Paul C. Buff, Inc. Technical Forum

Search here didn't like the word "Duration" as it's too common, so I have to go this route.

If one has two flash heads: One has a claimed short duration of 1/10,000 second. Second is much slower at 1/300 second. However, both read f/8 on the flash meter.

Does the slower one actually put more light onto the scene (for flash fill) than the faster firing one, even though the meter reads f/8 for both?

I don't think my flash meter accounts for the duration of the flash, and if it has a bearing on the overall exposure.

Tia.

Mack

This won't affect your exposure, unless your flash duration actually drops below your sync speed (eg syncing at 1/200 and your flash is 1/120) - even still, you would still get some exposure, but just darker than you should be getting at your flash output, because the shutter is opening and closing faster than the light is getting out.

Ok, first, I assume you mean each flash meters at f/8, rather than the two together.

Second, we have to know if the flash duration is t.5 measurement or t.1 measurement, for each light. The flash duration is typically a t.5 time, unless otherwise noted. This means the time that it takes for 50% of the flash to discharge. The time it takes 90% of the light to discharge is the t.1 time, and is typically 3x as long for voltage controlled or capacitor switching lights. So your t.1 time for the respective lights is roughly 1/3000 and 1/100 of a second. Some cases, when IGBT technology is used the really short durations are t.1 times (in the case of Einstein, for example, or speedlites).

So, assuming these are t.5 times, a x-sync shutter speed of 1/250 will capture all of light 1 but miss a fairly significant portion of light 2 (1/3 to 2/3 of a stop). A slower shutter speed will capture the same amount of light 1, but stay open longer to capture more of light 2, which means light 2 *could* meter higher at a slower shutter speed.

If these are t.1 times, then a 1/250s shutter speed would probably capture both in their entirety (or nearly enough) that both are contributing equally and neither would benefit from a longer exposure.

Lastly, it is important to note, if you are metering in the 1/10 stop scale, f/8.0'0 is essentially one stop less than f/8.0'9, as f/8.0'9 is just 1/10 of a stop below f/11. So when metering, pay attention to the numbers after the apostrophe (or they may just be smaller numbers, depending on the meter). Also, if metering in 1/3 stops, there could be a 1/3 stop difference, but the meter would not show it.

Thanks TS.

Where I am seeing this duration issue is in long exposures with a 6-stop ND filter.

I've set my exposure to 1/2 sec. @ f/11 ISO 100, 10 feet, through the filter and it looks good.

Now if I add a flash for a fill light, it look like nothing happened in the image at all (still dark). Yet the flash meter tells me to close down even more to maybe f/16 or f/22 from f/11 above. Flash was at 8 feet distant.

For whatever reason, the flash isn't doing anything at all when set to a slower shutter speed even though the flash meter wants me to close down more to make it darker as it reads the brief flash pop. That brief pop seems to have zero effect at slow shutter speeds through the ND filter which is puzzling me.

I'm thinking that at slow speeds of 1/2 second, the flash must have a duration to match the speed or it has no affect on the image, aside from the meter saying it does but not in reality?

I don't know if the reverse is true; i.e. if I open the diaphragm up and leave the shutter speed high?

Tia.

Mack

First, are you taking the 6 stop filter into consideration when metering the flash?

How are you triggering the flash, and are you using rear curtain sync?

If you are using rear curtain sync, why?

Just checked, rear curtain sync wasn't used. Whew!

My setup was two gray cards in a V: One in sunlight side, other in shade. Meter said 1/160 @ f/11 ISO 100. Image looks fine.

Now onto the flash one:
The flash GN had me place it at 8' for the f/11. Same settings of 1/160 @ f/11. Bit bright for this 2nd shot so maybe f/16 would be better, but the fill was okay. Flash meter agreed on the f/16 too and this 3rd image looks fine.

Now I put on the ND filter (to drop the speeds for water).
Put on the Lee "Little Stopper" 6 stop ND and dropped the shutter to 1/2 sec. @ f/11 and no flash, just daylight. Looks fine, other than gray card in shadow is very dark as expected. I just dropped the shutter speed the 6 stops needed for the ND filter.

Now comes the flash problem...

So now using the flash for fill at 8' was way too dark. Moved it in to 4 feet and still way too dark on shadow side of gray card. The flash meter wanted me to set f/22 at 4 feet, but resulting image is way too dark like no ambient or flash at all.

Left me confused as to why lowering the speed seems to have no effect with the flash (Gets too dark)? With the speeds up to 1/160 in sunlight, it's all normal and fine with flash. With the ND filter and slow speeds with the flash it goes dark for some reason. And following the meter's advice makes it way darker (Wants me to close down more since flash is so close, but the ND isn't being read here now too. hmmm...).


Considering the filter and flash, perhaps I need to meter the flash output through the Lee 6 stop ND filter perhaps at 10 feet and come up with some new guide number for fill? I don't know if the 6 stop ND filter also requires me to change the GN to six stops down too? That seems odd if the GN=80 normally in daylight, so a GN of maybe GN=0.2? (I ain't a f/stop math wizard. :? ) for flash? I know at 4 feet I had nothing at full power, but can't be that I need to be inches away for a fill light?

That's why I wondered if longer duration might also be part of this? i.e. if the flash was 1/2 second duration would I need to alter the GN too?

