Flash Power Comparison

Documented Flash Power

Guide Numbers

The guide number gives the (nominal) number of meters away a subject can be to be lit at that focal length and ISO, at f/1. Divide by aperture to get effective distance stopped down to the realm of real lenses — that is, the Metz 48 AF-1 is listed as providing full lighting at about 3.6 meters away at f/8. (That's approximately 12 feet, for those of us accustomed to the Imperial system.)

This really ought to be tested by someone with access to all of the flash units and a flash meter to give a perfect answer, but this is what I can do with the publicly-stated data. With some scouring of manuals and official web sites, I put together the following list of manufacturer-claimed guide numbers (in meters) for ISO 100 at various 35mm-equivalent focal lengths. I don't have any particular reason to distrust the numbers from Pentax, Metz, Sigma, or Promaster, but as detailed elsewhere on the site the single number given by Sakar doesn't appear to be honest.

Remember to divide by 1.5 to get real focal lengths for Pentax's current APS-C dSLR bodies. The list is ordered by guide number at 50mm, with the Sakar unit placed at its estimated position.

✗ indicates that this flash does not have this zoom step.
¹ with built-in or included wide-angle diffuser.
² This number comes from the Dörr version of the flash, and agrees with the general pattern of a 3× power increase from 24mm to 85mm. Projecting along this sequence gives 26, 29, 35, and 38 for the missing values in the middle. But see the next note for a caution.
³ Multiple reports suggest that the actual GN at 85mm is closer to 30-32 — about half of the claimed power. Sakar has not responded to requests for further information.
⁴ both Super and ST models.

Comparing Flash Power

Relative Power

The power in terms of light output is proportional to the square of the guide number. However, a simple comparison in this way isn't necessarily the most useful in practice. Generally the relative power of each flash is mostly consistent across the zoom range, but depending on how the flash is to be typically used, different factors may be more important. So, it's helpful to look at the data in a number of different ways.

Focused Power

Since the guide number at the narrowest reflector zoom setting is the biggest, flash makers like to focus on it, usually working it into product names.

The Metz, Sigma, and Promaster flashes have a reflector which can focus the burst more tightly than the Pentax and Sakar flashes — they go to 105mm (35mm full-frame), whereas the Pentax and Sakar models have zoom reflectors which stop at 85mm. (The zoom reflectors move in discrete steps rather than being continuous, so there's no particular benefit at until you go above the final step.) So, on a current 1.5×-crop Pentax dSLR, if you're at a (real) focal length of 70mm or above, the relative list looks like this, normalized to the Pentax AF360FGZ.

• Metz 58 AF-1 = 2.6×
• Pentax AF540FGZ = 2.2×
• Metz MZ-4i = 2.2×
• Sigma EF-530 DG = 2.1×
• Metz 48 AF-1 = 1.8×
• Promaster 7500EDF = 1.4×
• Sakar 952AF/PEN, et al = 1.4×
• Promaster 5750DX = 1.2×
• Promaster 7400EDF = 1.0×
• Promaster 5550DX = 1.0×
• Pentax AF360FGZ = 1.0× (Of course.)
• Metz 36 AF-4 = 1.0×
• Promaster 7200EDF = 0.72×
• Promaster 5250DX = 0.46×
• Pentax AF200FG = 0.31×
• K20D/K200D/K-7 built-in = 0.13×
• K100D/K10D/K2000/K-m built-in = 0.09×

The number for Sakar is based on their single published figure: their claimed power at maximum zoom of 85mm. Anecdotal reports suggest that the flash is actually significantly less powerful than this in practice. Therefore, I don't recommend putting much faith in it without further information.

Normal Power

The narrow beam guide numbers are interesting, but they're only relevant at higher focal lengths. This list represents the middle of the zoom range — a field of view which covers the "normal lens" range of 50mm equivalent focal length — that is, about 35mm on a current Pentax dSLR.

• Pentax AF540FGZ = 2.2×
• Metz 58 AF-1 = 2.0×
• Metz 54 MZ-4i = 1.8×
• Sigma EF-530 DG = 1.8×
• Promaster 5750 = 1.4×
• Metz 48 AF-1 = 1.4×
• Promaster 7500EDF = 1.3×
• Promaster 7400EDF = 1.0×
• Promaster 7200EDF = 1.0×
• Promaster 5550DX = 1.0×
• Pentax AF360FGZ = 1.0× (Of course.)
• Metz 36 AF-4 = 1.0×
• Promaster 5250DX = 0.66×
• Pentax AF200FG = 0.44×
• K20D/K200D/K-7 built-in = 0.19×
• K100D/K10D/K2000/K-m built-in = 0.13×

Since there is no data from Sakar, the 952AF/PEN can't be included here.

