What are uses for lens focal points?

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    Focal Points
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Discussion Overview

The discussion revolves around the uses and significance of focal points in lenses, exploring their measurement and applications in various optical contexts. Participants examine both theoretical and practical aspects of focal points, including misconceptions and specific applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the utility of focal points, suggesting they are not constant and primarily useful for specific applications like burning ants, while emphasizing the importance of focal planes for understanding magnification and numerical aperture.
  • Another participant seeks clarification on a misconception regarding the relationship between object distance and focal points, specifically how placing an object at the focal point of a convex lens results in an image at infinity.
  • A participant mentions the concept of measuring object distance from an imaginary point between two convex lenses, questioning the implications of measuring from the centers of the lenses instead.
  • One participant proposes that placing a light source at the focal point of a lens can produce a parallel beam, referring to this process as collimation.

Areas of Agreement / Disagreement

There is no consensus on the utility of focal points, with some participants questioning their relevance while others highlight specific applications. Misconceptions about focal points and object distances are also debated, indicating unresolved differences in understanding.

Contextual Notes

Participants express varying assumptions about the nature of focal points and their practical implications, with some relying on specific definitions that may not be universally accepted. The discussion includes unresolved questions about the measurement of object distances in lens systems.

mkbh_10
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Can you tell me the need & use of the pocal points on a lens , why do we need to have them , why do we measure them .

focal points can be used for various measurements but other than that ?
 
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A focal point is not much use at all, since it is not a constant point... maybe useful for burning ants, but that's about it. The location of the focal *planes*, however, are of use- they give you an idea of the magnification of an object, for example. They also help determine the numerical aperture, which sets the resolving power of the lens.

All lenses/lens systems have 6 planes of interest: 2 focal planes, 2 pupil planes, and 2 nodal planes. The optical field at the pupil and focal planes are related via a Fourier transform. Telecentric lenses place one or both of the pupil planes at infinity.

One common misconception I see is that the focal distance is confused with the working distance- that is, the assumption is made that a lens with a 200 mm front focal length needs to be 200 mm from an object to correctly focus it.
 
Would u please clear that misconception that u have stated , if the object is placed at the focal point of a convex lens then image is formed at infinity & if the object at infinity is focused then it will come to focus at focal point of a lens
 
Question related to combination of two convex lenses ?

The object distance (u) is measured from an imaginary point b/w the two lenses , why so , it does not exist in reality but we use it to find out (V) .

If the object distance is measured from the center of 1st lens or 2nd lens ,will it make any difference ?
 
I didnt get u ?
 
mkbh_10 said:
Can you tell me the need & use of the pocal points on a lens , why do we need to have them , why do we measure them .

focal points can be used for various measurements but other than that ?

One application is if you want to produce a "parallel beam" with a small bulb.
How do you do this? You put the bulb at the focal point of the lens. Have a look at http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter2_files/image040.gif . This is also called "collimating" the light from the bulb.
 
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