How does a flash head’s zoom setting theoretically affect guide number?
Asked 3/4/2019
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2 answers
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I understand guide number as the relationship between aperture and subject distance, and the ISO scaling is straightforward. What I’m trying to understand is the effect of a zoomable flash head.
If a flash narrows its beam as the zoom setting increases, then in theory the same flash energy is concentrated into a smaller area. At longer zoom settings, using a small-angle approximation, what relationship would you expect between flash zoom/focal length and guide number?
A simple geometric argument suggests that doubling the zoom setting halves the beam width in each dimension, reducing illuminated area by about 4×. That would imply about 4× illuminance, or +2 EV, which corresponds to about 2× guide number. Another way to think about it suggests guide number might scale roughly in proportion to focal length if efficiency stayed constant.
But real flash guide-number tables don’t seem to match these idealized predictions. Why not?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Imagining theoretically, twice the speedlight zoom mm would compute half size field dimensions with 1/4 the area coverage. Same flash power concentrated into 1/4 the area theoretically would be 4x brightness, which would be 2 EV increase at twice the zoom mm.
However, that simply does not happen (not even close).
The speedlight zoom is very simple, the reflector simply moves back and forth, and does not change curvature to focus. The front fresnel lens is the same constant at any zoom, it does not change focus. Plus the design includes excess safety factor to ensure complete coverage regardless of slight mis-aiming in hot shoe.
Real world, speedlight 4x zoom (like 24 mm to 105 mm) is typically near 2 EV increase. Which is also near 2x Guide Number (because Guide Number is lens fstop number x distance). 4x zoom might be 1.9x GN instead of 2, but 2x zoom might be 1.5x GN instead of 1.414x (not linear). I imagine every speedlight design varies a bit. It would be great if zoom could be computed with Guide Number, but that is simply not the real world. Zoom is Not a precise mechanism. Still a big plus even if a bit crude.
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
7y ago
0
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In an idealized model, narrowing the flash beam concentrates the same light into a smaller area. If beam width halves in both dimensions, illuminated area becomes about 1/4, so illuminance becomes about 4× brighter (+2 EV). Because guide number scales with the square root of light intensity, that would mean about 2× the guide number.
So, theoretically, increasing zoom could make guide number rise roughly in proportion to how tightly the beam is concentrated.
In practice, real speedlights don’t behave like that very well. Their zoom mechanism is usually fairly simple: the reflector moves, but it does not become an ideal focusing system, and the Fresnel lens is fixed rather than perfectly refocusing for every zoom position. Manufacturers also leave margin to guarantee full frame coverage and tolerate slight mis-aiming. As a result, efficiency is not constant across the zoom range.
So the main takeaway is:
- ideal theory: narrower beam → higher illuminance → higher guide number
- real flashes: guide number increases with zoom, but much less predictably than simple geometry suggests
A common real-world result is that a large zoom change produces only a modest GN increase, not the full ideal gain.
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