When photographic flashes were commonly operated in manual mode, they usually had a chart or dial on the flash to calculate the correct exposure based on the distance to the subject. Now that many flashes are designed primarily for automatic use, they may omit this aid, making it harder to use the flash in manual mode.
I thought it would be helpful, therefore, to make a spreadsheet that would generate a table of aperture settings and subject distances for any manual flash. You can download it here:
To use it, just fill in the top (blue) section based on the published specifications for your flash, and fill in the next (tan) section based on the settings you want to use for the chart. Print the spreadsheet, cut out the chart, and keep it handy.
If you have any corrections or feedback, please feel free to leave a comment below. Thanks!
Notes: At first, I intended to let the user pick a zoom setting for the chart that was different from the one in the specifications. The distance would be calculated using an ideal physics approximation: Doubling the focal length would reduce the illuminated area by a factor of 4, which would effectively double the guide number. But when I looked at real guide numbers for different zoom settings on the same flash, I found that this approximation doesn’t hold up in real life.
Also, I initially tried to make this a Google Docs spreadsheet that anybody could edit on the web. But I couldn’t find a way for people to edit their own “sandboxed” copy. Edits by one user affected my master copy, and were immediately visible to everyone else. This would cause chaos. So I apologize for making you download an Excel file!