Thanks for your answers but no one has explained WHY...
OK, softboxes are known to output soft light. We do like soft light.
What is soft light? It is light that diffuses the shadows, and wraps around the subject.
(see
Four Flash Photography Basics we must know - Soft Light )
How does it do this? It does it by being LARGE and CLOSE. (this is really the only way)
How does Large and Close do this? Say it is a four foot light (like a softbox or umbrella) that is say four feet from subject. The subject sees it big and in-their-face, it being around (trig: 2 arc tan(2/4) = ) 53 degrees wide. This means some light is coming to the subject from 26 degrees on the subjects right, and some light from 26 degrees on their left (and top and bottom too)... because the light is that large and close. There are many paths of light to the subject, from all areas of this large light. All these light paths from the Large source fill the shadows made by all the other paths... resulting very vague soft diffused shadows, actually more just gradient tones than shadows, but these tonal gradients show the shape of the curves of the subject (interesting appealing light). Lighting is all about the tonal gradients (which we call soft).
Learning to see this is what it is about.
All this assumes the softbox is OFF Camera, from an angle, normally maybe 45 degrees wide and high. This OFF angle (very different than what the frontal lens sees) makes shadows which the lens can see, to add interest. The light, being off to the side more, makes those shadows which the lens can see. This is the opposite of flat. However, large and soft makes the shadows very diffuse, gradient tones. We like that.
A light ON CAMERA is direct frontal light, no shadows. Everything is lighted extremely evenly, no shadows. There may be shadows BEHIND the subject (which we try to prevent), but there is NONE ON THE SUBJECT from flat frontal light. There are no shadows to be filled, to cause gradient tone shading, which shows shapes. The subject is lighted abysmally flatly (dull, flat, evenly, not interesting). Whereas, we instead dearly love soft gradient tones ON THE SUBJECT. This frontal try ain't it.
Diffusion of any smaller diffused light ON CAMERA can only scatter and change the angles of the light paths, which only makes the light go outward, sideways more, to now totally MISS the subject (wasted, no longer of any interest to us).
Any LARGE light (if close enough) has relatively extreme width (and height, ie, size) to be able to deflect inward TO the subject from all those angles, left and right, mentioned above. This light is NOT wasted, it is softly lighting the subject.
Said again, a few more times:
You cannot draw different paths to a spot on the subject from a small point source (which is harsh - dark sharp shadows). However, you can draw many paths inward to a spot on the subject from a large area close in front of them (which is soft).
So scattering diffusion of a small light can only be deflected outwards. It has no width dimension to deflect it inwards, that has no possibility, no meaning. From a small light, its scattered diffused light can only go outward, to miss the subject.
Whereas a Large light is soft because it has width able to deflect it inwards, to hit the subject from many different angles, filling the shadows from all the other paths. That is Soft. And Close makes it appear larger.
A Main light ought to be off camera (to intentionally make shadows from a different angle than the frontal lens, so the lens can see those shadows, to add interest, to show shapes (specifically, to NOT be flat). Maybe it is the sun, but the direct sun is small, only 1/2 degree in size, so it helps to diffuse it through a large cloth screen, which can then be relatively large and close. Same idea as putting a speedlight in an umbrella, makes it much larger. "Soft" is about "large", and "close" helps it be large.
Diffused main or not, Fill light ought to be frontal, to fill the same shadows that the lens sees, without making its own shadows. Frontal is flat, but fill is at very reduced level, to keep the main light as the primary light with major effect. Fill is not lighting it, the Main is doing that. Fill is just enough to reduce the shadows somewhat (reducing the contrast of this image). But we must leave gradient evidence of those shadows, that is what lighting is about.
A bare speedlight on camera can be wonderful fill for harsh sunlight main light. It should be reduced level, common rule of thumb is around -1.7 stops less than the main light. It should NOT be obvious in the photo that a light was used (it should not be making shadows itself), but its dimmer result is extremely helpful (to make the dark shadows lighter, more acceptable contrast).
Bounce flash indoors... the ceiling is like a large umbrella up there (soft diffused tonal light). Maybe not the most ideal place for it, but natural from above, and very soft, and beats flat direct flash by a mile (also tends to light up the rest of a normal size room). A small pullout bounce card on the flash can provide direct frontal fill (and adds important catch lights in the eyes). However, it should NOT be a big bounce card. It should not obliterate all the soft gradient tones of the bounce, with instead only its flat frontal light. What was the point of the bounce then? Bounce should be the main light, should be making shadows still visible, just reduced. The pullout cards are NOT too small - and we don't have to pull them all the way out.
