Are Stan Bray’s Optical centre punch dimensions correct?

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Are Stan Bray’s Optical centre punch dimensions correct?

This topic has 4 replies, 2 voices, and was last updated 18 February 2025 at 21:23 by Andrew Tinsley.

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18 February 2025 at 17:58 #783970
Andrew Tinsley
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@andrewtinsley63637
I thought it about time that I made an optical centre punch, as I am having some difficulty with aging eyes! Checking the MEW index there is an article by Stan Bray in the first MEW issue, reprinted in 25 year anniversary special. I was about to make a start when I noticed that the focal length of the device was approximately 50mm and the radius of curvature of the plano convex lens is given as 25mm.

This set some dim alarm bells ringing, My rusty optics say that the “thick lens” formula was 2F = R, where F is the focal length and R is the radius of curvature. I did a quick check and this is indeed the correct formula (derived with a little approximation!)

The implication is that Stan had used an incorrect formula of F= 2R to derive his dimensions? So has anyone made the optical centre punch and did it work? Alternatively am I making an obvious a misunderstanding of the optical theory?

Andrew.
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18 February 2025 at 18:11 #783978
Fulmen
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@fulmen
I just made two of them: https://www.model-engineer.co.uk/forums/topic/magnetic-optical-punch/

The first was 55mm long and will focus with R=25mm but not with R20. Thee second was 50mm long and works with R20.
18 February 2025 at 19:52 #783996
Andrew Tinsley
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@andrewtinsley63637
Hi Fulmen,

Thanks for that very useful information. So it would appear that Stan Bray’s dimensions are essentially correct, as you have shown practically!

It must be my interpretation of the thick lens formula that is faulty. I shall put on my analytical specs and see if I can get to the bottom of this puzzle.

Thanks again,

Andrew.
18 February 2025 at 20:14 #784002
Fulmen
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@fulmen
Not quite. The 2F=R would suggest that the 50mm lens shouldn’t work with R20 which it does. One thing lacking in your formula is the refractive index, that might be the difference.
18 February 2025 at 21:23 #784009
Andrew Tinsley
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@andrewtinsley63637
I was taking the refractive index as 1.5 for Perspex, before simplifying. I think that it should be a touch less, but that would not make a great deal of difference.

The full formula gets a touch complicated, but essentially it is

1/F = (Ni – 1) x ……………… where Ni is the refractive index. So (Ni – 1), is where the 0.5 comes from. After simplifying the rest of the equation, we finish up with.

1/F = (Ni – 1) { 1/R }

1/F = 0.5/R

F = 2R

So I got it wrong and Stan Bray was right. Looks as though I need more than new spectacles!!!!

A red faced,

Andrew.
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