Flash Calculations
Can't sleep... doing math. Math relating to flash calculations; for any of my photo friends does this make sense?
We know that the multiplier for stops is about 1.4.
I know that the guide number for my flash zoomed to 105mm is 190. And I know to calculate guide number is GN = Flash Distance To Subject x F-stop. Guide numbers reference a scene shot at ISO 100.
So lets figure my crazy math out here. I'm shooting people dancing about 15' away. The ceiling is about 12' high and lets assume nice and white. I bounce my flash up at about 60 degrees up. Assuming the distance my flash is from the floor is about 6' (my height plus the distance above my head) I calculated that the (center of the) flash travels about 16' before hitting the area above where the dancing is taking place it then travels the 12' down to the dance floor for a total of 28'.
If I pop a full flash that's 190 = 28' x f/stop. My resulting f/stop would be about f/6.7 ( I'm going to round to f/8 since I figure the half stop difference means I don't illuminate the floor so I'm shooting waist up photos).
So Full Flash ISO 100 @f/8 28'
That's not enough reach. I'd probably be barely illuminating heads and not to mention wasting batteries.
Let's drop to F/4. Two stops. That will give my flash, 44' worth of throw. That's better. But I'm still wasting batteries. More on that in a minute.
I'm going to increase ISO:
ISO 200 would give me 60' (44' x 1.4)
ISO 400 would give me 86' (60' x 1.4)
ISO 800 would give me 120' (86' x 1.4)
Now here's where I wonder if my math is actually off. All those values assume I'm popping a full flash. If I drop the power to 1/2 does that now mean ISO 800 drops to 60' of coverage? 1/4 drops me to 30'? 1/8 drops me to 15'? This almost makes sense to me since when I normally do receptions I shoot at about 1/4 flash and my target is about 10'-15' in front of me. Ceilings have been around 10' high from my estimation.
For fun I did some recalculation if I tilted my flash all the way up and closed the gap to 10' I get pretty much the same results erring more on the side of over-exposing rather than under exposing like above.
We know that the multiplier for stops is about 1.4.
I know that the guide number for my flash zoomed to 105mm is 190. And I know to calculate guide number is GN = Flash Distance To Subject x F-stop. Guide numbers reference a scene shot at ISO 100.
So lets figure my crazy math out here. I'm shooting people dancing about 15' away. The ceiling is about 12' high and lets assume nice and white. I bounce my flash up at about 60 degrees up. Assuming the distance my flash is from the floor is about 6' (my height plus the distance above my head) I calculated that the (center of the) flash travels about 16' before hitting the area above where the dancing is taking place it then travels the 12' down to the dance floor for a total of 28'.
If I pop a full flash that's 190 = 28' x f/stop. My resulting f/stop would be about f/6.7 ( I'm going to round to f/8 since I figure the half stop difference means I don't illuminate the floor so I'm shooting waist up photos).
So Full Flash ISO 100 @f/8 28'
That's not enough reach. I'd probably be barely illuminating heads and not to mention wasting batteries.
Let's drop to F/4. Two stops. That will give my flash, 44' worth of throw. That's better. But I'm still wasting batteries. More on that in a minute.
I'm going to increase ISO:
ISO 200 would give me 60' (44' x 1.4)
ISO 400 would give me 86' (60' x 1.4)
ISO 800 would give me 120' (86' x 1.4)
Now here's where I wonder if my math is actually off. All those values assume I'm popping a full flash. If I drop the power to 1/2 does that now mean ISO 800 drops to 60' of coverage? 1/4 drops me to 30'? 1/8 drops me to 15'? This almost makes sense to me since when I normally do receptions I shoot at about 1/4 flash and my target is about 10'-15' in front of me. Ceilings have been around 10' high from my estimation.
For fun I did some recalculation if I tilted my flash all the way up and closed the gap to 10' I get pretty much the same results erring more on the side of over-exposing rather than under exposing like above.