I know I should just try this myself but can anyone explain the difference in effect achieved between a large light source (softbox or umbrella) close to, verus far from the subject, say a head and shoulders shot.
Theoretically I know this: Light fall off would be much more apparent with the light closer. With the light farther away, the background and the subject would be more equally illuminated (less pronounced light falloff).
Closer light means a larger light source, farther light means smaller, harsher light.
Does anyone have a rule of thumb, something which works for them to determine how close to place the lightsource, other than, I need it close to get f11?
It sounds like you pretty well have it figured out already. What we are discussing is quality of light. I have used many light sources and they all have different qualities of light. I have a large softbox that when just outside of the frame( outside of seeing through the viewfinder) I didn't really like the light. When farther away it is better. When I used the medium softbox I like it a lot just outside the frame. I have a Halo Mololight that I really like a lot, same thing close up, just outside of the frame, however it is tough to feather because it will flare back in the lens. I have a Mola light, same thing, I like it close up. I almost always feather the lights to get the wrap around effect, thereby lessening the light falloff.
I use monolights because you can place the light where you like the quality of light, and then dial in the f stop that you like to use. I also keep my subject far enough from the background so that neither bringing the light closer nor father from the subject affects the background. On some power packs with a couple of heads you don't have this option and you have to move the lights to get what f stop you want. Also probably every different company's lights also give different results in the same light modifier.
Anyway it sounds like you know what you are doing, so I think you just have to experiment with each light source and find what you like.
Hope this helps,
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (blansky @ Jan 28 2003, 11:04 AM)</td></tr><tr><td id='QUOTE'>
I use monolights because you can place the light where you like the quality of light, and then dial in the f stop that you like to use. I also keep my subject far enough from the background so that neither bringing the light closer nor father from the subject affects the background. If you are using a power pack and a couple of heads then don't have this option and you have to move the lights to get what f stop you want. Also probably every different company's lights also give different results in the same light modifier.
Uh... You lost me here. It took a while, but I'm fairly familiar with the controls on the Dyna Lite Power Packs: Symmetrical - Asysymmetrical; 125, 250 500,watt-second power settings; and the overriding 3 stop Variator.
"Neither moving the light closer nor farther away affects the background"??? This is different with a "Power Pack" light head?
Two problems affect Monolites: Accessing the unit when it is "buried" in a softbox or "up high"(unless you have something like White Lightning's remote control), and the additonal weight of the unit as opposed to a separate "Light Head". That is a real consideration when hanging a light source on the end of a boom.
Older packs in particular (like my Normans) aren't capable of as fine adjustments as many of the newer ones, but that's more a question of the vintage of the equipment. Still, if you're just trying to get a variation of a half stop or less, moving the light closer or farther will have some effect on the quality of the light, but not an enormous effect, so it's not that hard to manage with such a system. I've seen after-market variators that can add 1/5 stop control to an older system, but I think of them as more of a convenience than a necessity.
Sorry if I was confusing. He said that his motivation for placement of his light was to achieve F11 which I think means that he has a power pack and heads that he can't control each head's power output exactly.
I merely tried to explain that I moved my heads to get the quality of light that I wanted and with my monolights could set the head to get whatever fstop I wanted. I haven't used Dynalights in a while and I don't know if they can do this or not, but it seemed his system couldn't. I also stated that my mainlights don't affect my background but didn't mean to infer that any other lighting system would.
I have used most company's lights at one time or another but now use Photogenic Powerlights and don't have a problem with either weight or accessibility whether on a boom or not.. Powerpacks and heads vs monolights is an ongoing debate that is mostly personal preference, except that if you have to move the lights closer or farther to get a favorite fstop, you can't completely control the quality of light.
The bottom line and answer to his question is closer equals softer and how soft is defined by the light modifier, and the light. What he prefers, is right for him.
The degree of contrast is directly affected by the size of the light source. You may consider it from this perspective. The sun is a very small light source...made small by the distance from earth. This leads to very harsh shadows on bright sunlit days. Conversly on hazy overcast days when the haze has broadened the apparent light source of the sun the shadows become more indistinct. Insofar as answering your question of how close a light source should be...How could anyone answer that?...it all depends on the effect that you want to accomplish. Sometimes one just needs to take the effort to try things after all of the discussion has been done. Good luck.
The closer the lightsource is to the subject the more diffuse it will be. The more farther away from the subject the more specular it will be.