How to calculate exposure with focal plane type flash?

[shutterfinger]

https://www.graflex.org/flash/navy/
Focal Plane Shutter in a Speed Graphic, Graflex SLR, or similar cameras sweep the exposure across the film the width of the curtain aperture in use so guide number should apply as it does with leaf shutters.

 

[BrianShaw]

Donald. Try a slightly different line of thinking. You are thinking about a LED that is timed to the focal plane shutter movement. At most that’s five time durations with setting 1 being the longest (and inclusive of 2-5). Assuming that a LED has no run up time to nominal performance, that’s a fairly easy on-off circuit and should be relatively easy to measure as the “shutter speed”. And it may simplify the exposure question. I’d call that a short-duration continuous light rather than a flash. A normal incident reading might give the correct exposure setting. Thinking out loud, so let me know if my thoughts are wacky.

 

[Mr Bill]

Almost all of the flash photography of interest is in that "gray area" -- too dim to hand hold without flash, but bright enough to get ghosts or blur with open flash.

I get what you're trying to do, but for me, personally, it's a steeper hill than I'd wanna climb. I'd be more inclined to use an electronic flash (obviously firing with a fully open fp shutter) which means you need to have the camera on a stand. And obviously if it's on a stand then you could use your other lenses, focusing on the gg. So those lenses could sync with any electronic flash.

If I were in your shoes, specifically wanting to shoot handheld, focusing via rangefinder, and use flash with the fp shutter, I would take a quick look at some of the cheaper, newer hot shoe flashes. Not well known is that many of the better hot shoe flashes have the ability to do multiple flashes over a given time. Albeit at a greatly reduced power setting, which may not be enough for you.

The first flash I ever noticed that could do this was the (expensive) Canon 580EX II, so if you look up the user guide you can see the terminology they use. As well as the limitations. Once you can recognize the lingo you could look for the much-cheaper knock off units.

What I'm thinking is, for example, to set the flash to fire 50 times at 1 millisecond intervals, or something along those lines. So the flash appears near continuous for roughly 50 milliseconds. Now, the downside is that the power has to be set really low, maybe 1/64th or lower, and it needs a cool-down interval of maybe a couple minutes (?) between flashes, and I don't know if it's a smooth enough output to prevent "chatter marks" on the image. But... if it's anywhere near close enough, power wise, it might be feasible to use a bank of 4 flash units, giving a 2-stop boost over one unit. So maybee....??

Obviously (?) LED arrays are gonna be much more efficient battery-wise (electronic flash puts out a lot of IR, which is all wasted energy). But you'll also have to engineer a way to get even lighting, which i think is gonna be more difficult than meets the eye. Offhand I'd say that the most obvious way is to bounce it off a diffused surface (aka white paper), but this is gonna spread the light over a wide area meaning a big loss, maybe several f-stops, with respect to your subject exposure.

Anyway, best wishes with your attempts.

 

[Donald Qualls]

Donald. Try a slightly different line of thinking. You are thinking about a LED that is timed to the focal plane shutter movement. At most that’s five time durations with setting 1 being the longest (and inclusive of 2-5). Assuming that a LED has no run up time to nominal performance, that’s a fairly easy on-off circuit and should be relatively easy to measure as the “shutter speed”. And it may simplify the exposure question. I’d call that a short-duration continuous light rather than a flash. A normal incident reading might give the correct exposure setting. Thinking out loud, so let me know if my thoughts are wacky.

Well, effectively that's what a focal plane flashbulb was in 1947. A near-continuous level of light for the 125-200+ ms of the Speed Graphic's focal plane shutter travel. And yes, I ought to be able to just measure the scene brightness with the LED unit powered up to get an exposure. The limitation is, with a power tool battery, there may be a pretty short time limit to draw the necessary current without overheating anything. On the other hand, since the light level should be pretty much constant as long as the LEDs aren't being current limited, I can probably use an external power source (like jumper cables from a car battery) for the measurements.

I get what you're trying to do, but for me, personally, it's a steeper hill than I'd wanna climb. I'd be more inclined to use an electronic flash (obviously firing with a fully open fp shutter) which means you need to have the camera on a stand. And obviously if it's on a stand then you could use your other lenses, focusing on the gg. So those lenses could sync with any electronic flash.

If I were in your shoes, specifically wanting to shoot handheld, focusing via rangefinder, and use flash with the fp shutter, I would take a quick look at some of the cheaper, newer hot shoe flashes. Not well known is that many of the better hot shoe flashes have the ability to do multiple flashes over a given time. Albeit at a greatly reduced power setting, which may not be enough for you.

The first flash I ever noticed that could do this was the (expensive) Canon 580EX II, so if you look up the user guide you can see the terminology they use. As well as the limitations. Once you can recognize the lingo you could look for the much-cheaper knock off units.

What I'm thinking is, for example, to set the flash to fire 50 times at 1 millisecond intervals, or something along those lines. So the flash appears near continuous for roughly 50 milliseconds. Now, the downside is that the power has to be set really low, maybe 1/64th or lower, and it needs a cool-down interval of maybe a couple minutes (?) between flashes, and I don't know if it's a smooth enough output to prevent "chatter marks" on the image. But... if it's anywhere near close enough, power wise, it might be feasible to use a bank of 4 flash units, giving a 2-stop boost over one unit. So maybee....??

Obviously (?) LED arrays are gonna be much more efficient battery-wise (electronic flash puts out a lot of IR, which is all wasted energy). But you'll also have to engineer a way to get even lighting, which i think is gonna be more difficult than meets the eye. Offhand I'd say that the most obvious way is to bounce it off a diffused surface (aka white paper), but this is gonna spread the light over a wide area meaning a big loss, maybe several f-stops, with respect to your subject exposure.

Anyway, best wishes with your attempts.

Multi-flash units, yeah. I'd thought about that. As I understand it, the ones that genuinely support "high speed sync" via multiple fire-and-quench cycles are all controlled by the camera. Pretty sure my 1940-ish Annie doesn't have the circuitry to do that. In fact, I need to look and see for certain, but I don't think the Anniversary Speed (designed before WWII, started production in 1940) has sync terminals for the focal plane shutter, so I'm limited to an add-on switch, either mounted on the FP trigger, or triggered by it. And yes, for what I've just spent on a Graflok upgrade (and still have to get a ground glass for it), I could have bought a similar condition Pacemaker that has sync terminals and a Graflok back -- but those are less common with the easily adjusted Kalart RF vs. the other side unit or the top one (which aren't easily adjusted at all).

All else fails, I can readjust the RF for one of my other lenses (all three in shutters with PC sync) and either shoot "normal" focal length or "wide and wider" (105mm and 90mm) with a conventional electronic flash -- though the dinky one I have that works would look very out of place on a Speed Graphic and lacks power to reach out as far as 20 feet even on ISO 400).

Even lighting is a question as well, though a little vignetting isn't a terrible thing for press/street style photography.