I have recently purchased a Nikon SB-28 flash for my Mamiya 645 1000s camera. Since I can use the flash only in the Manual mode, I am trying to create a guide in order to be able to calculate the distance that I can use my flash for. For my camera, I have an 80mm lens and most of the time I am using 400 ISO film. Regarding the aperture, I prefer from 11-22 F-stop normally. Now I found that the guide number of the flash is:

Guide number (ISO 100, m): 36 (with zoom headset at 35mm)

Since I am not familiar exactly with the term, how can I calculate the distance for my case? Any ideas? Is anyone familiar with any mobile application maybe that can do this automatically?

  • \$\begingroup\$ See this related question: How do I set exposure with a manual flash? \$\endgroup\$
    – scottbb
    May 10, 2020 at 16:49
  • \$\begingroup\$ But how can I make the conversion from 100 to 400 ISO and the zoom? \$\endgroup\$
    – Jose Ramon
    May 10, 2020 at 16:52
  • 1
    \$\begingroup\$ How can we answer the question if you don’t correctly specify the model number of the flash? Is it an SB-28 or SB-29? \$\endgroup\$ May 11, 2020 at 3:22
  • 1
    \$\begingroup\$ Hmm, ops I was so sure that I wrote it properly. \$\endgroup\$
    – Jose Ramon
    May 11, 2020 at 10:16

1 Answer 1


If the GN is 36 (meters) at ISO 100, it is x2 or GN 72 at ISO 400 (meters).

Specificaly new GN = old GN x square root(new ISO / old ISO)

Guide Number = distance x f/stop Number. This takes the Inverse Square Law into account.

So at GN 72 and f/16, direct flash exposure is GN 72 / f16 = 4.5 meters.

Or the other way for example, GN 72 / 8 meters is f/9.

This applies only to the same flash head zoom that the original GN 36 was for.

The flash manuals have a GN chart that shows GN for other zoom values, but I don't think the SB-29 zooms? See http://www.mir.com.my/rb/photography/hardwares/classics/NikonF5/Flash/SB29/index.htm for much detail about the SB-29. It says GN 36 is feet, meters is GN 11. GN meters is GN feet / 3.28, so ISO 400 would be GN 22 meters.

My web site has a GN calculator and much more GN information at http://www.scantips.com/lights/flashbasics1c.html which might be useful.

  • \$\begingroup\$ My flash is sb-28, not sb-29. There is one option for zoom. There is one zoom button to the flash, I guess you refer to that, right? \$\endgroup\$
    – Jose Ramon
    May 10, 2020 at 18:27
  • \$\begingroup\$ google.com/… \$\endgroup\$
    – Jose Ramon
    May 10, 2020 at 18:28
  • \$\begingroup\$ You previously said SB-29, but yes, the SB-28 does zoom. Its GN chart is on its manual page 56, and is GN 30/98 (meters/feet) for ISO 100 at zoom 24 mm zoom ... to GN 50/164 at 85 mm zoom. Double those for ISO 400. A confusion is that it can also do HSS which is a very different GN on page 51 (but HSS requires a compatible HSS camera). \$\endgroup\$
    – WayneF
    May 10, 2020 at 19:56
  • \$\begingroup\$ The SB-28 manual is at cdn-10.nikon-cdn.com/pdf/manuals/Speedlights/SB-28.pdf \$\endgroup\$
    – WayneF
    May 10, 2020 at 20:04
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    \$\begingroup\$ How can we answer the question if you don’t correctly specify the model number of the flash? Is it an SB-28 or SB-29? \$\endgroup\$ May 11, 2020 at 3:23

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