Fresnel Lens Question

[gafftapegreenia]

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?

[Van]

I believe you do it the same way you would for a standard plano convex lens, as essentially that is all it is. For depthe of lens you'd have to multiply the number of steps times the depth per step.

Van looks at Ship expectantly .......

[ship]

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..."

[ship]

All the above info on photometrics is from Sylabus For Lighting The Stage - McCandless. It's in the appendix.

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 Fresnel lens.