Odd question, why not just use the "
Photometrics Handbook?"
Plano
Convex Spot Lights:
250 -400 w. = 5 x 8" to 10"
Focal Length
500 - 1000w. = 6 x 8" to 12"
Focal Length
1500 w. = 8 x 10" to 14"
Focal Length
(don't remember the source of the above.)
Willard F. Bellman's "Lighting the
Stage - Art and Practice" c 1967 has possibly the best info on
Fresnel lenses and their design. p63-67 Three main types of lenses. Also visiting the Kopp Glass website might have more info on the
focal length of a specific
lens. Kopp is the normal manufacturer.
www.koppglass.com
Short answer... yes and no. Depends upon the
lens. For the most part, yes essentially it's a planoconvex
lens, but no, it often will have part of it's beam cut off or modified and if the more
convex the shape of the
lens as opposed to
flat, the more it diverges from the above PC lenses. Also, there is other types of
Fresnel lens.
Could sware I remember seeing somewhere a
base chart that shows
base beam spreads... Backstage Handbook? Yep - Backstage Handbook - or at least my
hand written notes into it...
The
lens is a form of
Plano Convex lens - normally 50 degree at flood and 20 degree in spot mode, or about half the diamater of the
Fresnel lens is the
focal length. (This given a normal
Fresnel lens.)
Further handwritten notes:
3.1/2"
lens has a
focal length of 2.1/4"
6"
lens has a
focal length of 3.1/2"
8"
lens has a
focal length of 4.3/4"
Don't remember where these notes came from.. Possibly
McCandless "Equipment for..." or "Sylibus for..."