Re: f? (Actaully Subject is about difraction and Poor IR baffling in lenses)

[Helmut Faugel TE <htf@xxxxxxxxxx>]

On Thu, 9 Nov 2000, Rolland Elliott wrote:
> It might be prone to diffraction, but I've never read anywhere that 
> described it as "SEVERE DIFFRACTION".

Well think about pinhole photography ;-)
 
> Instead of worrying about diffraction you should be worrying about what kind 
> of paint job your lens has. That's right, I said paint job! Many many lenses 
> are made out of aluminum that is anodized (anodized is a chemical process 
> that turns the surface of the metal black).

If you anodize aluminium with the normal procedures it does not turn
black. It turns black because some dyes are added, and anyone who has
experience know how black dyes can look like in IR: from bright white
to black.

BTW: A few years ago I designed a power supply and I used some
     black anodized heat sinks. After about one year with a constant
     temperatur of about 65 degrees Celsius the black color has been 
     bleached to grey. 

> I would bet quite a bit of money that the real culprit in many failed IR 
> shots is not diffraction, but poor lens paint jobs! So quit pickin on poor 
> old diffraction and start complaining to the lens designers!

And don't forget that anti reflection coatings are also less efficent
in IR and decreases image contrast.


Helmut

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