The Size-Distance Fallacy

There is a lot of information out there that says a larger light source is "softer" and a smaller light source is "harder." And that the "size" of the light source is also dependent on it's distance as that affects the "apparent/relative/effective" size. This is all true, but it is also rather misleading/oversimplified.

 
A light source is "hard" when the rays reaching the subject are effectively parallel. This occurs at ~5x size distance. Moving it farther away will *not* make it "harder." It only makes it "smaller" which will be evident in catchlights/reflections, but not in the "quality of light" per-se (you can't really make the light rays "more parallel").
 
And the same is true once a light source is "soft," where the rays are coming from many angles and are able to "see around corners" to provide "wrap." This starts to occur at ~1x distance for an equal size illuminated area. Moving the light closer does not make it significantly "softer." It again really only makes it "larger" which will be evident in catchlights/reflections, but not in the "quality of light" per-se (or not to a great extent).
 
For example, a 2ft softbox from 2ft is equally as soft as a 4ft softbox from 4ft away (for a 2ft illumination area). But a 4ft softbox from 2ft away isn't really significantly any "softer." What the larger softbox primarily gives you is additional control within that 2ft to 4ft range where the smaller modifier cannot provide the same softness. I'm not sure I can explain it easily/well without getting overly technical, but think of it this way... there is no real/major benefit or softness to be gained by lighting "farther around a corner" if there is nothing additional there to be lit.
 
The primary control you gain with a larger modifier is control over falloff/contrast, which is *huge* when it comes to controlling what an image is.
 
 
The real problem with small modifiers is not that they are "hard." The main problem is that they must be close in order to be "soft," and that severely limits your ability to control falloff/contrast/gradient. And smaller modifiers limit you to smaller subject/illumination areas to start with.
And the real benefit of larger modifiers is not that they are "softer." The real benefit is in the ability to control falloff/placement/gradient while *also* being/remaining soft.
 
In other words, it is not simply distance ='s size ='s softness. Rather it is size ='s softness (how hard a shadows edge is), and distance ='s falloff/contrast (how dark shadows are); and these two factors must be balanced for the desired result. You cannot simply move a smaller modifier closer in order to make it larger because that *will* affect the falloff and contrast, and there is nothing you can do to avoid that.
 
 
(For pictorial explanations see the other articles on size -vs- distance)
* You can use multiple lights to fill and control contrast
** You can make a larger modifier smaller/harder by feathering it's angle or masking/flagging.

If this raises questions for you, I'm more than happy to discuss it in the forums.
 
 
 
 
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