Tiffen screw-in filter are made of green glass (the glass looks green when viewe d from the edge). Their large professional-grade filters use clear glass (also k nown as water glass). Does anyone know if clear glass is really better than gree n glass? Is there a reason to spend twice as much for clear glass? I know Heliop an and B+W use clear glass, but they do not make all the colors that I want (58, 47, 47b).
The green tinge that is typical of soda-lime glass might be detrimental if used in lightly tinted color correction filters. It is for that reason as well as the lack of optical coating that professional motion picture cameramen and discerning still photographers might prefer the filters made of Schott glass. I believe the filters you reference are so dark to start with, I doubt you could tell the difference in the results anyway.
How to Measure magnification
How can I see how strong a magnifying glass is?
I bought this magnifying glass and I want
to know how to test it to see how strong it is. I hear a lot of people talk about
magnifying and how strong the magnification is, but I would like to know the true
magnification of my magnifying glass. I have a few of them and some seem stronger then
others. How can I rate these? How can I pin a correct number on mine? How do the companies
that make these come up with these numbers.
The magnifying power of a loupe works always together with the eye. The same lens has different magnification if used as reading glass, lens close to the e. g. newspaper, or used as a loupe, lens close to the eye.
If the lens is used as a loupe, the magnification (Gamma' L) is Gamma' L = a s / f. (a s is the normal viewing distance of the eye of 250mm.)
To measure the magnification power of a loupe focus a distante subject to the ground-glass of your LF-camera and measure the distance between the middle of the lens to the groundglass. This is the focal-lenght of the loupe. E. g. a lens with a focal length of 62,5mm has a magnification power of 4x. (250 / 62,5 = 4)
This is only true for the "normal" eye. A myopic, short sighted eye sees a larger magnification.
Last edited by Peter K; 11-Dec-2008 at 03:58. Reason: formula added
I think you will find the message about magnification is nothing but an advertising link to a website selling magnifying glasses ???Originally Posted by Peter K
That's a possibility Joanna, but my explanation works for LF-photographers too
Just to keep our nomenclature straight, water glass is sodium metasilicate (Na2SiO3) aka sodium silicate, which has nothing to do with filters. If the glass is "water clear" then it's optical glass.
They are ill discoverers that think there is no land, when they can see nothing but sea.
-Francis Bacon
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