Re: viewer lenses

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>Hello, all.  I am new to the list.  I'm so new, I haven't even taken a MF
>photo yet.  Greetings to all.

  Hi Tom. Welcome aboard.

>I've been hunting high and low for viewer lenses. I need to whip up a
>viewer before I can see my slides.  I did find some mounted 31x98 coated
>doublets at a reasonable price, but I'm wondering if the focal length is
>too long.
>
>Since I've so little experience, I don't know how visible the difference
>will be.  Is the 98mm of the viewing lens too far from the 80mm of the
>taking lenses?  Close enough?

  Well 98/80 = 22.5% stretch. If you've ever viewed
Realist/Kodak slides in a Red Button viewer (44mm lenses)
then you've seen 25.7% stretch. If it didn't bother
you then the MF viewing won't either.

>I also bought a pair of 36x92 achromats, unmounted.  I guess I'll see what
>I can build from them.
>
>Any thoughts?

  Alan Lewis is the expert on surplus lenses.
To summarize what he's said in the past (accurately
I hope), they may or may not work well as viewer
lenses (some may have tons of pincushion, some may
only be sharp in the center etc). Or they may
be surplus because they weren't manufactured right
(so they may not be achromats).

  That said, I have a pair of 36x82mm achromats from
Edmund Scientific that are very nice. Cost me $35 US
each though. Also a bit small for wearing glasses with.
I also have a pair of the 40x84's that Alan uses in the
MF viewer kit that Joel/RMM sells.They seem to be just
as sharp but definitely have more pincushion.

  Try what you have and later you may want to get
some different lenses.

---

  On a more wild suggestion track: I've always wondered
how some of the big buck 6x6 loupes would perform as
viewer lenses. Since you are a New Yorker, perhaps you
could drop in on B&H on a slow day (if they have any :)
and see if they would let you try out some loupes in pairs.

  One question is whether these 3x loupes are 83 mm
focal length or 125mm FL*. If you had one of your lenses
along you could compare see if the loupes were higher
magnification or lower.

  * There are two common ways of calculating magnifier
power: the first is 250/(FL in mm), the other is 1+250/FL.
Loupe manufacturers usually seem to use the second method
(since it gives a higher mag number). So the 3x 6x6 loupes
may not be a good choice for viewing.

S'll for now - Greg