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How Universally Useful is the Inverse Square Law?
 If a nail bomb exploded in outer space the number of nails that hit you would fall off according to the inverse square law. It isn't that each nail "fades" with distance, it's just that the scatter pattern spreads with distance. It's the same with light. Light really only obeys the ISL when it is expanding spherically from a point source. ISL falloff is not an innate property of light. For may daughter's 6th grade science fair experiment she chose to see what effect light modifiers have on the ISL. Just how badly does it fail when the source is not a point? I was kind of curious, myself... Feb 01 10 09:04 pm Link     Feb 01 10 09:04 pm Link So it's a pretty good approximation most of the time. Feb 01 10 09:06 pm Link For the soft box were you measuring from the fabric of the front or from the bulb? Feb 01 10 09:13 pm Link This inverse square law has ruled my life for the last 30 years.... mostly in a life before photography. It's very accurate. The only consideration is that if the off-axis light hits a reflector, it can contribute a bit more light back onto the subject, so that the light may not fall off as much as the inverse square law would predict. Reflectors are not always what you think of as reflectors... a white concrete sidewalk or driveway front of a subject, that's a form of a reflector... The more perfect the reflector (like a mirror) the more the contributing effect. the only exception to the inverse square law is columnated light, like from a laser. Feb 01 10 09:13 pm Link BendingLight wrote: Or from a fresnel lens or a parabolic reflector, or the walls of a fiber optic tube, or near a very large light, or inside a sphere, where exposure is equal at every point throughout the volume. Feb 01 10 09:17 pm Link Hoodlum wrote: front of fabric. Feb 01 10 09:17 pm Link A lot of things fall off according to the inverse square law, and in many cases you can approximate the whole volume/surface in question as a point in the center. Gravity's a good example - you can just treat it as if all the mass were in the center of the planet (yes, some points are closer or further away but they cancel each other out). For something like a softbox if you're close to it to really be accurate you would want to integrate over the points that comprise the surface. As you're further away it gets easier and easier to just treat it like a point source and be done with it. Feb 01 10 09:34 pm Link Good post. I like your technical posts. Do you have one regarding grids and how they change the shadow characteristics on a softbox? It makes the shadows harder. Feb 01 10 09:38 pm Link Antonio Marcus wrote: as it turns out, I do Feb 01 10 09:43 pm Link Doug Wade wrote: I think if you need to be super accurate then your weight will change with altitude or depth, but most people don't need that kind of accuracy. Feb 01 10 09:46 pm Link NothingIsRealButTheGirl wrote: holy crap. way to go man. haha. I've been thinking about grids a lot lately. They are not just for "keeping the light on the subject and off the background". Feb 01 10 09:46 pm Link NothingIsRealButTheGirl wrote: This is by far the most simple, easy to understand, and apt analogy for the inverse square law I've ever seen. It reminded me that this is the behavior of photons, which make up all light. A nail bomb in space is just deviously funny too Feb 01 10 09:54 pm Link Brett Michael Nelson wrote: +1 Feb 01 10 10:41 pm Link It's always nice to hear a good story before I go to bed. ;-)  Now the question becomes ....How Inversally Useful is the Universally Square Law? Feb 02 10 02:25 am Link Do you have one regarding grids and how they change the shadow characteristics on a softbox? It makes the shadows harder. With grids/honeycombs/egg-crates on a "smooth" light source, there is a range of distance that defies the inverse square law and yields virtually no fall-off. This range extends from the grid/honeycomb/egg-crate to the point in which the subject is being illuminated by all the cells of the grid/honeycomb/egg-crate. Feb 02 10 02:52 am Link |
