Have you ever measured your flash unit's output? It's an eye opener!

[Curtis N]

Getting to know my new Sekonic L-358, I decided to measure the output of a few flash units. The results I got were so far off from the manufacturer's specs that I had to double check that the units were on full power and the ISO on the meter was set right. I mean, I know the advertised guide numbers on flash units are usually a bit of an exageration, but the results I got differed by more than a stop! I would be interested to know if the Canon Speedlites are as far off from their specs.

The testing was pretty simple. I used a measuring tape to get the flash unit 10 feet from the flash, hooked the meter up to the flash with a PC cord & hotshoe adapter and hit the button. The outdoor measurements were taken at night. The indoor measurements were taken in a 12 x 15 room with an 8 foot white ceiling. The flash was about 5 1/2 feet above the floor, shooting horizontal across the room. I took three measurements at each setting and averaged the readings. The meter was set at 1/10 stop increments and the reading was mathematically converted to a simple number and multiplied by 10 to arrive at the guide number.

Some other observations:
The Sigma unit was very consistent across multiple measurements, never varying more than 1/10 stop.

The Sunpak unit was not so consistent. Waiting 10-15 seconds after the ready light came on generated up to an additional 1/2 stop. This same inconsistency was evident at 1/2 and 1/4 power.

[Lotto]

OK, Curtis, you GN numbers are too confusing and I am too lazy to to calculation. What F stop is equivalent to Sunpak's OEM spec of 120 at ISO 100 @ 10ft?

I can do a F stops measure with the Canon EXs.

[raymushgrush]

Before I measured the output from flash units I calibrated my light meter to my camera.

i.e. I created images with a kodak gray card totally filling the image and adjusted the shutter speed until the histogram in camera had a spike in the middle that told me I had a correct exposure. I also verified the image in photoshop, using the histogram which told me that the image had a density of 128 which is correct for a gray card.

Believe it or not, in bright sunlight at high noon with camera iso at 100, shutter speed of 1/100 and f 16 produced my correct exposure. I then adjusted my minolta light meter, in incident mode to gave me the same reading for the same conditions. I also did some exposures of just the gray card using flash outdoors at night and setting the camera as per what the calibrated meter told me to use and I was still getting corrrect exposure with the spike right in the middle.

I then checked the flash output outside on a moonless night with 10 feet between my sunpak 383 flash unit and the flash meter and I got a reading of f8 plus 1/3 or guide number of 90 with the sunpak 383 on full power (manual). I also checked my vivitar 285hv which has an advertised guide number of 120 (with the zoom set to wide angle) under the same conditions and it also got a reading of f8 plus 1/3. This is about 1/2 of the advertised power or 1 fstop less. I checked 4 different brands of flash units and they all got 1/2 of their advertised output under the same testing conditions.

Whenever I am outdoors or in a large room I figure I will get about 1/2 of the advertised power.

With any flash unit I have used with manual power settings I never get a full fstop difference between settings.

Yes, in my experience the ready light on the sunpak 383 comes on before it is fully charged.

Hopefully this is useful.

[Wilt]

OK, Curtis, you GN numbers are too confusing and I am too lazy to to calculation. What F stop is equivalent to Sunpak's OEM spec of 120 at ISO 100 @ 10ft?

I can do a F stops measure with the Canon EXs.

Lotto, Guide Numbers are not hard at all...the f/number in your question is f/12 (120/10 =12) !

[Curtis N]

Raymushgrush, thanks for chiming in. I did some initial testing of the meter with a gray card (see this thread (http://photography-on-the.net/forum/showthread.php?t=377475)), and I suppose I could go back and look at the RGB values on those shots. Glad to know I'm not doing something wrong.

