help with reflections

**eddie** · 14-Mar-2009, 05:36

hello all,

i am having trouble with trying to determine a lens formula using the reflections i see while shining a light in it.

i have a few lenses that i know what they are. petzval, RR, etc. i am still having trouble "seeing" it. and then to make it worse when i put the elements back into the barrel and try to do it assembled i am lost.

does it matter which end i shine the light from? am i looking for strong and weak reflections or big and small? how close should i hold the light? what kind of light works best? any tips on figuring it out with out disassembling the lens?

thanks for the help.

eddie

**Paul Fitzgerald** · 14-Mar-2009, 07:45

"how close should i hold the light?"

a single overhead 60watt bulb works well

"does it matter which end i shine the light from?"

not really, it's easier to check each cell alone by closing the aperture.

"am i looking for strong and weak reflections or big and small?"

look for strong and weak, large and small will differ by formula

have fun with it, practice makes perfect.

**Ole Tjugen** · 14-Mar-2009, 08:34

Strong reflections are form glass/air surfaces, weak reflections are from glass/glass surfaces.

Large reflections are from flat(tish) surfaces, small reflections are from strongly convex surfaces. Very large reflections can be from weakly concave surfaces.

When you tilt the lens/cell relative to the light, reflections from convex surfaces will move in one direction and those from concave surfaces in the opposite direction. Compare the front cell of a Petzval with a (front) cell of a RR to see this.