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In November 2014 I received a Sekonic L-308S exposure meter, specifically for use with electronic flash. So what's more obvious than to use it to measure the guide numbers of my flash units?

The setup is trivial: mount the flash unit on a tripod (in this case held in place by a camera), put the meter 1 m away and measure the flash aperture. It should be a direct readout as a guide number:


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Diary entry for Friday, 14 November 2014 Complete exposure details

 

One problem: this strange aperture/decimal readout. For example, 8.0/7 does not mean anything close to f/8. In fact, it's about f/10.2. Maybe. When you get to this level of detail, the problem is also that f/11 isn't really f/11, it's closer to f/11.31. The “odd” apertures f/1.4, f/2.8 and so on are really the “even” apertures multiplied by √2, which is 1.4142135623730951 or so. For normal use the abbreviations are close enough, but what happens when you go to tenths of a stop (or the 1/20th power of 2)? I can only guess, so I'm guessing that in this scheme of things f/11 is really f/11.3.

Here are the results for my flash units.

Unit       Advertised GN       Actual GN       % light
mecablitz 40 CT 4       40       27       46
mecablitz 58 AF-2       58       36.8       40
Meike MK-300       32       12.2       15
Olympus FL-LM1       7       4.4       40
Olympus FL-LM2       7       4.6       43
Viltrox JY-670       14       9.85       50

They're surprising: they're all much lower than advertised. I don't understand why this is the case, but there are strong arguments that it's not measurement error:

One possibility is that the assumptions about guide numbers don't work at such close distances. But that doesn't make sense for the Viltrox, which is a ring light designed specifically for this purpose. To test this hypothesis, I ran a second series of tests with the flashes at 2 m and 4 m. They do, indeed, show an improvement in the guide number, but not nearly enough.

The other thing of interest is how inaccurate the fractional intensities are. The Meike is particularly bad here: when set at 1/32 output, it actually outputs 15.2% of its rather poor maximum, or about 1/6. Even the mecablitz isn't spectacular. Strangely, my cheap studio flashes with analogue settings don't do too badly at ½ and ¼ power.

The tables below are the results of two separate sets of measurements. The first was a set of measurements of the various power levels, where available, at 1 m distance. The second, done later, was done at full power and at different distances from the exposure meter.

Preliminary results

This page remains a work in progress, and I'd be particularly happy for criticism that shows a flaw in my thinking. But at present it looks as if:

Viltrox JY-670

Tested at 1 m with negative powers of 2 powers. Results:

Power ratio             Raw GN       Real GN       Expected       Deviation %
1       1       8:6       9.85
1/2       0.50000       5.6:8       7.39       6.96       13
1/4       0.25000       5.6:1       5.80       4.92       39
1/8       0.12500       2.8:8       3.69       3.48       13
1/16       0.06250       2.8:1       2.90       2.46       39
1/32       0.03125       2.0:0       2.00       1.74       32
1/64       0.01562       1.0:9       1.37       1.23       23
1/128       0.00781       0.7:7       0.89       0.87       5

The second test was at 1 m, 2 m and 4 m. It does show a slight improvement. The last column is the proportion of the advertised guide number.

Distance       Raw GN       Real GN       Expected       Proportion %
1 m       8.0:8       10.56       14.00       57
2 m       4.0:9       5.46       7.00       61
4 m       2.8:3       3.11       3.50       79

Meike

Power ratio             Raw GN       Real GN       Expected       Deviation %
1       1       11:3       12.21
1/2       0.50000       11.0:2       11.79       8.63       87
1/4       0.25000       8.0:9       10.93       6.10       221
1/8       0.12500       8.0:1       8.28       4.32       268
1/16       0.06250       5.6:5       6.66       3.05       376
1/32       0.03125       4.0:5       4.76       2.16       386
1/64       0.01562       2.8:0       2.80       1.53       237
1/128       0.00781       1.0:9       1.37       1.08       60

The second test was at 1 m, 2 m and 4 m. Again, it shows a slight improvement. The last column is the proportion of the advertised guide number.

Distance       Raw GN       Real GN       Expected       Proportion %
1 m       11.0:3       12.21       32.00       15
2 m       5.6:4       6.43       16.00       16
4 m       2.8:8       3.69       8.00       21

Clearly this particular flash comes nowhere near achieving its promises.

mecablitz 58 AF-2

These measurements were taken with the reflector set to “105 mm”, which is the focal length for the specifications. I measured a guide number of 36.8, when it should have been 58. That's only 40% of the specification, or about 1⅓ stops difference! I didn't see any difference between 1/250 s and 1/125 s.