Lotto,
GN = aperture x distance
or
GN / distance = aperture
or
GN / aperture = distance.

so if you use a distance of 10 feet, it makes the math easy. ;)

Now for the really anal part:
f/5.6 is actually closer to 5.7. It's actually 2^2.5, or 5.656854249.
Each tenth of a stop is a factor of 2^(1/20).
So if your meter gives you a reading of f/5.6 and 2/10, the actual aperture is
2^2.5 * 2^(2/20) = 6.062866266
roughly. ;)

[PacAce]

Curtis, here are the results I got for the 580EX, 550EX and the Sunpak 333. I used your formula to arrive at the GNs for the flash. Note that I took two sets of measurements, one with the dome in and the other with the dome out. What surprised me was the fact that the measurements were higher when the dome was in. I was expecting the reverse to be true.

I only took measurements at full power at 24mm and 105mm because I have a very short attention span. :mrgreen:

209715

[PacAce]

BTW, I forgot to note that I used a Quantum Turbo 2x2 with the 580EX and the 550EX to get consistent readings. I couldn't plug the Turbo into the Sunpak so its readings varied at tad from reading to reading but not by much (a fraction of a stop, more or less).

[Wilt]

I just measured my Metz 54MZ

at full power, 105mm GN=110, 24mm GN=80

manufacturer's spec = 105mm GN=177


Lies! Damn lies!

[PacAce]

I just measured my Metz 54MZ

at full power, 105mm GN=110, 24mm GN=80

manufacturer's spec = 105mm GN=177


Lies! Damn lies!
It sure would be interesting to know how these manufacturers actually measure the flash output that they use for the product specs.

[Wilt]

Maybe all of them use calculators which are broken?! New math? Irrational numbers?

I just tested one of my Metz 45CL potato mashers, and its coverage is a fixed 35mm lens on 35mm format...its output is -0.3EV compared to the 54MZ at 105mm.
The 45CL has manufacturer's GN=148.

As for consistency of output generally pretty consistent...0.1EV for 54MZ and 45CL

[PacAce]

Maybe they round all their numbers to the next higher tens before they do the calculations. And then multiply the result by 2, round it to the next tens again and then divide by 2 to come up with the final result. :lol:

Oh, wait! That doesn't explain the 8 in the GN for the 580EX, though. :confused:

[Wilt]

How stupid that we are ALL making the same mistake...not realizing that they use Octal numbers in the counting!

[Curtis N]

Thanks, Leo and Wilt.
There is some small consolation knowing that Sunpak and Sigma aren't the only manufacterers in fantasy land, and there wasn't some fundamental mistake I was making.

Kinda makes you wonder what to do with the distance scale on the back of the flash unit, though.

[Wilt]

Kinda makes you wonder what to do with the distance scale on the back of the flash unit, though.

Do the arithmetic, then add EC +1 to what you set!

I think that flash specs are somewhere where Stereo Amplifier power specs were 40 years ago...universal lies that nobody could ever corroborate. Electronic snake oil.

My guess, seriously, is that they have a small room, not much in it that absorbs light, and they conduct the test in that. When I first ran my tests in a 7' narrow area where my computer is, the 54MZ measured out to have GN=256. Moved the test to a larger area 20', and the test measured GN=110 and the only difference was the width of the space.

[Titus213]

The first thing I thought of was the meters....Using a $240 meter to verify a $400 flash? Are we to assume the flash is off because the meter says so? Or could the meter be an issue?

At any rate I may take some measurements too - right after my nap.

[PacAce]

Do the arithmetic, then add EC +1 to what you set!

I think that flash specs are somewhere where Stereo Amplifier power specs were 40 years ago...universal lies that nobody could ever corroborate. Electronic snake oil.

My guess, seriously, is that they have a small room, not much in it that absorbs light, and they conduct the test in that. When I first ran my tests in a 7' narrow area where my computer is, the 54MZ measured out to have GN=256. Moved the test to a larger area 20', and the test measured GN=110 and the only difference was the width of the space.

