Hi Ricky Sharp,
The short answer, if I get your problem correctly, the amount of light will increase with the size (PBR-Area), so you have to adjust for that with the Intensity parameter.
Comparing real light with simulated light will always fall short. Any simulation that would come close to real light, would take with a normal computers forever. Light bounces trillion times and each Photon does its own little path. This can’t be done with any standard workstation in a reasonable time.
I think we both agree to the following initial statement:
If you bounce or filter (diffusion) a light source, that filter or surface becomes the new light source. These two options can also be combined, book light, etc. Since we can adjust those lights, a bounce or filter seems possible of course to recreate in Cinema 4D, but it increases render time. Some people even set up a bounce in Cinema 4D or some even a grid, etc. As with HDRi images, it is a simulation, and especially with out door HDRi it is far from being able to recreate the situation fully.
So, first question, have you adjusted the Light> Details Contrast and the Light Detail Falloff Angle? A flash light is typically sharper than the soft-box, pretty much in any quality.
Lets start with the option: bounce. The material settings are certainly crucial here, and yet we can have a reflectance of 100%, which is certainly not natural, even a mirror has less than 100%. So if you need a perfect match, you have to set those correctly. Since the surface is only simulated, the way a light will scattered will be also just simulated. It is physical based rendering, not absolute physical. We do not even allow for air particles to be included into the calculation, and many sets are using fog, haze, etc.
If we use a soft-box, which one? Which material is used to diffuse? (1/4 Tough Spun, Light Tough Spun, Light Opal, Full Tough Spun, Opal, a 216 [in 1/4, 1/2 or full, perhaps double] 1/4 Grid Cloth or a “Full Grid Cloth) Each of these will produce a loss of light, and each of those will produce a different light quality, noticeable on the penumbra for example. especially the grid cloth qualities allow for a little bit non-diffused light to come through and mix “un-filtered” light with diffused light, which is kind of remarkable. All of these, compared to an open face light can take up to two stops away from the light to begin with, on an average, like a full 216 -1 1/3 stops.
Again, each of these will change the light and its strength. Each soft-box has a different material, they are all different, more or less.
My tip, create an 0.18 material (AKA ~ 50%) or 18% and set up the light/material combination as it suits your needs. Explore all of the adjustments, e.g. color, as diffusion tends to produce a warmer light, especially the stronger filtrations. If you have your studio set up in a comparable way in C4D, you will know how much you need to adjust you light parameters in the future, to come closer.
From my experience, anything white is not a good idea to use as an adjustment reference, since white is not 100%, in integer values it might be or look in that way, but clipping all three channels makes them equally - but that is not congruent with reality. Besides that, even we can adjust the light temperature, Cinema 4D as well as most rendering engines, render in R, G and B not in frequencies, and certainly do not refract those frequencies at all. The point I try to make is, we simulate around a given sweet-spot of simplification and precision. We are getting closer, but real light is just too complex to re-create fully.
Light by itself is not visible, we can only measure it when it hits a surface, (including your light meter). The variables are large and any given number in Cinema 4D is given to come as close as possible to reality. IF we increase the light box (the Area light size) the strength of each area of the light source is equal to a smaller one. Given a certain (short) distance to a soft box or book light, etc, I would strongly argue against an Inverse Square Falloff [ISF], as the with an increase of distance we get more light (again, close to the filter and in the middle of the softbox), just to kill off a little bit the given simplification of light rules. But yes, ISF is certainly true most of the time, and again, not so much with parallel light sources, e.g., Laser as an extreme example.
To transfer the intensity of a flash to an equivalent of a soft-box, the ratio of size and intensity should be taken into account. But also the distribution after the “filtration” or bounce, since softboxes have typically silver material inside, how to account for that, since it is mostly crumbled. On the end, set the lntensity to the value you need, 100% is just never right, perhaps 10,000% or 0.1% might work more, that is your call, based on your material settings and scene set up overall.
All the best
