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Mark Sawyer
17-Dec-2012, 18:43
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
17-Dec-2012, 18:49
If I remember correctly, yes. But I believe there's some fall off after a certain number of pops.

Mark Sawyer
17-Dec-2012, 18:51
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
17-Dec-2012, 18:57
Watt seconds measure power into the flash head, not light out. Use a flashmeter.

Amedeus
17-Dec-2012, 19:14
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
17-Dec-2012, 20:05
I believe some higher-end meters will meter multiple flashes.

Mark Sawyer
18-Dec-2012, 00:13
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
18-Dec-2012, 04:15
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
18-Dec-2012, 05:01
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
18-Dec-2012, 05:07
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.

Scott Davis
18-Dec-2012, 10:06
Mark-

in playing around with wet plate and strobe, I found with a 2400 W/S pack at full power to a single head, I was hitting about f4.5/f5.6 on a single pop to get adequate exposure with a lens that opened to f4.5. So to hit your f14, you'd need to pop that 2400 W/S pack eight times to get there. While that's a pain, it's certainly better than 50 pops.

Bob Salomon
18-Dec-2012, 10:27
two heads into one generator on full power may have slightly lower light output than a single one (if you are generator power constrained)

This is true since the additional cables and added connectors of the two heads will reduce the output. And that assumes the same head design, reflector type and flash tube type. All of those variables will effect the output.

Mark Sawyer
18-Dec-2012, 12:08
Thanks all! There are quite a few variables, and I'm aware of most of them. Light meters are relatively useless for wet plate, especially when figuring out a new light source. Also, I'd be using the strobe "bare bulb" with no reflector, as I'm trying to get to a point source for more defined shadows. (I'm currently using an array of cfl's, and even straight on, they represent a pretty big area of illumination.) I'm just trying to roughly figure what power strobe is strong enough for close-up wet plate photography, where the effective f/stop is reduced for depth-of-field and bellows extension necessities.

patrickjames
18-Dec-2012, 13:15
Mark, many strobes have uv blocking built in. You may want to check that. Obviously that is undesirable for your purposes.

If you are doing small still lives (like your pixies) and want sharp light I would suggest you get a fresnel or a spot attachment for your strobes. It will dramatically increase the light falling on your subject as well as the control you will have. Firing your strobes bare-bulbed is hugely inefficient for your purposes.

Mark Sawyer
18-Dec-2012, 13:53
Thank you, Patrick, I'll look into it! A fresnel lens may be a good attachment for my CFL arrays, as the 16 bulbs in each occupy a 12x12-inch area, functionally a small soft bof, especially at close range.

patrickjames
18-Dec-2012, 17:07
If you want to tighten up the light on your CFL arrays try a grid, not a fresnel. I don't think a fresnel will help much unless you put it at the end of a box, in other words, move it away from the source. If you figured out the focal length of the fresnel it would help to get the light more focused.

It would be pretty simple to make a grid with a bunch of black paper.

I don't think you stated what type of strobes you are using otherwise I could point you in the right direction.

Jason Greenberg Motamedi
18-Dec-2012, 18:12
Mark, many strobes have uv blocking built in. You may want to check that...

I have done some testing with both Wetplate and Daguerreotypes, comparing speedotron UV blocking bulbs versus uncoated regular bulbs, and saw no significance in difference in exposure.

Mark Sawyer
18-Dec-2012, 18:26
If you want to tighten up the light on your CFL arrays try a grid, not a fresnel. I don't think a fresnel will help much unless you put it at the end of a box, in other words, move it away from the source. If you figured out the focal length of the fresnel it would help to get the light more focused.

It would be pretty simple to make a grid with a bunch of black paper.

I don't think you stated what type of strobes you are using otherwise I could point you in the right direction.

A grid would give a more directional light, but at a cost of intensity needed for wet plate. :( That's where I'm thinking your earlier suggestion of a fresnel might work, (I've found some 12x12" and 24x32" online). My strongest strobe currently is a 450-ws Bowens Mono 9000, though I also have some old Novatrons that go to 240-ws.


I have done some testing with both Wetplate and Daguerreotypes, comparing speedotron UV blocking bulbs versus uncoated regular bulbs, and saw no significance in difference in exposure.

I've noticed the newer "UV-blocking" glass on my studio window doesn't seem to affect exposures either. My strobes are all 20+ years old... they may pre-date the UV-blocking bulbs anyways.

patrickjames
18-Dec-2012, 20:44
Mark, the fresnel I was referring to for your flash is one like this- http://www.bowensdirect.com/index.php/fresnel-200.html

You could use a cheap plastic fresnel for your CFL array, which is what I assume you are referring to.

Ian Gordon Bilson
18-Dec-2012, 21:32
I ran up against this problem years ago -Product photography with under-powered strobes.
The rule is : If (A) is your base exposure at full power, to gain 1 F : 2 pops.
For another 1 F : 4 pops. Another 1 F : 8 pops. Another 1 F :16 pops. Another 1 F : 32 pops. You begin to recognize the progression..
Reciprocity was not a problem,but ambient light has to be controlled carefully.

Sevo
19-Dec-2012, 00:38
I have done some testing with both Wetplate and Daguerreotypes, comparing speedotron UV blocking bulbs versus uncoated regular bulbs, and saw no significance in difference in exposure.

That probably is due to the not explicitly UV blocking ones being plain glass, which by itself is a powerful mid to far UV blocking filter. There is already less near UV than blue in the light of a unfiltered plain glass flash tube - which may still throw off the whites on colour film (hence "UV blocking" flash tubes), but in terms of blue sensitive alternative materials we're talking of half stops.

If you want or need more UV, get quartz tubes, if there still are any around - in the past these used to be made for process purposes (offset plate exposure units) and as drivers for discotheque blacklight strobes.

Struan Gray
19-Dec-2012, 00:41
If you want to maximise UV and near-UV a plastic Fresnel isn't necessarily going to be the best option. Better would be to get hold of a large parabolic mirror and put the strobe at the focus. That gives you wide but parallel light, which mimics sunlight. You can also focus/defocus by moving the strobe in relation to the mirror.

There are, I'm sure, expensive light shaping tools from the photographic suppliers. I've used an old searchlight mirror my father-in-law just happened to have lying about the garage.*

Multiple pops of a short-duration flash can be affected by the 'intermittency effect'. It's related to reciprocity failure, in that the film emulsion 'heals' the development centres faster than you can make them. The result is that although two pops give twice the light of one, they don't necessarily give twice the exposure. It's usually an issue once you get up to a whole passel of pops, and it's emulsion-dependent, but I'd guess fifty is enough. Thus, if fifty is a calculated exposure, you'll probably need more. If it's experimentally determined, no worries - unless you change the aperture significantly.


*NB: this was tinkering with the kids, not doing wetplate, but the idea is sound.