Proper use of a loupegreenspun.com : LUSENET : Large format photography : One Thread
This originally came from Bob Salomon at HP Marketing, via a usenet discussion. I send it on with his okay. I thought it might be of interest largely to large format newcomers; it seems like really good advice.
"To use a loupe properly you must do the following:
1: Loupe must be focusing type! 2: Remove the lens from the camera 3: Point the lensless camera at a light source 4: Place the loupe on the ground glass 5: Focus the eyepiece of the loupe till the grain pattern of the ground glass is sharp 6: Replace the lens
You now have a loupe that is focused on the grain of the ground glass. Not on the back side of the glass.Using a non focusing loupe or an improperly focused loupe, or a reflex viewer you are focused on the back side of the glass and are out of focus by the thickness of the ground glass and possibly the fresnel package."
-- Ellis (firstname.lastname@example.org), December 11, 1998
To compound the focusing problems with a loupe, frenel viewing screens used together with short focal length lenses send the light rays out at an angle to the ground glass plane, e. i., not at 900 to the ground glass. This can make it difficult to use the loupe as it is off-axis to the colimated rays coming from the frenel. My trick is to turn my loupe over and focus. That way I can adjust angle and distance from the frenel/GG package manually to achieve a good image from the loupe. When the frenel rings are in sharp focus, you are focussed in the right place. This works only with short-barrelled loupes. Long barrels keep your eye too far away. ;^D>
-- Doremus Scudder (ScudderLandreth@compuserve.com), December 11, 1998.
When this happens I flip my Schneider 4x around and look through it backward and match my viewing angle to the angle of the light coming through the lens and back. I can then focus on the image formed on the grain in the groundglass.
-- Ellis (email@example.com), December 11, 1998.
What I found worked well was to invest all of 9.95 in a pair of drug store magnifiers 3x or whatever the aging eyes need. Lot easier and just as effective as a loupe (and a hell of a lot cheaper).
-- Pete Smith (firstname.lastname@example.org), December 13, 1998.
And the answer is: Ellis. You are so correct Ellis in realizing that the light rays first of all are always seen on the front of the GG unless you have it in backward. That's the reason the GG is frosted. To give the light a place to scatter thereby creating an image. And secondly the light rays do not stop at the GG and make a turn to come straight out perpendicular to the GG only. If you want to get the brightest image you can when focusing the image then follow the angle that the light is coming through the lens past the GG and look along that same angle. Put on a lens and stop down to f8 and try both ways. You will see a brighter image looking at the GG at the same angle as the light rays are traveling through the GG. Try it. That is why I have used a short loupe to focus and not a focusing loupe like a Toyo or schnieder. They require you to put the front of the barrel "on" the GG. When you are at the corners you experiance light falloff. When you look along the angle of the light rays going through the GG you can on longer focus. I use an old 80mm enlarging lens which works quite well.
-- james (email@example.com), March 28, 1999.