Negative Concave Lens - Creating a Beam Expander

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Discussion Overview

The discussion revolves around the concept of a negative concave lens, particularly in the context of creating a beam expander using various lenses. Participants explore the definitions, characteristics, and implications of lens types, focusing on the confusion surrounding the terminology and measurements associated with concave lenses.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant seeks clarification on what constitutes a negative concave lens, expressing difficulty in finding such a lens for their project.
  • Another participant suggests that the term "negative concave lens" is somewhat tautological since concave lenses inherently have negative focal lengths.
  • Some participants discuss the implications of focal lengths in lens formulas, noting that a positive focal length corresponds to convex lenses while a negative focal length corresponds to concave lenses.
  • There is mention of the potential for confusion in lens markings, with some lenses labeled with negative values and others with positive values, possibly due to manufacturer differences.
  • A participant emphasizes the importance of understanding the size and sign of the focal length in characterizing lenses, asserting that a negative lens diverges light.
  • Another participant introduces the concept of "concavo-convex" lenses used in spectacles, which adds to the discussion about lens shapes and their applications.
  • One comment diverges from the main topic, discussing lens bending as a process that can alter lens characteristics without changing their fundamental properties.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement regarding the terminology and characteristics of negative concave lenses. While some agree on the tautological nature of the term, others maintain that it is relevant in specific contexts. The discussion remains unresolved with multiple perspectives on the definitions and implications of lens types.

Contextual Notes

There is ambiguity regarding the definitions and measurements of lenses, particularly concerning how different manufacturers label their products. The discussion also highlights the complexity of lens characteristics beyond simple categorizations.

tryingtolearn1
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I am trying to create a beam expander. I purchased 3 double convex and 3 double concave lens (200mm, 300mm and 500mm). But for the diverging concave lens I need a negative -500mm lens. What exactly is a negative concave lens? I am unable to find anything on amazon or any other site that has a negative mm concave lens.
 
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tryingtolearn1 said:
What exactly is a negative concave lens?
There seems to be no dearth of info on the Innertoobs about it. What is confusing you?
1605224546745.png
tryingtolearn1 said:
negative -500mm lens
I don't know this for a fact, but I'm pretty confident in my deduction. Its focal point will be 500mm behind the lens:

1605224689625.png
 
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DaveC426913 said:
There seems to be no dearth of info on the Innertoobs about it. What is confusing you?
View attachment 272550
I don't know this for a fact, but I'm pretty confident in my deduction. Its focal point will be 500mm behind the lens:

View attachment 272551
I see, ty! The thing that confused me is that I have seen concave lens that is marked with the measurement of -75mm and other concave lens that are marked with 75mm so I didn't know what the difference was between the two. It may have just been a particular manufacture who marks there measurements that way.
 
tryingtolearn1 said:
What exactly is a negative concave lens?
The statement is a bit confusing as negative and concave effectively mean the same thing.

A concave lens has a negative focal length so calling it a "negative concave" lens is somewhat tautological.

A convex lens has a positive focal length.

When you use the formula 1/u + 1/v = 1/f, f will be a positive number for a convex lens and a negative number for a concave lens.
 
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Frodo said:
The statement is a bit confusing as negative and concave effectively mean the same thing.

A concave lens has a negative focal length so calling it a "negative concave" lens is somewhat tautological.

A convex lens has a positive focal length.

When you use the formula 1/u + 1/v = 1/f, f will be a positive number for a convex lens and a negative number for a concave lens.
Again, I'm not an expert in optic, but...

Concave and convex describe a face of a lens. Every lens has two faces.

It is possible to have two different lenses - both having a concave and a convex surface - yet one can be positive and one negative.

Look at examples 3 and 4 in the pic I provided:

1605293238748.png
 
Thank you for the clarification though I think you are off topic.

The original post said "double convex" and "double concave" and asked "What exactly is a negative concave lens?" I was answering this limited case - what is a negative double concave lens? My answer is then correct.

The important fact for the OP to take away is that the size and sign of the focal length characterises the lens (ignoring the aperture). A positive lens converges a parallel beam of light to a focus. A negative lens diverges a parallel beam of light.

I think my comment still stands in that "negative concave" is tautological. As there is no mention of convex it suggests both faces must be concave making the negative superfluous. And, if one is allowed to use "negative concave" presumably one can also use "positive concave" which, is, I think, impossible with two concave faces.
 
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Many spectacle lenses are ‘concavo convex ‘ to keep a strong positive lens away from your eyelashes.
‘
 
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Off topic, but you can distort a lens in a process known as lens bending where the lens keeps essentially the same characteristics.
lens bending.png
 

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