Wide-Angle Power

The relative power level of each flash at different zoom levels is relatively consistent: the 50mm chart above is so close to the ones for 35mm, 70mm, and 85mm that there's no point in graphing them separately. The only exceptions are at the narrowest zoom position as detailed above, and at the widest setting.

The fixed-reflector flashes are set at a 28mm coverage angle (35mm equivalent), and can't go wider than that without a diffuser, while all of the zoom flashes except the Metz 36 AF-4 go to 24mm. At higher focal lengths, the inability to produce a more focused beam severely penalizes the fixed-reflector packages. This is particularly unfair to the tiltable-reflector Promaster flashes, because it is common to use a non-zoomed setting when using bounced light — so this value may be your typical use case.

Here are the fixed-reflector flashes, normalized to the Pentax AF360FGZ set to 28mm:

• Promaster 7200EDF = 1.9×
• Promaster 5550DX = 1.9×
• Promaster 5250DX = 1.2×
• Pentax AF200FG = 0.83×
• K20D/K200D/K-7 built-in = 0.35×
• K100D/K10D/K2000/K-m built-in = 0.25×

Note the oddity with the Promaster figures — the 7200EDF does not zoom but Promaster claims a GN of 100' at its fixed 28mm coverage, the same as the 7400EDF zoomed to 50mm. E-mail from Promaster confirms that the 7200EDF does indeed have a more powerful strobe to compensate for its lack of zoom. In fact, by the manufacturer-stated numbers, at 28mm it is more powerful than all but the most expensive offerings from other companies.

Relative Flash Coverage

Flash Burst Profiles

While looking at power ratings numbers directly can be useful, another practical approach is to look at the total area each flash could cover at any zoom setting. That is, consider the flash to be fixed in one place and examine the possible places a single subject could stand and be appropriately lit. This takes into account both flash reflector zoom angles and the raw power needed to reach into the distance.

The following flash burst profile images use the actual correct angles for zoom settings. The shape and proportion of the profile remains the same regardless of aperture and ISO, but of course becomes smaller. And obviously light doesn't really work like this: the hard-edged cutoffs are a fiction — but a convenient one.

Relative Potential Flash Coverage

The flash burst profile concept can be boiled down to a single number by measuring the total potential area of flash coverage. The following list shows this area relative to that covered by the Pentax AF360FGZ. Since this incorporates all of the published numbers for each flash, the value is less susceptible to rounding errors or other unintentional inaccuracies than the other charts.

Keep in mind several things when looking at these numbers. First, the data is entirely based on manufacturer-stated numbers which may be faulty in multiple ways: the guide number could be overstated, and the angle of coverage at a given zoom position could be not completely adequate. Second, this particular number is not a measure of raw flash power. It's a partial indicator of the versatility which a certain amount of power can give you in combination with the zoom reflector of a particular flash.

• Pentax AF540FGZ = 2.3×
• Metz 58 AF-1 = 2.2×
• Metz 54 MZ-4i = 2.0×
• Sigma EF-530 DG = 1.9×
• Metz 48 AF-1 = 1.5×
• Promaster 7500EDF = 1.3×
• Promaster 5750DX = 1.1×
• Pentax AF360FGZ = 1.0× (Of course.)
• Promaster 7400EDF = 1.0×
• Promaster 7200EDF = 0.93×
• Promaster 5550DX = 0.93×
• Metz 36 AF-4 = 0.88×
• Promaster 5250DX = 0.60×
• Pentax AF200FG = 0.41×
• K20D/K200D/K-7 built-in = 0.18×
• K100D/K10D/K2000 built-in = 0.13×

Again, since Sakar does not provide complete guide number data, their flash cannot be included in this comparison. However, if we extract from the one number the do give, the coverage value would be about 1.4×. If we instead use a less-trusting estimate (based on actual tests), it's only about 0.8×.

If you need more power than this in a single flash, the non-hot-shoe mount Metz flashes which can use the Pentax P-TTL SCA module come in at 1.7× for the 45 CL-4 (with a much higher guide number at 35mm than any of the above), and an impressive 3.9× for the 76 MZ-5.

More Information

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