How do you do that Spotlight/Light Beam effect

Started 2 months ago | Discussions thread
Ed Shapiro
Ed Shapiro Regular Member • Posts: 328
Re: How do you do that Spotlight/Light Beam effect

Rico Tudor wrote:

Of course strictly speaking the inverse square law ONLY applies to the theoretical point light source, but is a good rule of thumb for most photographic light modifiers, but not so good for focused or parallel light.

Laser light is indeed coherent but also collimated—the characteristic required to project a "spotlight" over some distance with sharp edges. Unfortunately, that beam is only pencil wide! Similarly, a theoretical point source could be collimated by a followspot or projector fixture but the beam diameter would equal that of the front element: again, unworkable for human subjects. All lighting instruments for photographic use are very far from providing a point source, and the spotlight effect is created by focussingthe optical stop. Inverse square is then applicable in mathematical terms, and demonstrable by simple experimentation. The sun is not a point source, either: shadow edges quickly become fuzzy with distance, and projecting an image of the sun with any lens will obey inverse-square. Try it.

The sun is my favourite light source- at 93-million miles away, it does a heck of a job.  Condensing solar light and heat with a lens was my specialty as a kid going through my  '"pyro" stage. Saved me lots of money on matches for setting fire to cockroaches!  I did have a so-called point light source in a graphic arts enlarger that I purchased second-hand.  Sharp and heck and contrasty for line copy but showed up too much dust and gran- changed it up for a cold-light housing.

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Ed Shapiro- Commercial and Portrait Photographer. Ottawa, Ontario Canada

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