Modifiers do not make lights larger

We often say that an umbrella or softbox makes our light source larger and therefore "softer." But this is somewhat misleading. If we were actually increasing the size of the light source at the same power we would expect the brightness of the source to increase and the size of the brightly illuminated area to increase. Neither of these things happen.

A modifier does not change the size of the light source *unless* it is constricting the light source. This is because the modifier is NOT the light source, the flash is. Without a modifier it works like this.

You can see that due to the direction of the light it can only reach areas directly facing the light and the edges/ transitions to shadows are abrubt. We call this type of lighting "hard."

One might also think that the size of the modifier affects the area of coverage. This is also (mostly) untrue unless the modifier is constricting the light source. And this is also because the modifier is NOT the light source and we are not changing its size. You can use a 30inch umbrella or a 7ft umbrella (of the same design) from the same distance and they will cover the same area. And in fact, the differences in the "quality of light" due to the physical size of the modifier will be relatively subtle. This is why you can get reasonable results from smaller modifiers (i.e. diffusion domes, smallish umbrellas) for location and event work.

In this image the red lines represent the front face of two different sized modifiers. You can see that the smaller modifier is also closer to the light source. The net effect on the coverage of the light source is no different than what the larger modifier will be. And the light source is not actually any larger. This will be true as long as the light is not being constricted by the modifier. This is why every modifier of the same type and design will give the same coverage and light falloff *regardless of size. And this is also why smaller modifiers or bare flash works equally well for "event" type photography where you can't typically get close enough to benefit from a larger modifier.


So what is it about larger modifiers that makes them "softer?"

First, one has to understand what a "point light source" is. Generally a source of light can be considered a point source if the resolution of the imaging instrument is too low to resolve its apparent size. What this effectively means is that the source has to "appear to be" one photon in size. 

Everything we use is a "collection of point light sources." And each point source is emitting light in a pattern. (In actuality, the first image with the unmodified flash should show a collection of point light sources)

When we use a light modifier what we are actually doing is redirecting the light into a "larger collection of point light sources." This is the "larger" we talk about and it is the cause for what we actually see when using modifiers.

When we do this we change which areas of the subject can be reached by the light and this is what gives us "wrap" and fills in shadow areas.

When we change the distance of the modifier to the subject we are also changing what areas can be reached by the light. The change in distance is what actually changes the size of the light source. When we change the distance we do see a change in the size of the brightly illuminated area (which is inverse to the area of coverage) and the effective power of the light; exactly what we would expect to see with changes in size. If we are going to work with the lights closer, and also have adequate coverage, then we need a larger modifier.

This is important to understand becuase the distance at which a modifier is used is *more important* than the size of the modifier being used, and it will have a greater effect on the quality of light we get from it. When using a modifier the rule of thumb is that the "sweet spot" distance is equal to the long edge (or diagonal) for a rectangular shape, or the diameter of a round modifier. This is the distance that will allow the maximum "wrap" with the minimum falloff. It is also worth noting that within a distance of approximately 2x the diameter/diagonal of the modifier the inverse square law does not apply and light falloff occurs at a rate more like an "inverse double law." This means you can actually use a modifier from a shorter distance with *less* falloff than you might have at a larger distance. 


We tend to think of light as if it is a point light source "beam of light," but it's not. If it were "a beam of light" distance would have little effect on "wrap" or "softness" qualities. And "feathering" the light would be "detrimental" if anything.

But instead we have this larger "collection of point light sources" which is illuminating the area from multiple directions simultaneously. This is why you generally don't want to point a modifier directly at the subject. What you generally want to do is "feather the light" similar to what is shown in the image above.  What that does is make the light able to reach more of the subject facing towards the camera. It also allows the excess light to hit less of the areas where we don't want it (for example the background). And it allows us to redirect the excess light to where we might want it (for example by using a reflector opposite the light source to fill in the dark side).


So, everything you have ever heard about the size of a modifier affecting the quality of light is true (for somewhat different reasons and with somewhat different results). BUT, when selecting a modifier size, the DISTANCE from which it will be used and HOW it will be use (for what effect) are both critical considerations. Because those considerations DETERMINE the size and type of modifier required.






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