual image
5. real ${\ displaystyle -f ${\ displaystyle b <0}$ ${\ displaystyle B> G}$ not inverted virtual image
6th Borderline case; on the lens ${\ displaystyle g = 0}$ ${\ displaystyle b = 0}$ ${\ displaystyle B = G}$ not inverted real image
7th virtual; right of lens ${\ displaystyle g> 0}$ ${\ displaystyle 0 ${\ displaystyle 0 not inverted real image