? re: ED glass (1 Viewer)

I'm sure this is going to prove a monumentally stupid question, but I still just have to ask it.

As I understand it, when light hits the objective lens of an optic it has not yet been "split," nor will it be until it reaches the prisms inside the optical barrel.

So, assuming that's true and since nothing bad has happened to the light at the objective lens, how does an ED glass objective lens help anything?

Two completely unrelated issues lucznik.

Objective lenses suffer from chromatic aberration even if there is no prism in the optical path, and there is destructive interference in roof prisms no matter what kind of objective glass is used.

Henry

I probably should have guessed that. I have, after all, noticed CA in some of my eyeglasses and they certainly don't have any prisms involved.

So then, what is it that actually causes CA?

You can Google plenty on the subject. The information here is mostly OK, except for a promotional bias toward the particular ED glass that Zeiss calls FL:

http://www.zbirding.info/Truth/colorfringing/color.htm

Or you can find out more than you ever wanted to know here:

http://www.telescope-optics.net/chromatic.htm

Wikipedia has a good article on CA including it's causes (and why high Abbe number glass helps).

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

lucznik said:

I probably should have guessed that. I have, after all, noticed CA in some of my eyeglasses and they certainly don't have any prisms involved.

So then, what is it that actually causes CA?

Click to expand...

So called dispersion. The speed of light (actually any electromagnetic wave) in any environment other than vacuum depends on wavelength (and thus on color in case of light).The refractive index of a medium is a simple function of speed of light in that medium: n=c/v where v is speed in a medium and c is a speed in vacuum. The refraction angle depends on n, thus depends on the color of light. When passes through any piece of transparent medium – f.i. glass - light refracts twice on two interfaces between air and glass. On each interface it is ‘split’ into different wavelengths (colors) – it means that every color propagates in slightly different direction. If top and bottom surfaces of that piece of glass are parallel the ‘split’ effects on them compensates and different colors propagates in the same direction. If the interfaces are not parallel – as it is in every lens – compensation is never complete and different colors propagates in slightly different directions. For example – if the parallel beam on light passes through a lens the red color beam propagates in direction angled 12 degrees to the optical axis, and the blue in direction 12 degrees and 10 minutes to axis. It means that these two will be focused in different points. That is what we see. You do not need a prism to split the light. In the prism these effect is magnified but it is present in every lens…