Reciprocity failure and flash

Hmmm… maybe this is a dumb question, but… wondering if reciprocity failure is different for strobes vs. long exposures.

Say I am shooting a pinhole camera with an f stop of 128 (OK, that's more a zone plate f-stop, but anyway) that would require a minute or two in sunlight - reciprocity charts or personal experience would come into play.

But say I'm shooting with the same camera in the studio; my meter tells me that I need one pop of the strobes at f45 for correct exposure. This would translate to 3 more stops exposure at F128, or 8 pops of the strobe.

I swear back in my 4x5 ektachrome product shooting days, I never considered reciprocity for multiple flash pops. But since each pop is, what, maybe a 400th of a second when using a big pack & head rig (not one specialized for very short duration bursts), the actual "exposure time" for 8 pops is actually about 1/50th of a second (if my math is right). Even though the lens is open for recycling, one usually turns off the modeling lights to avoid polluting the image with tungsten, so very few photons are hitting the film.

Am I thinking this through correctly? Kinda thinking about zone plate nudes… if I can find someone to sit still that long...

What's the output rating at full power? What guide number do you get at the preferred film speed? IIRC(I haven't used studio strobes in years)don't you divide GN by distance to get F-stop? Then all you need do is convert that info to get exposure. I think, like I said, it's been years. What you need to do is figure exposure time derived from the info on hand. You may only be doing a half second to one second exposure including the flash burst.

If I remember correctly short time reciprocity exposure for film kicks in on the order of 1 x 10-4 seconds [1/10,000 seconds] or shorter.

Rick A said:

What's the output rating at full power? What guide number do you get at the preferred film speed? IIRC(I haven't used studio strobes in years)don't you divide GN by distance to get F-stop? Then all you need do is convert that info to get exposure. I think, like I said, it's been years. What you need to do is figure exposure time derived from the info on hand. You may only be doing a half second to one second exposure including the flash burst.

Click to expand...

Well, I have power packs from 800 to 2000 watt seconds. To get the f-stop, I use a flash meter - calculating exposure with softboxes (2 layers of diffusion and grids), flags, nets, cards, reflectors, diffusion panels, etc... would just be a guessing game vs. one pop and glance at the meter. Even in the film days, we metered and then shot a polaroid or two or ten. Shutter speed is moot unless I'm doing mixed lighting. I generally have enough power that I only need one pop of the strobes unless I'm doing very small aperture 4x5 (but these days I generally shoot digital). Wondering about using strobes with pinhole or zone plate with film though.

What I'm wondering is if an exposure takes, for example, 8 bursts of the flash, in a dark studio with the lens open for the duration of the flashes plus the recycle time, does reciprocity play a role? I'm guessing not, since almost no light is hitting the film between flashes and that light isn't a meaningful part of the exposure.

Reciprocity failure happens when the light intensity at the film is either:

1) much lower than "normal"; or
2) much higher than "normal".

The first case is what we normally encounter - exposures that of necessity are much longer than normal, because the light intensity at the film is so low.

But the second case can happen too - especially with "automatic" flashes that have extremely short duration at high intensity.

Your pinhole/zone plate will protect you from the latter case though, through its f/128 opening.

Intermittency effect is a form of failure of reciprocity. In addition, exposures less than around 1/1000 of a second suffer from additional loss of reciprocity. I don't know of any good way to predict these effects. Edgerton used empiric data (from sensitometers of his own design).

ic-racer said:

Intermittency effect is a form of failure of reciprocity. In addition, exposures less than around 1/1000 of a second suffer from additional loss of reciprocity. I don't know of any good way to predict these effects. Edgerton used empiric data (from sensitometers of his own design).

Click to expand...

The intermittency effect is a form of low intensityreciprocity failure, and realistically, it can only be predicted from emperic data.start by doubling the exposure,that can never hurt with B&W negative film. I sugest using a flash meterand compensating through the ISO setting.A flash meter able to account for several flash bursts is best.

what happens?



is your subject static ?

p.s. image stolen from the web.

RobC said:

what happens?

is your subject static ?

p.s. image stolen from the web.

Click to expand...

Multiple pops is to use very small apertures, or if you're not running 4800WS packs with dual-tube heads, etc., not for special effects. Though usually for tabletop work, it's 2 or 4 pops…

RalphLambrecht said:

The intermittency effect is a form of low intensityreciprocity failure, and realistically, it can only be predicted from emperic data.start by doubling the exposure,that can never hurt with B&W negative film. I sugest using a flash meterand compensating through the ISO setting.A flash meter able to account for several flash bursts is best.

Click to expand...

Well, there's a simplicity to doing multiple pops with pro packs that have consistent output - if you want to be at at F32 and have a flash reading of F22, you trigger the flash 2 times, just simple math and as the count increases it's easy to figure half or quarter stops for bracketing with E6. I'm thinking for zone late or pinhole, I may need something like 8-15 pops. To double an 8 pop exposure in that case, you'd do 16.

But I'm leaning towards the idea that reciprocity won't be an issue - since no light is reaching the film during the recycle time. I do have a list of oddball stuff I want to test (like, can I wash the halogenation layer from some B&W films before shooting, if I take an old zoom lens and paint all the internal black surfaces silver what will that look like, stuff like that), looks like I'll have a testing day before long.

I have made this question a couple of time and always I was told that even with flash and almost no light ritching the film during the flash recycle, it will have a reciprocity faillure.

I was told that with about 8 flash pops sugested by the lightmeter I would actually need 3 or 5 time more pops.

My conclusion was that very little people, or almost nobody, who answer this questions actually ever have done it and know exactly the answer.

At the moment I am just waiting James release his 6x6 pinhole camera for me to purchase and then do my own test with it.

Are you sure your camera is f 128? Use this pinhole camera calculator.

http://www.mrpinhole.com/calcpinh.php

M Carter said:

Multiple pops is to use very small apertures, or if you're not running 4800WS packs with dual-tube heads, etc., not for special effects. Though usually for tabletop work, it's 2 or 4 pops…




Well, there's a simplicity to doing multiple pops with pro packs that have consistent output - if you want to be at at F32 and have a flash reading of F22, you trigger the flash 2 times, just simple math and as the count increases it's easy to figure half or quarter stops for bracketing with E6. I'm thinking for zone late or pinhole, I may need something like 8-15 pops. To double an 8 pop exposure in that case, you'd do 16.

But I'm leaning towards the idea that reciprocity won't be an issue - since no light is reaching the film during the recycle time. I do have a list of oddball stuff I want to test (like, can I wash the halogenation layer from some B&W films before shooting, if I take an old zoom lens and paint all the internal black surfaces silver what will that look like, stuff like that), looks like I'll have a testing day before long.

Click to expand...

a simple test would be to create an indoor scene with a gray cardand at night,expose it with1,2,4,8 and 16 pops.Then, check the density of the card and see how it relates to the number of pops.my bet is it will behave like a low-intensity reciprosity curve,showing a max efficiency(max density/exposure time or pops) somewherebefore max exposure.