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How to I compare guide number?

Is there a table somewhere on the internet (sorry my internet backup is running behind )

I want to compare apples with apples, obviously I cant compare the following directly, I need to factor in focal length, but I dont know the math, for example;

Nikon SB-800
Guide Number: 125 @ ISO 100 (35mm.),
to a

Metz mecablitz 76 MZ-5 TTL Flash
Guide Number: 76 @ ISO 100 (105mm.),

OR
Metz mecablitz 45 CL-4 TTL Flash
Guide Number: 148 @ ISO 100

does anyone have a nice link to a table, or a brand comparison site?
thanks and regards
Oz

Oz,

It is not the focal length of the lens that matters but rather the coverage of the flash. If the flash has some sort of zoom capability, changing the zoom will affect the area over which the light is spread and so the intensity and guide number.

You should be able to directly compare the strobes assuming both are in metres or both are in feet. If the strobes have zoom capabilities, compare them at the same effective focal length.

If you have the SB-800 set at 35mm but use a 105mm lens on the camera, the guide number will still be 125 @ 100 ISO. This means if you are 5m from the subject, the lens will need to be at f/25.

Ideally, set the flash to be a little wider than the lens and then use the guide number at that coverage.

Oz,

I'm not sure if you have these:

SB-800 Manual: http://www.nikonusa.com/pdf/manuals/Spe ... SB-800.pdf
Page 42 has a table of guide numbers for each zoom head position as well as the flash output level.

Metz 76 MZ5 digital Manual: http://www.metz.de/en/service_support/s ... ad.160.pdf
Page 140 has the guide numbers for each of the zoom settings.

Note that the GN you quoted above for the SB-800 is in feet. The GN is only 38m @ 35mm and ISO 100.

The GN for the Metz 76 is 45.5m @ 35mm and ISO 100.

Magic feedback, thanks so much, answered my questions with your references!!!

So, from those two tables, I can see that at 105mm, 100ISO the SB800 to Metz76, have GN 56 and 76 respectively.

Now, thats a difference in GN of 20. So, is the difference mean that the metz76 is 35% more powerful (=20/56) or i the GN not linear?

So, from those two tables, I can see that at 105mm, 100ISO the SB800 to Metz76, have GN 56 and 76 respectively.

Now, thats a difference in GN of 20. So, is the difference mean that the metz76 is 35% more powerful (=20/56) or i the GN not linear?

The flash is a point light source and so the amount of light reaching the subject is determined by the inverse square rule: double the distance and only a quarter as much light reaches the subject; triple the distance and only get one ninth; etc.

The amount of light passing through the lens is also governed by the inverse square rule based on the diameter of the aperture. The f/stop is the ratio of the focal length to the diameter of the aperture. If you double the f/stop you get one quarter of the light passing through the lens.

Guide numbers are used to determine the aperture based on the distance the flash/strobe is from the subject. So, the relationship between light output and guide number is also governed by the inverse square rule. If the guide number of one flash is half that of another, the first flash puts out only one quarter as much light as the other. Each doubling of the light output results in an increase in the guide number by the square root of 2:
1.414.

76/56 = 1.357

1.357 squared is 1.84

The Metz76 puts out around 1.84 times more light than the SB-800.



Of course, if you don't care about the theory, you can see that the Metz76 is about twice as power as the SB-800 by determining the f/stop at 3 m. With the Metz76, you'd have an f/stop of around 25 (76/3) and with the SB-800 it would be 18 (56/3). That is a one f/stop difference so the Metz is putting out twice as much light.