Flash Unit Beam Candlepower Seconds?

[StrangleMeRandy]

I have a Kodak Master Photoguide (the silver edition) and in the flash page it refers to BCPS, which isn't listed in my flash unit's specs. Is there a way to calculate or find it based on guide number or something? My specs start on page 22 of the manual below (25 of the actual pdf).

http://www.cameramanuals.org/flashes_meters/minolta_auto_360px_flash.pdf

 

[AgX]

Your flash shlould have a nominal output of 60Wsec .

 

[Ian C]

The manual shows an ASA-100 guide number of 36 in meters (118 in feet) at full power. The calculated full-power rating is 2785 BCPS.

A Kodak guide book shows

G = squareroot(Bs/20)

Where

G = guide number (in feet)

B = BCPS rating

S = ASA film speed

For example, a Paul Buff 5000 flash is rated at 5000 BCPS. So for ASA 100,

G = sqrt(5000*100/20) = 158 (feet) = 48 (meters)

Note: 1 foot = 0.3048 meter

Here is the Paul Buff 5000 manual. It claims a guide number of 160, which is a roundoff of the calculated GN = 158 (feet) above. [see 2. The Guide Number Method in the PDF]

https://www.paulcbuff.com/manuals/retired/flash-units/wl5k10k.pdf

We can also solve for B from G and s as

B = (20G^2)/s

Using the above example,

B = 20*(158^2)/100 = 4993 which is approximately 5000 BCPS.

The value is exact when the full precision of the calculated value of B = 158.113883008 is used.

Watt-second (Ws) ratings are not directly related to light output. This is simply the amount of electrical energy stored in a flash unit’s one or more capacitors. Flash units vary in efficiency in converting stored energy into light. Thus, the Ws stored-energy rating isn’t a reliable indication of light output. US makers tend to use Watt-seconds in rating studio flash. European makers use the SI unit joule. There’s no difference.

1 Watt-second = 1 joule.

 

[AgX]

Interesting post, Ian.

Over here one does not use BCPS, it is even unknown.
Wattseconds are typically used in comparing flashes with exchangable reflectors. The guidenumber has lost its practical value since manufacturers of on-camera flashes use it in the most misleading way...

Using Wsec for on-camera flashes gives an idea how to relate them to studio flashes.

 

[StrangleMeRandy]

Thank you both for the detailed responses!

 

[wiltw]

Using Wsec for on-camera flashes gives an idea how to relate them to studio flashes.

Unfortunately the correlation between Watt-sec and light output is NOT a good one!
Years ago Chimera be published a comparison of 1000 w-s units from a wide variety of mfgr and Dynalite output was 1EV higher than another well respect pro brand!

 

[AgX]

But good enough to give an IDEA.

The Wsec relate to the charge of the main capacitor. One may argue on losses up to that charge transferred into photogenic light. But a great part in the end plays the reflector and the tube mount..

 

[wiltw]

But good enough to give an IDEA.

The Wsec relate to the charge of the main capacitor. One may argue on losses up to that charge transferred into photogenic light. But a great part in the end plays the reflector and the tube mount..

We agree on the IDEA. BCPS was better than W-s simply because the BCPS was indicative of actual light output, while Watt-seconds was more distantly relative in the conversion efficiency of electricity to light.

BCPS suffered from the fact that the Coverage Angle was not considered, but merely the measured intensity of the light at a standard distance. Two different flash units could have same BCPS rating, but one head might cover 100 degrees of area while the second head only covered 60 degrees of area. But W-s suffers from the same limitation.

 

[AgX]

But W-s suffers from the same limitation.

And guide numbers too...

With all these units we still have to do our own thinking... which likely is not the worst in times of full-auto and artificial intelligence.

 

[wiltw]

And guide numbers too...

With all these units we still have to do our own thinking... which likely is not the worst in times of full-auto and artificial intelligence.

Well, when a manufacturer like Canon provides a Table of Guide Numbers for different FL settings, you know the relative output regardless of angle of coverage which is set.
But all too many manufacturers (the Asia cheapie brands come to mind) provide NO INFORMATION but assume the user is always going to be using nTTL!