Optical Glass ABBE , MTF, Aberration and using them as flat filters

Mustafa Umut Sarac · Jul 10, 2014

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 ?

Umut

Xmas · Jul 11, 2014

Mustafa Umut Sarac said:
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 ?

Umut

Hi Umut

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.

Noel

ps the lens designer has a PC application program that designs an optimum lens and tells him the expected MTF

Mustafa Umut Sarac · Jul 11, 2014

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 ?

Umut

Xmas · Jul 11, 2014

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?

Mustafa Umut Sarac · Jul 11, 2014

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 ?

Xmas · Jul 11, 2014

The optical glass is different from window glass.

http://www.britglass.org.uk/optical-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.

Noel

DREW WILEY · Jul 11, 2014

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.

DREW WILEY · Jul 11, 2014

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.

DREW WILEY · Jul 11, 2014

Back to it - there's a tremendous amt of interest nowadays in optically coating window glass for low-e energy efficiency purposes. The fundamental problem is that ordinary float glass is not flat enough to accept an even coating. The biggest US (if not international) mfg of float
glass is PPG, and they estimate that less than 1% of their product is suitable for even a basic single vacuum-deposition coating. This I'm not
guessing about, though my exact figures might be a bit out of date. The company I work for is a major window distributor and service contractor - the biggest in the Western US, in fact, though I run a different dept myself. Then optically-coated picture frame glass started showing up. Some people tried this in enlarger carriers, with so-so results. But in front of a lens? No way.

Optical Glass ABBE , MTF, Aberration and using them as flat filters

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