Optical Glass ABBE , MTF, Aberration and using them as flat filters
Is there such a thing like optical glass MTF curve ? Do flat cut pieces of glass have a specific sagittal tangential MTF Curve ? We coukl order a optical glass filter from schott and go on from there ?
Is refractive index change per color , also a aberratiion description , how we can know what aberration would the glass give ?
Originally Posted by Mustafa Umut Sarac
You need to visit library for book on optics.
The refractive index is a measure of how much a prism bends light
The dispersion a measure of how much the red and blue light bend varies
The abbreviations in a lens are a function of the curved surfaces,
- aspheric surfaces reduce this problem.
- very high refractive glass also reduces this problem because for the same light bend the optic surfaces are less curved.
ps the lens designer has a PC application program that designs an optimum lens and tells him the expected MTF
Last edited by Xmas; 07-11-2014 at 05:24 AM. Click to view previous post history.
Noel , do flat glass disperse also and any dispersion is a aberration and any aberration reflects as MTF graph.
Am I correct at that point ?
A multi coated uV filter might just show on a good lens on a MTF machine (as reduced contrast).
But other than that a plane glass has no effective aberration, though the Nikon rangefinder 35mm /1.8 circa 1958 used a plane corrector plate..., not simple.
The IR cut filters that some used on Leica M8s were horrible even on 5cm lenses... but that was light reflected of sensor. Very prevalent in UK in spring and fall with low Sun even with lens hood.
A night shooter needs to remove a multi coated filter or get ghost images of light reflecting from film surface as above.
Please can you ask easier questions?
The basic idea behind high quality glass or gel filters is "optical flatness", meaning theoretically/ideally the target is no refraction, no reflection (ie maximum transmittance of desired wavelenghts/spectrum) and no dispersion or other aberrations.
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Thank you Michael. Michael , is it true for both wave theory and ray theory ?
Is cheap window glass and expensive glass the same. Sounds like not normal but I dont know ?
Last edited by Mustafa Umut Sarac; 07-11-2014 at 11:20 AM. Click to view previous post history.
The optical glass is different from window glass.
The aspheric glass used for the smaller diameter optical elements has
very high refractive index
is like honey just above its melt temperature
will solidify after crush between precision ceramic blanks in hydrulic ram press with out crazing
and only need final polish when it cools
two or three rare earths work this magic.
There are many different types of optical glass as you know. But yes, high quality glass filters are different than windows. There are also coatings to consider. Glass optical filters are usually coated or multi-coated with special coatings both to minimize reflection (maximize transmittance) and for protection. Since a system is only as good as the weakest part, high quality filters from Schneider etc. (most use Schott glass) are designed to be as good as the lenses they are put on.
Originally Posted by Mustafa Umut Sarac
I went through a lot of this pain when I made some negative carriers a few years ago. I talked a lot with Schneider in the U.S.
You have to go to the specific manufacturer of the glass (Schott, Hoya, etc). They'll have the relevant specs. How you interpret that kind of
data requires some background. There are many glass types, and how they're coated alters certain properties. It's a complex subject, and I'm
not an optical engineer, but have had some rather thick resource books on hand from time to time, and have spoken to various engineers
concerning personal projects. I don't think MTF is a relevant variable when working with blank materials. You start with different properties.
Oh the other question, cheap window glass and expensive window glass ain't the same. All kinds of that stuff too, for all kinds of different
purposes. But I wouldn't use any of it on an optical device of any type.