Watt-second equivalents...

[Mark Sawyer]

Forgive me, but this is something I'm just not familiar with, and a search didn't turn up much.

For a given strobe, say 100 ws, would 2 pops be the equivalent of a single pop from a 200 ws strobe? And would 10 pops be the equivalent of a single pop from a 1000 ws strobe?

[brian mcweeney]

If I remember correctly, yes. But I believe there's some fall off after a certain number of pops.

[Mark Sawyer]

If I remember correctly, yes. But I believe there's some fall off after a certain number of pops.

Possibly due to reciprocity failure, I'd guess. Thanks, Brian!

[Dan Fromm]

Watt seconds measure power into the flash head, not light out. Use a flashmeter.

[Amedeus]

Forgive me, but this is something I'm just not familiar with, and a search didn't turn up much.

For a given strobe, say 100 ws, would 2 pops be the equivalent of a single pop from a 200 ws strobe? And would 10 pops be the equivalent of a single pop from a 1000 ws strobe?

Not necessarily as the efficiency of a flash bulb increases with peak current. The typical economical 200 Ws strobe will most likely operate at a higher peak current and as such convert more electrons in to photons everything else being the same.

Dan is right with respect to the spec being power into the flash lamp and not out of the head. Production, design boundary conditions, age etc etc ensure that things are not alike when it comes to optical output.

Use a flash meter indeed if you want to be sure about any equivalent you are looking for.

[Vaughn]

I believe some higher-end meters will meter multiple flashes.

[Mark Sawyer]

Thanks guys, but I'm trying to figure out how to scale up to the power I'd need for a strobe with wet plates. Currently I'm at 50 pops of a 450 ws monolight for a lens at an effective f/14 (after belows extension is figured in).

[Leigh]

Hi Mark,

Watch out for reciprocity failure.

Everyone knows that it occurs for long exposures, but it also occurs for very short exposures, as with flash.

I've never shot wet plate, so I have no idea what the reciprocity characteristics are.

If your results don't turn out as expected, this is one point to consider.

- Leigh

[Bob Salomon]

100WS is input. Just the design of the reflector will change the output of a 100WS flash. Use a 45° reflector and compare the output to an 80° reflector and there will be over one stop output difference at the same WS rating.

ECPS, BCPS and guide number are outputs from a flash and can translate to an f stop. Watt seconds can not be converted to a f stop. You would need to know lots of other specs besides the WS to figure out a f stop.

Or use a flash meter.

[Sevo]

Not necessarily as the efficiency of a flash bulb increases with peak current.

So does the tendency to self destruct on one hand, and that to fail to trigger on the other. That is, reasonable designs are current controlled.

What is right is that two heads into one generator on full power may have slightly lower light output than a single one (if you are generator power constrained) - but in practice this is mostly made up for by the better placement a multi head set-up allows. Beyond that, it is more a matter of manufacturer of flash and tube, reflector shape and tube wear - it is not possible to use manufacturers figures to replace measurements, even less so when dealing with flashes of different type.