Using electronic flash with LF cameras

[tonyowen]

Response to posts 73 thru 80

For clarity, the Cobra D650 has a GN of 46 at 100 ISO [meters] when set at the 85mm zoom. This is what I'm using.
Therefore, the GN at 6 ISO is 11.75 one stop down gives me a GN of 8.30 [???]. But why should the GN be one stop down at 6 ISO but not at 100 ISO or 50 ISO??

Whereas my estimated effective GN of about 4 for multigrade paper negative based on results [Post #70] infers an ISO of less than 1 for the paper

Quote from the Cobra manual "Flash duration Manual 1/1000th sec at full power" - again this is what I'm using.

I'm assuming that the flash output is daylight not tungsten. Therefore, given that multigrade paper is [often/mainly] exposed with tungsten light and given that film is panchromatic film, but paper is orthochromatic [Pere] might this explain the paradox I have between the results of the three media types????

Do we continue this "multi-logue"??????

regards all round

Tony

[Dan Fromm]

But why should the GN be one stop down at 6 ISO but not at 100 ISO or 50 ISO??

Because flash manufacturers are, to be polite, optimists. To be impolite, liars.

[tonyowen]

Because flash manufacturers are, to be polite, optimists. To be impolite, liars.

sorry Dan I phrased my comment on #81 wrongly.

The film images at 100 ISO and the digital images at 50 ISO taken using the Cobra flash seem okay.
Hence if the flash GN is [seemingly] correct for ISOs 100 & 50 then why not at other ISOs.
Surely any optimism will be linear.

I agree with your comment regarding (all and any) manufacturers and their sales/publicity department.

regards
Tony

[Dan Fromm]

Tony, I've had flash meters for years. The only flashes I've tested with meters -- testing with negative film has many frailties, testing with reversal is better but not perfect -- that matched their makers' claims were Minoltas. All of the others ran around one stop under.

[tonyowen]

Tony, I've had flash meters for years. The only flashes I've tested with meters -- testing with negative film has many frailties, testing with reversal is better but not perfect -- that matched their makers' claims were Minoltas. All of the others ran around one stop under.

Dan how does your comment "Because flash manufacturers are, to be polite, optimists. To be impolite, liars. " relate to flash meter manufacturers in terms of the [claimed] accuracy, precision and repeatability of their products??????

regards
Tony

[Dan Fromm]

Their claimed GNs are usually one stop more powerful than measured. Repeatable, yes if allowed to recharge between full power pops. Accurate, no.

[hsandler]

I’ve found a reference that states: As a rough rule of thumb, multiplying the square root of the (wattsecond) Ws number by 4 gives a indicator of the metric GN at ISO 100.

Therefore assuming your shutter is set at 1/60th second and your studio lights give 1500w power

We get 1500 x 0.017 = 25Ws

Therefore GN = SQRT(Ws) x 4 = 20

Both of my flash guns have a higher GN at 100 ISO

However, as stated above this is a rough rule of thumb.
regards
Tony

Tony, I think Phil probably meant he used three 500Ws flash heads, not 500W, as he was writing about flash, not continuous lights. So take the 1/60s shutter speed out of the calculation. That gives a much higher GN for his setup, using your rule of thumb, than your two flashes. Couple that with the effective ISO of the paper being much less than 6 when using flash due to its orthochromatic response and possibly reciprocity (paper not intended for brief intense flash) and I think your results are explainable. Your equipment seems to function; you just need a lot of light.

[Bob Salomon]

Tony, I think Phil probably meant he used three 500Ws flash heads, not 500W, as he was writing about flash, not continuous lights. So take the 1/60s shutter speed out of the calculation. That gives a much higher GN for his setup, using your rule of thumb, than your two flashes. Couple that with the effective ISO of the paper being much less than 6 when using flash due to its orthochromatic response and possibly reciprocity (paper not intended for brief intense flash) and I think your results are explainable. Your equipment seems to function; you just need a lot of light.

It is a meaningless formula since WS is not a power rating, it’s an input rating and the output will vary with each makers pack rated at 1500 WS plus the formula does not provide for the angle of coverage.

WS is pretty much like horsepower. Horsepower will not tell you your miles per gallon or time to 60mph or time and speed in the ¼ mile.

All you can assume from hp is it probably uses more fuel per mile, it probably can do a fast 0 to 60 and perform well in the quarter mile. But it can not and will not let you assume how well or how much fuel. Watt seconds is the same, it is nice for advertising but it can not convert, by itself, to a useable GN or aperture!

[Pere Casals]

It is a meaningless formula since WS is not a power rating, it’s an input rating and the output will vary with each makers pack rated at 1500 WS plus the formula does not provide for the angle of coverage.

I agree, the angle of coverage is basicly variable because we always use a kind of light modifier, the shape or the reflector itself modifies that angle...

WS rating is not to adjust aperture, but to compare capability of different strobes.

To me the WS is the most important parameter of an strobe, because it tells what we can do with it, but of course WS does not tell the aperture depending on power selector position, this is told by the flash meter...

[Tin Can]

Full disclosure: I paid full retail for my collection of PCB studio lighting. I like it, some don't.

The founder, now deceased, explains a lot about flash on this page.

https://paulcbuff.com/Studio-Flash-Explained.html

And i often use 10 fingers to count stops up and down, after I run out I use paper and pencil. Pencil as I make mistakes...