Why guide numbers are (mostly) meaningless.
1 month ago
Because questions about guide numbers have been popping up frequently as questions here, I think there is something important that people new to using flash need to understand: Claimed guide numbers are pretty much meaningless.
I say that Guide numbers are mostly meaningless, especially when it comes to speedlights because we don't know how those numbers are arrived at.
If they are merely calculated by the manufacturer based on summing up component specifications, that usually has no relationship to reality.
If they are calculated by actual testing and measurements, the manufacturer needs to state the circumstances and the instrumentation used for testing. For instance, was it in a large space with dark walls, floor, and ceiling or a small room with highly reflective walls, floor, and ceiling? If the speedlight has a zoom feature, what beam angle was the light set to?
The problem for consumers is that there is no industry-wide accepted method or set of guidelines manufacturers must follow for what kind of tools and methodology must be used to measure the results.
To establish a set of guide numbers for your light for your camera, you must test and then double-check your results. This has been the case going back to at least the late 1970s.
Bottom line: when it comes to claimed guide numbers: distrust and verify.
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