people say about softbox, the bigger the better, you can always back it up to get the same effects.
but, Beauty dish has to be within certain feet in order to see the certain effect.
for beauty dish, if i want to do head shot and half body shot,
do i need two different size dish? or
i get the bigger one and back it up a little?
even then, does a bigger one give the same effect as a smaller one when doing head shot?
The unique character of the BD comes from the disk in the middle which creates a "dead" zone in the middle of the pattern which can be feathered. For example in a butterfly configuration where a dish is commonly used the light winds up physically closer to the forehead than the eyes and mouth if the subject. Aiming the "hole" in the light at the forehead keeps it from getting overexposed when the eyes/mouth are correctly exposed.
As you move the dish further way the hole fills in and the pattern become more even and by the time you get it far enough back for the full-length shot the difference between it and an umbrella will be slight. By way of contrast an umbrella used close is hotter and more direct in the center than the edges which also allow variation in lighting via feathering but also gets more uniform as distance is increased, in part due to the inverse/square relationship of distance and intensity: the further away the light is the more slowly it falls off front to back.
A large SB or octobox at the same distance would control spill better. Bigger isn't always better. On a dark background where the light doesn't go is as important as where it does and smaller modifiers allow more precise control.
My BD configuration (white interior, frosted glass diffusion disc) does not assume magical properties at a certain distance. Rather, it provides a beautifully smooth distribution of light, and a nice catchlight. With increasing distance, the shadows harden and the catchlight shrinks with no abrupt transition. There's plenty to like in the 3-10' range.
