I searched, googled and yahooed the net for an answer to this simple question, and couldn't find one. So, for those starting or amateurs or semi-professional or professional photographers that need to know how much money they would need to save to buy all the lighting power they would ever need, please read forward.
Introduction
"But I was thinking of a way To multiply by ten, And always, in the answer, get The question back again." - Lewis Carroll.
Let's remember that in photography, everything, exposure related, works either by doubling or halving or thirding, if that's a word, which is not.
So, if you go from f/11 to f/16, or from 1/125s to 1/250s, you are halving the light that enters the photographic plane. And if you go from f/11 to f/8, or from 1/125s to 1/60, you are doubling the amount of light that enters the plane. The same with ISOs, from ISO 100 to ISO 200 it's the double of light, and from ISO 400 to ISO 200 is half the light.
Multiples flashes from a single flash head
So, lets say that for a particular situation the required exposure dictates f/16 at 1/60s at ISO 100 but your flash head can only give you enough light for f/11. What do you do?
You flash the subject twice. Two flashes at f/11, twice the amount of light, will give you an f/16 exposure, the rest of the factors remaining equal. And of course, assuming your subject does not moves.
That was easy, wasn't it?
So, how many photography flash heads do I need?
"Begin at the beginning and go on till you come to the end; then stop." - Lewis Carroll, again... I'm sensing he knew something about photography.
Now, instead of one flash head, we will start adding more units. Until there is no use in adding more. And then stop.
We are going to use the worst case scenario in which all flash head units fire at the same power. For the remaining of this discussion, all of our flash heads will fire at f/8.
The shutter speed and the ISO do not matter at all. But if you need more numbers, assume it is 1/250, ISO 100.
One flash head
Let's start with one flash head at f/8. So, you have an exposure setting of f/8, 1/250s, ISO 100.
Two flash heads
Now we add a second flash head firing at f/8. What happens when we fire both flash heads simultaneously? We get double the light.
If we get double the light, what aperture should we dial in to get a proper exposure?
f/11. Right? If we are flashing double the light at f/8, then we need to close 1 f-stop to let only half the light pass thru. Thus, f/11.
So now, two flash heads firing at f/8 will gives us an exposure for f/11. Each light now contributing the half of light, or 50%.
Three flash heads
We add a third flash unit firing at f/8, and we fire all three at the same time. Each light now contributes 1/3 of the light, or 33.33%.
So, we would need to close down the aperture to f/13, or half a stop.
Tricky? From one to two flash heads we got double the light. But from two to three flash heads, we are only adding 50% more light. So, we need to close down half a stop. To f/13.
Four flash heads
Now we add a fourth flash head unit at f/8, and fire all four simultaneously.
Each flash unit now contributes 1/4, or 25% of the light. However, by adding a fourth flash unit, we are effectively doubling the amount of light that was provide by two flash units.
And, we will need to close down the aperture to f/16, or one full stop.
Recapitulating, from one flash unit at f/8, we added a second one for double the light and an exposure of f/11. We have now added two more, for double the light, and an exposure of f/16.
So far so good.
Five flash heads
We now add a fifth flash head unit, at f/8. Now, each of our five flash units, all firing at f/8, contribute each to 1/5, or 20% of the total light.
This means that our fifth flash head only adds 25% more light to our former setup of 4 flash heads. In photography, the equivalent of of 25% more, or less, is 1/4th of a stop, but we can only adjust for 1/3rd. So, we close down the aperture to f/18, one third of a stop less.
Let's pause for a moment and remember that from f/16 to f/22, there is one full stop. A half stop would be at f/19. And if you count in thirds of a stop, then it would be f/16, f/18, f/20, and f/22.
Six flash heads
What happens when we add a sixth flash head unit firing at f/8, just as the rest? Each flash unit would contribute to 1/6 or 16.6% of the total light.
However, a sixth flash unit will only add 20% more light to our former setup of 5 flash units.
And it just happens two things: one, that the maximum error in exposure allowed in photography is 1/3rd of a stop; and two, as a result of the former, we have no way to close down the aperture to a finer setting other than 1/3rd of a stop.
Thus, a sixth flash head unit adds nothing in terms of light quantity and exposure.
Diminishing returns, an economist would say.
So, again, how many flash head units do I need?
Five. Cinq. Cinco.
That's all the lights you would ever need in your life.
Well, not quite.
Thanks to input from seasoned photographers at the Nikonians Speedlight forum, I came to think that these calculations are good for all five lights pointing at a spot. There is no need to trow more light to a subject-spot.
That spot could either be an apple or a person or a row of persons, but certainly, 5 lights will not suffice to light larger subjects such as a truck or a building.
Also, for a small studio, for instance, one would need, say, 5 lights to light the subject, plus at least 2 more lights to light the background evenly.
For larger subjects, either indoor or outdoor, or for images where there are several subjects at different planes in the frame, one would get to need as many flash units as there are stars in the universe. Photographers lighting images with either several subjects or larger subjects, use an average of 12 flash units, or even more.So, no. There is no good answer to the question here presented. One could do great images with a single flash unit, or with five, or twelve or 43. It all depends on the subject and the requirements.
However, I still think that there is no need to use more than 5 lights all pointing to a single subject, a person, or a sub-subject, a tire in a car, for instance.
They say Einstein said "Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.". Well, I can't neither be sure about the number of flash units needed.
Eduardo Machuca
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