Talk:Guide number

List of guide numbers
Recently added a rather long list of guide numbers. I removed it but reverted me. I see this list as unencyclopaedic - it serves no purpose in explaining what guide numbers are or how they work. Surely people who have those models of flash either already know what GN their flash does, or could easily look it up in the manual. Or the flash is automatic and they don't need to know. The only purpose I can think of is for bragging rights (my GN is bigger than yours!) or comparing models before buying.

Wikipedia is not a shopping guide, nor an indiscriminate collection of information. The article already gives a few examples for guide numbers. A few better examples would be welcome, but this large list is not useful. --Imroy (talk) 06:39, 1 February 2008 (UTC)

What does "exposed correctly" mean in the article? A correct exposure highly depends on the subject's tone not just the distance, which makes "exposed correctly" a subjective term. "correctly expose a 18% grey subject" would be more accurate (assuming that's what manufacturers base the GN information the provide on) 81.253.15.86 (talk) 00:44, 17 December 2009 (UTC)


 * (Responding eight years later!) A list like that is why we have list-based articles. In this case, we have Flash comparison, so we don’t duplicate information here that is going to rapidly change anyway. Greg L (talk) 20:30, 5 August 2017 (UTC)

Strobist Reference
I don't see the link to the Strobist page justified. I think it adds no real value. —Preceding unsigned comment added by 60.241.91.114 (talk) 05:39, 4 January 2011 (UTC)

Exposure Duration
It might be nice to add a comment why exposure duration is not relevant to GN (because GN relates to the total energy of light emitted by the flash, regardless to how long it takes to emit that energy). —Preceding unsigned comment added by 60.241.91.114 (talk) 05:42, 4 January 2011 (UTC)


 * (Responding six years later): The duration of the flash is intimately linked to the illuminating power of the flash device. The shutter speed, however, is largely irrelevant. This has now been properly addressed in the article. Greg L (talk) 20:32, 5 August 2017 (UTC)

Wrong too many ways to count....
The authors have the concept of a guide number wrong, as well as the specifics. Too much to edit, it needs to be rewritten.

For example; the opening sentence has an error: "The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor sensitivity and angle of view." This is incorrect, a guide number is Not affected by angle of view.

Under the next topic, "Value" there is another major error: "The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose film or a digital sensor with the specified sensitivity." A guide number is the product of ANY distance to subject and proper aperture.

I could continue, for instance; a guide number is NOT a measure of distance, and so is not qualified as either "feet" or "meters." The last sentence is completely erroneous, a guide number has nothing to do with the reflectance of surroundings (although the exposure does.) I'll have to give this some study, but it's so wrong in so many place it cannot be fixed by limited edits. Truthhurts34 (talk) 00:21, 20 February 2013 (UTC)


 * (Responding four years later): I couldn’t agree more. The article was beyond repair. I threw it out and started anew. The issue of “feet and meters” should have been clearer in the original article. Guide numbers are scaled differently for use in the U.S. because photographers measure distances in feet in the U.S. As there is no single, universally accepted way that all manufacturers abide by to express guide numbers when marketing to the U.S., some manufacturers of flash devices use nomenclature that misleading makes guide numbers appear to be a unit of measure for distance. This confusing-looking practice has now been properly explained. The industry-wide practices in this regard may suck, but it is what it is. See This article requires rewriting, below. Greg L (talk) 17:12, 6 September 2017 (UTC)

Distance section - link to "The Inverse Square Law - Myth Busted"
Link [2] to the blog article "The Inverse Square Law - Myth Busted", as well as the text related to that article, is of very low quality. In fact, the author of that blog article now admits at the top of his page that his article is fundamentally flawed. It should be removed as a link, and the text under "Distance" should be edit to reflect that.

Source: http://www.diyphotography.net/the-inverse-square-law-cheat-sheet-myth-basted — Preceding unsigned comment added by 208.184.67.190 (talk) 02:39, 19 September 2013 (UTC)


 * I agree. In fact, the original author retracted the claim in a followup post. http://www.diyphotography.net/the-inverse-square-law-experiment-done-right-myth-unbusted/ -- therefore, I'm going to edit this article to remove the misinformation. — Preceding unsigned comment added by Mattdm (talk • contribs) 17:09, 4 January 2015 (UTC)

This article requires rewriting
Sorry, authors, but I found this article very confusing. I think it needs to be rewritten. It confuses me and I know how to use guide numbers. — Preceding unsigned comment added by 88.109.93.115 (talk) 11:45, 9 March 2014 (UTC)


 * (Responding three years later!): Indeed, the article was in a very poor state from back when you saw it (perma-link). It was confusing, had errors, and had serious omissions. It was the product of an error-ridden, poorly written article to begin with plus various people’s good-faith efforts to repair it. ’Twas crap—as noted by others in some of the above threads.


 * Moreover, after you read the article and made your above comment, some editor from England who apparently forgot that Americans measure distances in feet did an ill-advised drive-by shooting on the article to suggest (by invoking an ISO standard) that guide numbers can only properly be expressed A) in meters at B) at ISO 100 (perma-link here).


 * Also, the article lacked an illustration, which as they say, is worth a thousand words. Adding one gave me an excuse to use my old, 1970s, all-manual Micro-NIKKOR 55mm f/3.5 macro lens to shoot the Vivitar 285’s circular calculator image. You can see the pitch of the fabric used for the silkscreening. It was also an excuse to shoot the “lens & tape measure” photo.


 * I’ve updated the article to its latest state as of this writing (perma-link). It is now 100 percent new. I also added a much-needed section on how guide numbers are affected by flash zoom level (perma-link to section here). I couldn’t find that information anywhere else; the answer is, there is no precisely fixed relationship.


 * It was also an opportunity to get to the bottom of sync delays with flashbulbs (perma link) as there was a dearth of information on the subject. Greg L (talk) 17:00, 6 September 2017 (UTC)