rpavich wrote:
I got an ebook about portraits and one main message that struck me was this...find a simple, repeatable light set up that will work in many situations. Don't keep reinventing the wheel each time you shoot.
His advice in the book was this:
One shoot through umbrella at 45 deg to subject; that is 45 deg up and 45 deg to the side.
One umbrella opposite of that umbrella behind subject.
Both flashes on same power (or I should say putting out the same power at the same distance.)
Front umbrella at 3 foot distance
Rear umbrella at 6 foot distance (which effectively cuts the power in half.)
Simple, easy to understand, and repeatable.
Here is my "before work bath robe portrait" sample.
I got an ebook about portraits and one main messag... (
show quote)
Great portrait and the system works as you can see. One minor problem is in the math in that the second light at 6 feet is ineffect not half as bright! All artificial light sources work the same way as far as exposure goes. Unfortunatly digital only users are spoiled by having auto flash exposure so never ever learned the inverse square law or used guide numbers for calculating correct exposure.
All flash equipment has a guide number. A typical guide number might be 96. To get exposure spot on you measure or guess the distance from the light source to the subject, say for example 10 feet, you divide the guide number by the distance and the answer is the F stop you should set. In the above example 96 /10 feet =F9.6 . However if you do the same cal for a subject 5 feet away the answer would be F16.
The problem comes when the distances are closer in the postster's example of 3 and 6 feet you get the following results 96/3 feet = F32 while at 6 feet the calcs are 96/6 feet = F16 which is 2 whole F stops not double .
Look up inverse square laws. But when you get a result like this from a standardized set up who cares about the math haha. When you get it wrong with digital you can see its wrong and fix it, with a sheet of 10x8 film you get it wrong and it cost you in excess of $35 plus your subject has long gone by the time the transparencies are returned from the lab as most of the ex pro photographers will know! Congratulations on the portrait it is superb! Well done.
Ian
http://www.paulcbuff.com/sfe-inversesquarelaw.phpInverse Square Law
The common misconception is to think that moving a light twice as far from the subject will result in half the light and a one f-stop difference in illumination. This is incorrect.
It must be remembered that light is projected in a 2D pattern up/down and right/left. If you consider a reflector that illuminates, say, a 10-foot circle at a 10-foot distance, the area illuminated is pi times radius squared or 11,310 square inches. If the distance is doubled to 20 feet, the projected pattern will be a 20-foot circle and the area of that circle will be 45,239 square inches four times as great as the first example. Since the light must now illuminate four times the surface area, its intensity is one fourth.
Thus the square law propagation of light:
The intensity of light varies according to the square of the change in distance from light to illuminated surface.
So, if you increase the distance by two, the light intensity becomes one fourth 2 f-stops, not 1f. If you move the light in to one half the distance, the intensity increases by 2 f-stops.