Size vs Distance

You will often hear that modifiers make your light source larger, and that larger is better. But I disagree to some extent.

In Modifiers do not make lights larger I explained that what modifiers actually do is collect the existing light spread and break it into many more smaller light sources; a "larger collection" of light sources if you will.

And you will often hear that the "sweet spot" distance for a light source is equal to it's diagonal or diameter measurement (some say 1.5x that measurement). That's because this is the distance at which the light becomes "soft." And the reason the light becomes soft is because the larger collection of light sources can see around the edges of an equally sized subject. 


But that's NOT necessarily what we want. Changes in lighting distance matter as much, if not more than the size of the light source.

In these images the camera settings were kept constant. The exposures were adjusted by changing the flash output power in conjunction with the changes in distance. None of the exposures have been modified. The full image can be clicked on to open at 1024x resolution.


This first image was taken using a 5" reflector with a single layer of diffusion from a distance of 8ft. Flash power was 1/32+.3

We have small catchlights in the eyes, strong/hard shadows, specular reflection on the lip.


Now I've swapped the 5" modifier (19.5 sq in) for a 2ft x 3ft (864 sq in) double diffused softbox; increasing the level of diffusion and increasing the size by over 44 times. Power was set at 1/8.

Now the lighting is a little softer, but certainly not 44 times softer. And most of this softness is due to being in a small room. It's mostly spill bouncing in as fill. A 5 inch, 23 degree reflector "focuses" the light a lot more compared to a softbox so there is a lot more spill.

We can tell that is the case by looking at the speculars and shadow characterisitics.

 reflector left, softbox right

We have increased the size of the catchlight and specular highlight on the lip only marginally. And we have softened the shadow transition but not nearly as much as one might expect. If I had any spill control the differences would be notably less significant.


Here I've moved the softbox up to the "sweet spot" of 4ft; increasing the relative size by 2x length and width (4x the area). Power was set at 1/16.
Note that the power requirement is *not* following the Inverse-square Law. The Inverse-square Law says that the power reduction required should be two stops for half the distance, but it's actually only about one stop.


8ft left, 4ft right

The catchlight and specular highlights are larger. The shadow transition is only slightly softer due to the spill helping the 8ft image. And  we are starting to introduce more contrast into the image due to light falloff.


For this image I've again reduced the distance by half to 2ft. Power was set at 1/32.


 4ft left, 2ft right

Again the catchlights and highlights have gotten larger. Shadow transitions are softer/larger. And the contrast has increased (the shadows are deeper/darker).


And finally from 1 ft away. Power was set to 1/64th.


 2ft left, 1ft right

All of the same changes have occurred with a notable refinement of the image. What's more, the overall character of the light has completely changed to more of a "soft glow" with the specular catchlights becoming translucent. Obviously, one foot is painfully close to the subject; and that's due to the softbox being relatively small... that's why professionals use a lot of very large modifiers.


Here I've put all of the softbox crops together in sequence to help make seeing the differences easier.


And here they are combined into a video with the image changing in five second intervals. Also note the change in the BG lighting... the "subject" was only about 1ft away from the BG.


So, just because you have "soft light" it doesn't mean you are done... changes in distance can make a big difference in refining the final result.



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