Mack

Shutter speed has no effect on flash intensity like it does on ambient light (assuming you are below x-sync, and in some cases, a tad below that).

Lets say you have a 1/60s shutter speed. All of the flash is discharged in the first 1/300s (assuming t.1 time). So in a dark room, a 1/125s, a 1/60s, or a 10s shutter speed will look exactly the same, because there is no more light hitting the subject after that initial pulse because the flash has exhausted all of its energy.

In bright ambient light, no additional flash will hit the subject with longer shutter speeds, but ambient light will continue to accumulate. In your scenario, the 6 stop filter stops 98.5% of all light, flash and ambient. You lengthen the time ambient can accumulate to compensate for this loss, but you do not give any compensation to the amount of light cut from the flash contribution. IF you metered through the ND filter, you would probably have a more reasonable measurement.

To compensate for the 6 stop loss, you can move the light from 8' to 1', increase the flash by 6 stops, or a combination of the two, or use a more efficient modifier like the OMNI or silver HOBD (and maybe distance or power, depending).

Incidentally, physically moving the light from 8' to 5.6' is one stop. Moving again to 4' is another stop, then to 2.8' and 2', 1.4', and 1' are all one stop increments. You may notice a convenient pattern here. :)

Thanks for your help. I really appreciate it.

For whatever reason, I thought the flash would be part of the ambient and whatever I used for a ND filter would apply to the overall scene as well as the flash. Didn't treat it as a double-exposure.

For fun, I placed an Ultra WL 1200 flash behind the 6 stop ND filter at 10 feet and fired it at the meter. It read close to f/2 so I guess I will try maybe a GN=20 and maybe even a GN=10 today and see how that goes. Somehow it seems very odd that a flash would need to be maybe 1 foot away at f/11 assuming GN=11, but if that filter absorbs 98.5% of the light I guess it might be right. A Lee 10-stop ND or that 15-stop ND by Singh-Ray may make any flash fill impossible.

Aside, I saw where Kenco (sp?) flash meters have some "Flash as a percentage of ambient" analyzing mode. Their website shows some bell curve (the flash portion) over the block for the ambient.

I'd be curious if those 2-second long-duration burn "flashbulbs" made by Meggaflash for destruction testing would change the exposure since they flood the scene much as daylight and would be treated as daylight and not a fractional part of the exposure as with electronic. At $75 per bulb, I don't know, but 140,000 lumen/seconds is their claim (or 280,000 lumens for the total 2-second burn?). If sunlight is 20,000 lumens, those things must be super bright (2.5x that of sunlight?). I cannot find info on what 140,000 lumen/seconds equates to some EV or Guide Number either. Trial-and-error at $75/pop seems sort of outrageous, but Meggaflash may like it ($$$).

I guess reading the scene with the flash meter and using the heavy ND isn't going to work since the flash doesn't have a ND on it, and why my readings are f/22 (Flash too close) yet they are getting darker by using the flash reading. Guess I'd need to ND the sunlight and ND the flash "both" and leave straight-shot from subject-to-camera for the flash meter to work properly. I'll just use the low GN recommendations today and ignore the meter, other than for the daylight setup.

Thanks again. This strong ND filter thing gives me a headache but I like the challenge. Gave me something to think about and try today using a far lower GN too.

Mack

Think of the ND filter as reducing your ISO. Assuming you are shooting at 100, the ND filter reduces this, effectively, to ISO 1.5.

The fact the MeggaFlash bulbs burn for 2 seconds vs. 1/200s is not, in and of itself, the reason it is so bright. 5-X3200s (or 10 AB1600) would also produce 140,000 Ls, but do so in 1/300s. (duration may be a by-product of why it is so bright, or it may be by design to take that long).

If you have two images. In one image, you have 10,000 Ls of light discharged in 2s and a shutter speed of 6 seconds, you will get the same result as an image that has 10,000 Ls of light discharged in 1/300s and a shutter speed of 6 seconds. You have 6 seconds of ambient light, and 10,000 Ls of flash.

Metering is fine to get your daylight/flash ratio. The ND filter will drop both sources in ratio. Using ISO and/or aperture, you can raise the exposure of both simultaneously. If you use shutter speed to raise the ambient, you will need to use flash power/position to raise the flash contribution.

I assume you are using a longer shutter speed specifically for the visual effects of a longer shutter speed (ex. flowing water). If not, what is your end goal?

You are right, it is moving water. Good catch.

My AM test got me down to around GN=25 for a fill. Daylight meter had me at 1/500 sec. @ f.6.3 ISO 100.

With the 6 stop ND, 1/8 sec. f/6.3 ISO 100.

With flash at 4' it showed a fill with the 6 stop ND 1/8 sec. @ f/6.3. So I am guessing I need GN=25 with the modifier and flash.

I'd like to get down to 1 sec for the moving water, but that puts me at 1 sec @ f/19 (From meter reading), and with a GN=25 for the flash. So flash is about maybe 15 inches flash-to-subject?

I'd like to open the lens more than f/19 at 1 second, but that puts me using a 10-stop ND and a whole new flash GN too (GN=0.25 which is almost on top of the subject?).

Think I need more power somewhere. :o

Fwiw, I also got a color shift using the Lee 6 stop "Little Stopper" filter, but seems that is a common issue too as they are not truly neutral gray. I think I can manually set the Kelvin in the body to compensate for that without taking a lumens/power hit and a whole new GN.

Mack