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01-07-2018, 01:04 PM   #1
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What is the flash energy of the AF-500FTZ?

I have not found in any Pentax data or in published reviews the flash energy (watt-seconds or joules) that the AF-500FTZ flash emits. It would be a project to work backwards from the guide number to derive an approximate value that would be rife with dubious assumptions. Anyone who has seen a published value please feel free to respond.

Thanks

01-07-2018, 01:35 PM   #2
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QuoteOriginally posted by kaseki Quote
I have not found in any Pentax data or in published reviews the flash energy (watt-seconds or joules) that the AF-500FTZ flash emits. It would be a project to work backwards from the guide number to derive an approximate value that would be rife with dubious assumptions. Anyone who has seen a published value please feel free to respond.

Thanks
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01-07-2018, 01:46 PM   #3
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QuoteOriginally posted by kaseki Quote
I have not found in any Pentax data or in published reviews the flash energy (watt-seconds or joules) that the AF-500FTZ flash emits. It would be a project to work backwards from the guide number to derive an approximate value that would be rife with dubious assumptions. Anyone who has seen a published value please feel free to respond.

Thanks
With most on-camera electronic flash, energy output is seldom published since it has little or no practical application for calculating exposure and motion stop ability in the field. Instead, guide number and flash duration are more typical. Such is the case with the AF-500FTZ. A lux meter to measure power might be a good way to actually measure output with a back calculation to get watt-seconds.

Addendum: The watt-seconds of a flash is actually the energy consumption on discharge, not radiometric output. For that measurement of radiometric emittance (watts/m^2 source surface area) is more appropriate, though correction to allow for actual spectral output might also be needed.


Steve

(...none of my flashes have energy as part of their specification...)

Last edited by stevebrot; 01-07-2018 at 02:07 PM.
01-07-2018, 02:19 PM - 1 Like   #4
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For anyone stumbling on this thread and curious about the topic, here is a little more information.
  • Studio strobes power is usually given in watt-seconds
  • Speedlights and on-camera flash power is usually given as guide number
  • The basic calculation for Ws is based on the energy consumption of the flash:
    Ws = ((capacitance in farads)2/2
    Ws = ((capacitance in farads)*voltage)2/2
    ...or something similar. I could not find an unambiguous equation.
There is a also a short article for compare and contrast of Watt-second vs. guide number that I have found helpful at:
Watt-Seconds vs. Guide Numbers | DPanswers
I found this paragraph from the article particularly helpful:
QuoteQuote:
In terms of energy consumption, a 60 Watt tungsten bulb burning one second, a 600 Watt halogen light being lit for 1/10th of a second, and a xenon tube electronic flash burst consuming 60000 Watt for 1/1000 second all equals 60 Ws. However, a xenon tube is much more efficient converting energy into light than the other two, so even when fitted with the same reflector, the 60 Ws electronic flash will produce more light than each of the other two. Translated into GNs, the GN of the electronic flash in this example would be greater than the GN of the tungsten bulb or the halogen light.
A conventional conversion from guide number (GN) to Watt-second is:
Ws = (GNm/4)2
Ws = (GNft/13.2)2
Note that the flash tube efficiency (see quote above) is not factored in.

Also...
Understanding Watt-Seconds | AdoramaTV

Steve

(...will note that Watt-seconds might be more useful in calculating flash battery life...)


Last edited by stevebrot; 01-07-2018 at 03:02 PM. Reason: Capitalization
01-07-2018, 03:07 PM   #5
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From Guide nummer to Ws must be another equation, there must be something which contains the focal length. Today the guide number of a flash is given at the maximum zoom extend of the flash. Before any calculation it must be normalized to a standard focal length.

Then the relationship mentioned above might be correct.

Compare Flash Guide Numbers for Different Focal Lengths: Open Talk Forum: Digital Photography Review
01-07-2018, 04:27 PM   #6
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Aha! Some have seen the method to my madness. I am actually interested in determining [a measure of] Quantum Turbo Battery life by loading the unit such that the drain on the battery is similar to that which the flash would induce if one wasted flash lamp life going through hundreds of flashes. Because the voltage is about 330 Vdc, I suppose the easiest method would be to buy and gut a raggedy example of an AF-500FTZ to find out what size capacitor it uses. (1/2)CV^2 is correct for storage energy, and most of it will go into the lamp if not quenched with TTL.

