[John Layne]

Hi

I need some help designing a simple lens.

The lens will be injection molded in Acrylic, and will be used in Toy. Picture below details Diameter and thickness. I need to know what the radii of of the lens faces should be to achieve a magnification of 2.5?

If anyone here with some optical knowledge could offer some help it would be much appreciated.

(ignore the radius of 316.51 it's just a wild guess)

[John Layne]

After reading through a number of articles on
Wikipedia. I think, what I require is an optical power of 2.5 dioptres.
Similar to over the counter reading glasses.

[hdesbois]

Do you know the refraction index of your material? Acrylic is often listed at 1.49.

try the lensmaker's equation found here: http://en.wikipedia.org/wiki/Lens_%28optics%29



and in your case 1/F = 2.5, n=1.49 (Refractive index)
be careful with the signs of the radii: it may be tricky!


H.D.

[hdesbois]

Ok, I found 1.96 m radius, both faces. I madea quick LWO file with a lens of theses specs, and it seems to work ok. PM me if you want it.
Regards,
HD

[hdesbois]

update:
I made a quick test:

with 22.5 mm lens of acrylic, two faces 0f 1.96 radius each.
Lens is about 15 cm above the book, and the camera with 55 mm lens is roughly 20 cm from the lens.

HD

[John Layne]

Thanks for the help,

I ended up using the Lens Makers Formulae, and found I needed radii of 392mm to achieve 2.5 dioptres. Someone from the sci.optics forum also ran an analysis using Zmax for me and achieved similar results. Magnification is subject to the distance of the eye from the lens, in my first post I stated a magnification of 2.5x this was somewhat misleading, what I actually needed was 2.5 dioptres (similar to off the shelf reading glasses)

Quote from the Zmax guy below and his files attached.

"I took a cut at this, but I had to assume the wavelengths of operation and the object size.

Given:

Power 2.5 Diopters = 400 mm EFL
Magnification (MAG) -2.5X
Material Acrylic (PMMA)
Aperture Diameter 22.5 mm
Lens Center Thickness 2 mm

The combination of EFL and MAG set the object to lens distance, the lens to image distance, and the object to image distance (1956.86 mm).

Assumed:
Visual Spectral Range C e F wavelengths (0.6562725, 0.546074, 0.4861327 microns)

Object size 2 mm diameter.

The attached analysis shows the imagery is pretty poor over the visual spectral range. For just the green mercury e-line (0.546074), it's diffraction limited."

His comments about pretty poor, in this situation are more than good enough.

Before I received his email I came to a similar conclusion via
http://hyperphysics.phy-astr.gsu.edu/hb ... enmak.html




Other interesting reading.

http://en.wikipedia.org/wiki/Magnification

http://en.wikipedia.org/wiki/Magnifying_glass

http://en.wikipedia.org/wiki/Diopter

[hdesbois]

Thanks for the useful links. Seems I badly miscalculated.
HD