open flash calculations

[Luís Pereira Ribeiro]

Hi all, I usualy work with open flash using a Sinar P2 and Broncolor's both flahses and meeter and it's not unusual to photograph with multiple pops. Although I don't really need it for my work I've been searching an old formula for calculating the number of flash pops recquired for a given f. number; I used to have it but I lost the book and I can only remember that it had something to do with "somethin's" log/2 log. Is it possible that someone knows what I'm talking about? Any answers will be greatly apreciated.

Luis

[Emmanuel BIGLER]

Luis : with little maths the principle is as follows.
Two flash pops are equivalent to doubling the exposure time, i.e you need to close one f-stop.
Assume that you read f/5.6 with one flash pop. Two flash pops will allow you to close by one f-click i.e. f/8. Four flash pops : shift by two f-clicks. So the number of f-clicks to stop down is equal to the base 2 log of the number of pops.
In multiplicative terms, the new f-number required with "N" pops is obtained by multiplying the original f-number for 1 pop by the square root of N.
Example : initial reading f/5.6 for one pop, with 10 pops, what will be the final f-number ? sqrt(10)=3.16 times 5.6 yields f/17.7 in between f/16 and f/22.

A good ol' analog slide rule with the direct reading of squares roots and multiplicative factors by a single translation of the central ruler would be the best tool to use ;-);-)

[Armin Seeholzer]

Or even easier to buy a Gossen Mastersix or an other good exposure meter were you can count multible pops. Or ask the meter how many pops you need to get f 45!

[Leonard Evens]

The formula you are thinking of may be 2*log (N)/(log(2)), but it doesn't have much to do with the number of pops. It relates the f-number to the number of f-stops.

Emanuel's answer was right for pops.

[Luís Pereira Ribeiro]

Thank you all for your replies, specially Emmanuel, that's what I was looking for.

Luis

[Ralph Barker]

Although the calculation above is correct as a starting point, in practice you'll find that additional exposure is often required once you get beyond about 4 pops, depending on the film's reciprocity-failure characteristics. What one would think might be a four-pop exposure might need five or even six pops to produce the expected density, for example.

You might call this reci-pop-ricy failure. ;-)

[Scott Walton]

As Ralph states, 2 pops no problem, 4 pops usually ok too but there comes a point (usually in the 8-10 pops and if your doing that either switch to a higher speed film or add more lights!) you will reach the need for adding pops to cover reciprocity-failure and each film manufacturer will be different. Testing is a good thing!