As to the relationship of guide number to flash power, even ignoring flash efficiency (input electrical power to output radiant power in the spectral band that film cares about), guide number will depend on beam shaping, if any, and certainly the AF-500FTZ has not just beam shaping, but variable beam shaping. Measurement of total flash output from which one might infer flash input requires an integrating sphere unless the flash radiant intensity as a function of angle is known (and preferably simple, as in a cylinder shape).

So the bottom line as supplied by stevebrot is that speed lights are not usually quantified by energy, and I shouldn't expect anyone to know the W-s value for the AF-500FTZ.

Thanks everyone for your help.

P.S. joergens.mi: Where do those Pentax proud owner icons come from?
01-07-2018, 04:34 PM   #7
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QuoteOriginally posted by joergens.mi Quote
From Guide nummer to Ws must be another equation, there must be something which contains the focal length. Today the guide number of a flash is given at the maximum zoom extend of the flash. Before any calculation it must be normalized to a standard focal length.
Good point. While focal length is often used, the actual factor for calculation is the coverage area. Convention for non-focusable flash is coverage to accommodate the FOV of 50mm lens on 24x36mm format.


Steve

01-07-2018, 04:40 PM   #8
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QuoteOriginally posted by kaseki Quote
Aha! Some have seen the method to my madness. I am actually interested in determining [a measure of] Quantum Turbo Battery life by loading the unit such that the drain on the battery is similar to that which the flash would induce if one wasted flash lamp life going through hundreds of flashes. Because the voltage is about 330 Vdc, I suppose the easiest method would be to buy and gut a raggedy example of an AF-500FTZ to find out what size capacitor it uses. (1/2)CV^2 is correct for storage energy, and most of it will go into the lamp if not quenched with TTL.

As to the relationship of guide number to flash power, even ignoring flash efficiency (input electrical power to output radiant power in the spectral band that film cares about), guide number will depend on beam shaping, if any, and certainly the AF-500FTZ has not just beam shaping, but variable beam shaping. Measurement of total flash output from which one might infer flash input requires an integrating sphere unless the flash radiant intensity as a function of angle is known (and preferably simple, as in a cylinder shape).

So the bottom line as supplied by stevebrot is that speed lights are not usually quantified by energy, and I shouldn't expect anyone to know the W-s value for the AF-500FTZ.

Thanks everyone for your help.

P.S. joergens.mi: Where do those Pentax proud owner icons come from?
Wow! What an awesome way to bring the thread full-circle

We have had similar questions in the past and the usual intent was to compare speedlights to low-end strobes to guide in a purchase decision. It is possible that one or more users on this site may have disassembled an AF-500FTZ and might know the capacitor specification.


Steve
01-07-2018, 06:17 PM   #9
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The studio question reminds me of why one would want an energy number -- blasting into an umbrella pretty much eliminates guide number from the discussion because the umbrella determines the angular pattern. I have some Paul Buff White Lightening lights from the distant past that I just dug up the literature for. (I'm ancient; they're only distant.) The higher power one has the following three parameters: 10000 BCPS, 250 W-s, and GN 220 ft at iso 100. For actual illumination in the eye's spectral band, the candle-power-seconds value is most pertinent, but we don't usually get that information with speed lights for comparison.

So lets do a dirty approximation based on guide numbers and beam widths. The Paul Buff beam width is about 60 degrees, and the 500's beam width at 28mm zoom position is listed as 53 x 70 degrees, which I'm going to declare as equivalent pattern-weighted steradians. The 500's guide number at this zoom is 32m or about 100 ft. Hence, ta dah, the energy used for the 500, assuming the same lamp technology, etc., has to be that of the Buff lamp times the square root of the ratio of guide numbers, or 250 x root{100/220} = 169 W-s. Given that the Buff lamp is exposed with minimal losses and the 500's lamp is behind a plastic lens and may not be enveloped similarly by its reflector, the 500's input power may need to be higher, perhaps as much as 200 W-s.

