Question re. the innards of an eyepiece

[Probablynot]

I can understand how several lens elements, individually, can affect the path of light through the eyepiece.

What I don't understand is how, or why, there would be a difference between (say) two lenses cemented together, and a single lens made to the same (combined) design.

Are the lenses of a doublet or triplet cemented only at the edge? Or does the light have to pass through the cement? Is the cement as optically good as the glass?

Can somebody please explain? (I recall we 'did' prisms and lenses in Physics classes some 60 years ago, but I don't think they covered these points!)

[rwilkey]

I guess it cound be something to do with the refractive indices of the glass, plus you quite often see that the elements are made of different types of glass and coated respectively, this would be the case in ED glass elements and so on, where the coatings differ.

[coulthamst]

The wikipedia page on "lens" will go some way towards explaining the effects of lenses of differing types and configurations.

[Macavity]

I suppose, in essence it's analogous with:

Achromatic lens - Wikipedia, the free encyclopedia

To supress Ghosting - internal reflections?

The light rays don't travel (deviate) through a big enough of air (oli) gap to significantly change the basic optical idea...

[Capricorn]

2 lens have 4 faces. Call them F1 F2 F3 F4 as you go from left to right.

If the lens is cemented then F2 = F3.

If the lens is seperated then this is not necessary.

So it allows more flexibility in designing the doublet.

2 lens cemented together allow 2 different glasses to be used so this enable an achromatic lens to be design and built. 2 wavelengths are made to come to the same focal plane, in a single lens all wavelengths come to different focal planes - lots of chromatic aberation.

They are not cemented at the edge only the whole face is cemented to the mating face.

[Probablynot]

Thanks Capricorn. I was overlooking the fact that different glasses could be used, with different refractive indexes (indices?). I can now see the point of combining lenses into doublets etc.

I'm still curious about the cement though. Presumably it has optical qualities of its own which would have to be taken into account, even if the cement film between the lenses is microscopically thin. Oh, and Macavity's hint at oil-filled units brings in another complication! I think I'd better stop asking, now that you've kindly answered my primary question!

[michael.h.f.wilkinson]

It is not so much differences in refractive index, but differences in dispersion that matter most. Dispersion is a measure of how the refractive index of glass changes as a function of the wavelength of light. Suppose I have a weak negative lens made from a high dispersion glass, and I combine this with a strong positive lens made from low dispersion glass. The positive lens will focus blue closer to the lens than red. When I insert the the negative lens, it puts both the red and blue focus away from the pair of lenses, but pushes the blue out more (because of the higher dispersion). If I do the calculations right I, I can get a net effect red and blue focusing at the same point.

Before the invention (at Carl Zeiss) of anti-reflection coating, it was best to shape the two lenses in such a way that they could be cemented together, because this removes two glass-air surfaces, which each would reflect 5% of the light. This would not only lead to light loss, but also to loss of contrast through internal reflections.