Power ratio             Raw GN       Real GN       Expected       Advertised       Deviation %
1       1       32:4       36.76             58
1/2       0.50000       22.0:4       25.27       25.99       41       -5
1/4       0.25000       16.0:6       19.70       18.38       29       15
1/8       0.12500       11.0:5       13.08       13.00       21       1
1/16       0.06250       8.0:8       10.56       9.19       14.5       32
1/32       0.03125       5.6:8       7.39       6.50       10.3       29
1/64       0.01562       5.6:0       5.60       4.59       7.25       49
1/128       0.00781       2.8:4       3.22       3.25       5.13       -2

At other focal lengths I had:

Focal length       Raw       Measured       Advertised       Ratio
105       32.0:4       36.76       58       40%
85       32.0:3       35.51       52       47%
70       32.0:0       32.00       46       48%
50       32.0:0       32.00       42       58%
35       22.0:5       26.16       35       55%
28       22.0:2       23.58       31       58%
24       22.0:1       22.78       29       62%
12       16.0:0       16.00       20       64%

Why are the results for wider angles better? One reason is a limitation of my test setup: I'm receiving light not just from the flash, but also bounced off the surroundings.

The second tests were at 1 m, 2 m and 4 m. The last column is the proportion of the advertised guide number.

24 mm

Distance       Raw       Measured       Advertised       Proportion %
1 m       16.0:9       21.86       29.00       57
2 m       8.0:9       10.93       14.50       57
4 m       5.6:2       6.00       7.25       69

105 mm

Distance       Raw       Measured       Advertised       Proportion %
1 m       32.0:1       33.13       58.00       33
2 m       16.0:2       17.15       29.00       35
4 m       8.0:4       9.19       14.50       40

While these results show a slight improvement, they're inconsistent. Here are my raw results:

      1 m       2 m       4 m
24 mm       16:9       8:9       5.6:2
28 mm       16:9       11:0
35 mm       22:2       11:3
50 mm       22:7       11:7
70 mm       22:9       16:0
85 mm       22:9       16:1
105 mm       32:1       16:2       8:4

These numbers are very difficult to read. I should produce a graph showing the real relationships, and maybe I'll do that some time. For the time being, though, a couple of comparisons:

In the first column (1 m), the results for 35 mm, 50 mm, 70 mm and 85 mm are almost the same, ranging from GN 24 to GN 30, or an increase of 25%. According to the manual, the guide numbers should be 35 and 52, an increase of 48%.

But in the second column things are different. Here the result for 35 mm is GN 12.21, close enough to half the value for 1 m, which looks correct. But the guide number for 85 mm, which should be 15, is closer to 17.

I still need to think this one through, but it looks as if the answer might be in inaccuracies in positioning the reflector. I ran the first column changing the focal length manually from 24 mm to 105 mm, and then ran the second column from 105 mm to 24 mm. Does that make a difference? I need to investigate more.

mecablitz 40 CT 4

This unit has an advertised guide number of 40. Here are the measurements:

Distance       Raw       Measured       Advertised       Proportion %
2 m       11.0:6       13.54       20.00       46
4 m       8.0:0       8.00       10.00       64

Olympus FL-LM1

This flash is bundled with the Olympus E-PM2, and it doesn't even get its own page. It's described on the pages of some, but not all models with which it is delivered, such as the E-PM1. It claims a guide number of 10 at 24°/200 ISO, which implies 7 at the more standard 21°/100 ISO. Here's what I got:

Distance       Raw GN       Real GN       Expected       Proportion %
1 m       4.0:3       4.44       7.00       40
2 m       2.0:7       2.55       3.50       53
4 m       1.4:0       1.40       1.75       64

Olympus FL-LM2

The FL-LM2 came with my Olympus OM-D E-M1. It's very similar to the FL-LM1, and I suspect the differences are mainly cosmetic. Again it's advertised with the same guide number 7, and the measurements are close enough to be the same within a reasonable margin of error:

Distance       Raw GN       Real GN       Expected       Proportion %
1 m       4.0:4       4.59       7.00       43
2 m       2.0:7       2.55       3.50       53
4 m       1.4:1       1.45       1.75       69

Studio flash

These are cheap 110 Joule studio flash units with analogue adjustments. By accident I made these measurements with the exposure meter set to 24°/200 ISO, so the comparison values should be 1.4 times lower At ½ and ¼ settings they're not too inaccurate, but minimum (marked ⅛) is closer to ⅙ power.

The other interesting thing is that using the umbrella instead of direct flash reduces light by almost exactly 1 EV. I had expected more.

Direct

Power ratio             Raw GN       Real GN       Expected       Deviation %
1       1       22:1       22.78
1/2       0.50000       11.0:8       14.51       16.10       -19
1/4       0.25000       11.0:2       11.79       11.39       7
1/8       0.12500       8.0:6       9.85       8.05       50

With umbrella

Power ratio             Raw GN       Real GN       Expected       Deviation %
1       1       16:1       16.56
1/2       0.50000       11.0:1       11.39       11.71       -5
1/4       0.25000       8.0:1       8.28       8.28       0
1/8       0.12500       5.6:5       6.66       5.86       29

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