200 W-s at 330V requires nearly a 4000 microfarad capacitor. This seems to me to be asking a lot of the available volume in the AF-500GTZ. But then, we are asking a lot of this method of scaling. I guess the capacitor value is what needs to be determined.

---------- Post added 7th Jan 2018 at 20:37 ----------

Addendum: The Buff pattern looks very Lambertian, and if so would encompass pi steradians. The 500's pattern may be sharply truncated at the specified angles, and be closer to 1 steradian. This would lower the capacitor value to the ballpark of 1200 uF at 400V, a still large but maybe plausible to package value.
01-07-2018, 08:33 PM   #10
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QuoteOriginally posted by kaseki Quote
Aha! Some have seen the method to my madness. I am actually interested in determining [a measure of] Quantum Turbo Battery life by loading the unit such that the drain on the battery is similar to that which the flash would induce if one wasted flash lamp life going through hundreds of flashes...................
You've got several issues doing it this way. First, the flash uses somewhat more than any given amount of flash energy since the inverter circuit which supplies the high voltage isn't 100% efficient. That same inverter will run even though the flash capacitor is fully charged, eating more battery energy that doesn't go into the rated Ws or Joule output of the flash.

Calculating the flash discharge energy from the scene brightness is wrought with room for errors. As you pointed out, the concentration of flash power by the reflector and Fresnel lens is unknown and is a key factor as to how much of the flash energy is directed out of the flash. There is also the conversion efficiency of the flash tube to consider since it will not convert all of the stored energy to light. Then there is flash duration which is varied by the flash circuit and it's anyone's guess where that is set for a given discharge (unless manual full output is selected). If the full discharge is not used (discharge % unknown), the stored energy value is meaningless. Then there is the inverter efficiency which is unknown.........and there's some capacitor leakage (though small) which eats battery energy.

What this boils down to is that it would be far easier to tap into the battery circuit and measure current drain for a full discharge and determine battery life from there. Even this is pretty difficult, because the current starts high and decreases (as the flash capacitor charges), and battery life varies depending on current (longer for low drains, and shorter for high drains - Peukert's law). (If you were to plot and integrate the voltage & current curves product for a charge cycle, it would give you a close approximation of the flash energy not taking inverter efficiency into account.)

Better to just trip the flash (manually set to maximum flash power) X number of times with fully charged batteries (Quantum Turbos of course), until they quit and average a few trials to come up with average battery life. That still has room for error since the life you determine will depend on the frequency at which you trip the flash (and you might melt down the flash and/or the batteries in the process if that frequency is too high).

Anyway, it's a good challenge but let your neighbors in on what you're doing so they don't think you have a gun fight going on in your house.

P.S. You're probably in the neighborhood of 200 to 500 Joules per flash at full power with the AF500FTZ.

Last edited by Bob 256; 01-07-2018 at 09:26 PM.
01-08-2018, 12:01 AM   #11
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QuoteOriginally posted by kaseki Quote
P.S. joergens.mi: Where do those Pentax proud owner icons come from?
I found them here on the forum and put the appropiate ones together.

In former times all GN where done at ISO 100 an 50mm focal length.
Here are the numbers for my AF540FGZ
Iso 100:
mm GB
85 54
70 50
50 45
35 39
28 35
24 32
20 21 with spreader.
ISO 200: GN will increase by 1.4142
ISO 300: GN will double


Metz 58
ISO 100
mm GN
105 58
85 52
70 46
50 42
35 35
28 31
24 29
18 25

You see even with a higher GN the Metz is "weaker" than the pentax I don't think the GN 3 difference is that important in light, but you see how th customer is given a "wrong" information, if he don't know the exac circumstances.

Last edited by joergens.mi; 01-08-2018 at 12:34 AM.
01-08-2018, 10:32 AM   #12
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Thanks for the further insights. W.r.t. "Bob 256" first paragraph inverter comment, with the QTB feeding 330 Vdc to the flash directly, the inverter in use is that of the QTB.

