How does one figure out hte focal length of a Frensel Lens? Is there any manufacture code or information I am not aware of? Can you use the same method as you would with a Plano-Convex lens to find focal length?
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 Fresnellens.
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...
Good book, (mostly on theater building design and a bit on how lighting design was done to supplement "A Method of Lighting the Stage".) Just got my 1962 re-print of the 1930's edition in from Amazon. The 1962 version is possibly better in going into more modern types of fixtures and dimmers - or at least the concepts behind what we have today, though I would love to see an origional edition.
Both the Plano Convex and Fresnel charts have larger lense specs. listed if anyone is interested for further info - but the differences between the two lenses is presumed to be very accurate given the source. Thus while similar a PC lens is not a Fresnellens.