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

["Rolland Elliott" <rolland_elliott@xxxxxxxxxxx>]

It might be prone to diffraction, but I've never read anywhere that 
described it as "SEVERE DIFFRACTION".

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). Under visible light the lens 
looks just fine. HOWEVER, under IR illumination anodized aluminum looks like 
regular shinny aluminum!

Would you buy a camera lens who's insides were painted a nice shinny silver? 
Probably not. Most photographers would realize this could easily lead to 
internal reflections within the lens and image degredation. Something I have 
experienced with a few of my lenses.

Yet many IR photographers shoot with lenses that for all intensive purposes 
(at least for IR photography) have shinny reflective interiors.

And it doesn't stop there. Many hoods with black felt lining show up as 
hoods with white felt lining under IR illumination.

I have a Mamiya RB 100-200mm lens and it's internals are nice and shinny 
under IR light. It would be a shame if Mamiya's highly touted APO lenses 
exhibited this same problem. Or if Hassy's new line of lenses with 
"improved" internal baffling actually were nothing more than IR reflectors!

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!

Peace, Rolland

>     This might be a basic question, but here it goes.
>     I know that infrared light is prone to severe diffraction so I try
>to avoid f stops of 16 & 22.  My question is what f stop will help
>correct the diffraction problem? f/8 ?

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