Well, that's the odd thing, Wilt and Curtis. When I set the aperture and ISO according to the specs so that the distance scale read 10 ft, the exposure was a little "hotter" than I would have liked. I then increased the aperture to what the light meter recommended and that resulted in an even brighter picture; way brighter than I really care for. :confused:

[PacAce]

The first thing I thought of was the meters....Using a $240 meter to verify a $400 flash? Are we to assume the flash is off because the meter says so? Or could the meter be an issue?

At any rate I may take some measurements too - right after my nap.

My meter cost me more than my 580EX and the readings I got with it collaborate what Curtis and Wilt got.

[PacAce]

Here's another tidbit of info. When I set up a mid-gray board as the target 10 feet away from the flash and spot meter that with the light meter, I get a reading that is more than a tad above the specs at 24 mm (130 read vs 92 spec) and very close at 105 mm (200 read vs 190 spec). Me thinks the flash makers are probably using reflective measurements instead of incident.

[Curtis N]

Me thinks the flash makers are probably using reflective measurements instead of incident.Perhaps, especially if they're getting surface reflections as happened to my in my initial testing (http://photography-on-the.net/forum/showpost.php?p=3925781&postcount=1).

[PacAce]

Perhaps, especially if they're getting surface reflections as happened to my in my initial testing (http://photography-on-the.net/forum/showpost.php?p=3925781&postcount=1).

I had to go out of my way to avoid the surface reflections in order to get numbers that came close to the specs. I actually ended up taking the readings off from one side. Measurements taken with the meter perpendicular to the surface produced an exaggerated reading much higher than the specs.

[FlashZebra]

My take is that when it comes to the guide numbers for a flash, the design engineers go to lunch and the marketing departments take over.

This is not an issue of meter accuracy, it is an issue of the ready accommodation of marketing department to just lie in what they think are very creative ways.

I also find the idea that the measuring instrument's cost has to somehow be commensurate with the cost of the item being measured as a very odd one.

The measuring instrument should be chosen based on it's own intrinsic accuracy and the accuracy needed. Not based on how much the measuring instrument costs with regard to the item being scrutinized.

I have routinely used a 2 dollar metal scale to measure dimensions on parts or assemblies that cost 1000s of dollars.

Alternately, I have used very expensive optical comparators to measure dimensions on plastic parts that cost less than a penny.

Enjoy! Lon

[Rudeofus]

My take is that when it comes to the guide numbers for a flash, the design engineers go to lunch and the marketing departments take over.

Since I heard that E-TTL is programmed to underexpose by about 1 stop (to avoid blown out highlights they say) I assume that flash manufacturers took advantage of this fact and bumped up their guide number ratings. A flash that would be GN41 for correct exposure suddenly turn out as GN58 for -1 stop exposure ...

If you add the fact that flash manufacturer's GN measurements "accidentially" account for reflections of a room (which just happens to be really small and has bright white and highly reflective walls), you quickly reach those inflated guide numbers.

This is not an issue of meter accuracy, it is an issue of the ready accommodation of marketing department to just lie in what they think are very creative ways.
It's not lying, it's just "creative representation of the measurement results" ;)

I also find the idea that the measuring instrument's cost has to somehow be commensurate with the cost of the item being measured as a very odd one.
No, but an expensive instrument can be assumed to be quite accurate. Precision is expensive, so it's unlikely that a 10$ contraption measures flash light accurately and with reproducible results.

I have routinely used a 2 dollar metal scale to measure dimensions on parts or assemblies that cost 1000s of dollars.
If you don't need extreme accuracy, that's the way to go.

Alternately, I have used very expensive optical comparators to measure dimensions on plastic parts that cost less than a penny.If being off by 10ppm incurs high costs, you'd better use expensive instruments. In this case it's not the cost of the part but the cost of making an error which counts.

[DocFrankenstein]

It's common and usual.

I'm getting less than manufacturer's rating from my vivitars also. I think about 80 feet would be right. Those 4AA batteries don't give you much power and that's the reason why a lot of pros have quantums and monoblocks.