# Thick Lens focal length and surface facing the object

Hello Forum,

In the case of an ideal, thin lens (free from any aberration), it does not matter which face of the lens is facing the object. The results will be identical.

What if the lens was an ideal (no aberration) thick less with the H planes, etc...? Would it matter which lens surface is facing the object? Would the effective focal length change if we turn the lens around?

If we considered aberrations, I know that in the case of a plano-convex lens, if the incoming illumination is collimated, it is better to have the curved surface face the source to reduce spherical aberration. A plano-convex is even better than a bi-convex lens with that type of illumination to reduce SA...

thanks
fog37

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Simon Bridge
Homework Helper
Look up "back focal length" and "forward focal length"... is that what you are thinking of?
In general, a thick lens can be thought of as a system of lens components ... then you are asking if a system of lenses could be constructed so the view is different depending on which side you looked through it.

Bandit127
Gold Member
Are you asking about the difference in the simple equations for lens power both with and without reference to the lens thickness?
https://en.wikipedia.org/wiki/Lens_(optics)#Lensmaker.27s_equation

If so they do not care about aberrations, spherical or otherwise.

Simon is talking about compound lenses which require a more complex analysis.

And why are we in an electrical forum anyway? I will ask the mods to move this thread to a more appropriate forum.

Simon Bridge
Homework Helper
A thick lense can be modelled as a series of optical components, esp in the paraxial approximation. So no, I am not talking about a compound lense specifically but more pointing op towards a way tomanswer the question.
More generally, any optical system can be modelled via its transition matrix.

What if the lens was an ideal (no aberration) thick less with the H planes, etc...? Would it matter which lens surface is facing the object? Would the effective focal length change if we turn the lens around?
If it is indeed "ideal" with no aberration, then by definition it wouldn't matter which surface is facing the object. It only matters when the lens is not "ideal" as in your example of a plano-convex lens and collimated light.

Khashishi
A carefully designed aspherical lens will only be close to ideal from one side. If you turn it around, it will become very non-ideal, unless the image and object distances are the same (in which case you have symmetry). A rigid lens can only be made to ideal for a given object distance and image distance. If you change those distances, you have to change the lens. That's why camera lenses use complicated compound lenses-- to try to give good images over a wide range of object distances.

The EFL will change if you turn the lens around.

Simon Bridge
Homework Helper

The EFL will change if you turn the lens around.
The Lensmaker's Equation gives the same focal length for either direction.

Khashishi
Hmm, you might be right. But if I look through my eyeglasses backwards, I feel like the focal length has changed (and the aberrations go all to hell).

Hmm, you might be right. But if I look through my eyeglasses backwards, I feel like the focal length has changed (and the aberrations go all to hell).
Are you looking through the same lens when you turn them around?

Simon Bridge
Homework Helper
Why not check? Do some math?
Reversing the direction through the lense amounts to reversing the order of the ray transfer matrixes.
If you check your eyeglasses, you'll see one surface is concave and the other convex, with differen't radii, and there's a gap netween them.

https://en.m.wikipedia.org/wiki/Ray_transfer_matrix_analysis