Introduction to Imaging with Lenses

Jiefei WangOPTI 521

December 02, 2008

Overview

• Introduction• Objects & Images• Focal Length• Imaging Equation• Image Movement

Introduction

• OPTI 521: Opti-Mechanics– Optics determines where to mount optics, and

how the elements need to move– Mechanical system holds the optics together to

allow it to:• Focus on objects at different locations• Work over different thermal environments• Change field of views• Survive vibration/sock

Scope

• Paraxial Optics– Valid for points “near” the optical axis– Ignores off-axis affects

• Single Thin-lens– Zero thickness– Refract light perfectly at one plane

Objects & Images

• The object is the item of interest that the optical system will image

• Object radiates the light that is collected and imaged by the optics.

• Image is formed when all the rays of the light are focused by the optics.

Focal Length

Object Distance = infinityImage Distance = Focal Length

• Focal point is the point at which rays from infinity comes to a focus.

Imaging Equation

Object

Image

Focal Point

Object Distance Image DistanceFocal Length

Lens

Object Position vs. Image Position• As an object moves

towards the lens from infinity the image moves further away from the lens.

• Image movement is more sensitive to object movement when object is close

Focal length is 300mm

0 20 40 60 80 1000

2

4

6

8

10

12

14

16

18

20

Object Distance (m)

Ch

an

ge

in I

ma

ge

Dis

tan

ce (

mm

)

Change in Image Distance vsObject Distance

Object Position vs. ∆Focal Length

• To keep the image distance fixed, the focal length needs to change.

• Again the change in focal length is more drastic when the object is close 0 20 40 60 80 100

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Change in Focal Length VsObject Distance

Object Distance (m)

Ch

an

ge

in F

oca

l Le

ng

th (

mm

)

Initial focal length is 300mm

Summary

• Imaging Equation:

• Image tends to move more when the object is close to the lens

• Keeping the image in focus due to object movement becomes harder as the object gets closer to the lens.

Questions?