My initial goal was to avoid operating the flash a massive number of times while achieving the purpose of detecting by experiment when a QTB battery was degrading relative to its new-ish state. I believe that this can be achieved by reasonable steady state loading of the QTB, although pulse loading could be more stressful. The exact number of flashes produced is irrelevant; first because I would never use that many before recharge, and second because flashes would likely in real life have various flash powers. I just wanted a definition of 'reasonable' that had some connection to the set of AF-500FTZ I have. I could argue for 100 W-s every 10 seconds (10W) as an average load on the QTB and measure the time from full charge to shut off of the 100% charge green light (or one of the other lights). A value measured a year from now would be compared to a value measured now. It is not practical to disassemble the QTB to measure the battery directly.

QuoteOriginally posted by joergens.mi Quote
Originally posted by kaseki Quote P.S. joergens.mi: Where do those Pentax proud owner icons come from? I found them here on the forum and put the appropriate ones together.
OK! Copying is the best compliment. Now to scour the forum for a 645n version.

---------- Post added 8th Jan 2018 at 13:26 ----------

Looks like the motherlode exists in a sticky in the DLSR forum. No 645n though. I'll have to modify one of the existing 645 DLSR images.
01-08-2018, 01:23 PM   #13
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Image modified and inserted into signature profile. Maybe it will show up eventually.

---------- Post added 8th Jan 2018 at 15:24 ----------

Looks like it is not retroactive.
01-19-2018, 07:54 AM   #14
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Update

Capacitor sizes. I haven't found a junk AF-500FTZ to dissect, but here are some characteristics reported for some other comparable flashes.

Nikon SB800 ~ 75 W-s from 1400 μF @ 325V
Nikon SB600 ~ 54 W-s from 1000 μF @ 330V
Nikon SB24 ~ 76 W-s from 1400 μF @ 330V
Minolta 320X ~ 61 W-s from 1000 μF @ 350V

Quantum Turbo Battery characteristics. The Quantum Turbo Batteries can be separated into two groups. The early group has a red 25% remaining battery capacity light and three green lights, whereas the late group has all four lights green. The early group output is 330 Vdc. The late group output is 330V pulses with a period of roughly 180 ms. Both seem to recharge the flash in about the same amount of time, but an accurate method of measuring the time is a gedanken experiment in progress. The two pins for external power seem to be isolated from the capacitor, so it is not trivial to just measure the capacitor voltage as it is charged. I'm hoping that the state of charge will show up in the voltage-time history of the QTB output. I need another Quantum part, presently on order, for the next experiment.

The implication of the two different outputs is that any battery life testing will require different circuits.
01-19-2018, 09:58 AM   #15
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QuoteOriginally posted by joergens.mi Quote
I found them here on the forum and put the appropiate ones together.

In former times all GN where done at ISO 100 an 50mm focal length.
Here are the numbers for my AF540FGZ
Iso 100:
mm GB
85 54
70 50
50 45
35 39
28 35
24 32
20 21 with spreader.
ISO 200: GN will increase by 1.4142
ISO 300: GN will double
...................
Here are the AF500FTZ numbers (meters) for 100ISO:

85 50
70 47
50 42
35 36
28 32
24 30

(at ISO400 they double)

The energy output doesn't depend on internal or external batteries as long as the flash capacitor voltage is the same. I believe that it is in either case for the AF500FTZ.. It would be an easy matter to calculate the energy output if you know the size capacitor that the AF500FTZ uses.

There is no one equation for determining output power from a guide number - it changes with every flash unit, lamp, and flash reflector design (including flash zoom factor). Best case would be an equation that delivers an approximation with considerable error for some flashes. The way the flash draws power from an external pack has a lot to do with how long the pack will last. It's very much dependent on when that pack decides it's empty (might be the 50% charge point or it might be the 10% charge point). Whether the inverter is in the flash or the external pack still means the battery power has to go through an inverter and its efficiency comes into play (exception is when the batteries themselves provide the high voltage as was the case with some old flashes which used 300 V batteries).

Last edited by Bob 256; 01-19-2018 at 10:22 AM.
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