Flash power in f stops versus fractions

chrisgeary

chrisgeary

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Could someone explain to me why you meter in f stops but the head power is in fractions?

I'm sure this has been discussed many times, so please direct me to appropriate threads. Its a tricky subject to search for.
 
and because you can't really have the flash in stops because the actual light hitting the subject is also dependant on flash to subject distance which falls off using inverse square law which means that double the distance = quarter the light falling on the subject.

So you could have a flash on a power setting of f8 (if you called it stops) but the light hitting your subject might just be f4......move it closer and at the same power you have f5.6 on your subject.

Sorry I'm not explaining that well but they may as well have a dial that goes up to eleven :)

So to have it in stops would actually be a misnomer. :)
 
I think that expressing reduced power in fractions is just the way it's always been done - and it works!
Historically, the very first studio flash heads didn't have any adjustment, then as the specs improved they were marked with something like full/half. It's more complicated now because many heads adjust down to 1/32nd, but the few Companies who have set up the adjustment to show stops rather than fractions have just produced something that's very hard to actually use.

In reality of course, we just take meter readings and adjust up or down without really looking at what the power indicator says
 
Hmm that makes sense. So the meter reading spawns off some internal maths then, rather than a particular value on the head? I guess I assumed the flash meter had some knowledge of distance.
 
Nope, it just measures light :)

It is useful to understand some of the physics though because I know that if I change ISO from 100 to 200 I need to lose a stop off the flash power (assuming the same distance). If I'm on 1/2, than means dropping to 1/4.

The flash to subject distance is not linear but inverse square law so it drops off a lot faster than you might think. It's not a double distance/half power but double distance quarter power.
 
what are they really saying?
Click to expand...

Just that there is twice as much light on the BG as the subject.
 
The background is 1 stop brighter than the subject.

F8 to f11 = 1 stop
F8 to F16 = 2 stops
etc

:thumbs:

also as dazzajl says....
 
chrisgeary said:
Ok. So when I see people talking about their background at f11 and the subject at f8, what are they really saying?
Click to expand...

Just to dot the i and + the t...
That the correct exposure for the subject is f/8 and the correct exposure for the background is f/11, therefore because only the subject gets the correct exposure, the background is 1 f/stop overexposed.

f/stop has really become a generic term now, and just means a doubling or halving of exposure (rather than a specific term)
For example, in outdoor photography "I gave it a stop more" just means "I gave it twice the exposure" which could mean half the shutter speed and the same aperture, just as easily as the more precise use of the term, which would be one stop wider aperture.
 
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