Negative Concave Lens - Creating a Beam Expander

[tryingtolearn1]

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.

 

[DaveC426913]

What exactly is a negative concave lens?

There seems to be no dearth of info on the Innertoobs about it. What is confusing you?

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:

 

[tryingtolearn1]

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.

 

[Frodo]

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.

 

[DaveC426913]

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:

 

[Frodo]

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.

 

[sophiecentaur]

Many spectacle lenses are ‘concavo convex ‘ to keep a strong positive lens away from your eyelashes.
‘

 

[Frodo]

Off topic, but you can distort a lens in a process known as lens bending where the lens keeps essentially the same characteristics.