fall-off

[Phil Rhodes]

Is it me, or is it not so much about the change in lighting anyone wants between a wide and a closeup, it's more about the fact that you can't create the lighting in the wide that you'd want for the closeup? You can't have enormous diffusion six inches from someone's earhole then shoot them full length, but if it were possible to create that closeup lighting for the wide, anyone would just do that and get everything they needed.

[Simon Osaji]

I believe so, and also I think it’s getting them look somewhat close and not look drastically different to the eyes.

[David Mullen ASC]

Sure, what works for a wide shot in a room might not work for a close-up, and you can't get the close-up lighting in the wide shot. On the other hand, wide shots also need to be lit well and work on their own too, they don't just exist as a vehicle for getting into the close-up.  Often I've had a hot slash of light on some shiny furniture that puts a small reflective kick of light onto the actor's face that is only visible in a close-up, so it's not distracting in the wide... but usually I flag it off in the close-up.  Silverware on a table is a classic example, you get in close with the camera and then you notice a spot of light on the cheek or chin, etc. from a reflection.

I think every cinematographer would love to light a wonderful wide master and have that lighting look great on the close-ups as well with no changes. Things would go a lot faster on set and there would be no concern with matching. Sometimes you manage it or get pretty close.  David Watkin was notorious for beautifully lighting a room and not changing things for the coverage unless absolutely necessary. It's not a bad approach by any means, it just means forgoing a certain level of glamorization of the actors.

[Gabriel Devereux]

https://gabrieldevereux.com/calculator/
 

Here’s how to calculate large near-lambertian (matte) source propagation.

A simplistic view of it - explained crudely for the umpteenth time.

1, 2 or however many light sources can be illuminating the reflector. It is assumed these light sources are at an acute angle to the normal of the reflector and the surface is near lambertian.

The reflectance value of muslin is approximately 95%/0.95. It then models the reflecting soft surface as being composed of many point sources, m*n (the amount of point sources scale with the size of the reflector), whose total light output is the same as the total luminous flux of the original light sources (minus losses) If the total luminous flux of the sheet is L, then the luminous flux of a single point source is L/(m*n).

These point sources emit light isotopically at a solid angle of 2pi steradians (only in front of the surface). The algorithm computes the luminance at some point z on the axis perpendicular (the normal) to the reflector passing through its centre. To this end, it simply needs to find the luminance produced at this point by each point source, apply certain laws such as lamberts cosine, and then sum these values to get the total luminance. 

Note that the number of point sources, m*n, can become arbitrarily large. However, for m*n => 50 the approximation is almost perfect (note there are approximately 7*7 (49) point sources per m**2 so this algorithm is accurate for sources larger than 3’ x 3’). 
 

The rest is basic geometry. 
Fall-off and contrast is not related. Fall-off is a term, I believe, taken from radiometry. A very poignant topic for cinematography.

Contrast in our world is artistically relative. How you interpret the two is up to you but, it’s probably easiest to view the two separately.

Also if you have any questions about the above calculator please do ask. It’s all reasonably simplistic math. 

[Simon Osaji]

For instance when using big frames to light a master from the toes to head, when you go in for CU, do you just place a 4x4 frame in front of this big frame for the CU or you bring in additional light and still leave the light from the big frame turned on?

[David Mullen ASC]

Just depends.

If the 4x4 frame is not too heavy, like an Opal, and would be larger relative to the subject compared to the distant physically larger frame, then sometimes that works as a fast way of softening the key further. But if the 4x4 frame wasn't going to be any larger relative to the other frame, then the light isn't going to be much softer unless that larger frame was too light so the lights behind it were creating hot spots rather than filling the frame, but at that point you'd be better off just adding the 4x4 frames between the lights and the large diffusion to make the light fill it more evenly, because it's the size relative to the subject that determines softness.

If the 4x4 frame was relatively larger compared to the distant frame but you wanted something heavier like Full Grid Cloth, then you'd probably add some light behind it to compensate for the light loss. And sometimes you might want to bring it slightly forward to wrap around the face a little more too, so that would require both a diffusion frame and a light behind it.

There are no rules.