# Tag Info

Accepted

### Flash guide number f/stop calculation

However, 25 is NOT 2/3 of the way between 16 and 32. You just have to remember that the scale is exponential, not linear. 4 2/3 stops allows 24.66 more light, and 24.66 = 25.28132..., or roughly 25. ...
• 31.7k

### Are guide number rules still valid over unusually high distances?

I guess there would be some some small amount due to absorption in the atmosphere. But in theory there's not much complicated to it, and I don't see why it wouldn't still be the same. The formula ...
• 143k

### What is the theoretical relation between zoom length and guide number for a flash?

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 ...
• 12.9k
Accepted

### Why does the Guide Number have a distance given with it?

I don't understand what it means by 196.85' (60 m) ISO100 at 200 mm position You're looking at a unit where the flash head zooms to change the area that's illuminated. The head moves toward the front ...
• 31.7k
Accepted

### Godox V860II Guide Number at different zoom steps

A guide number (GN) is the power of the flash to illuminate a subject correctly at ISO 100 at a specific distance using a specific f-number. Normally this distance is 10 ft for easier calculations, ...
• 24.9k

### What is the guide number of a smartphone "flash"?

The guide number of a flash is determined as the F-number multiplied by the distance that gives a proper exposure. The guide number is different for different ISO sensitivities, but they are typically ...
• 6,760

### Why does the Guide Number have a distance given with it?

Guide Number is about the Inverse Square Law, which describes the way the direct flash intensity falls off with distance. At 2x distance, the flash is reduced to 1/4 intensity. At 3x distance, ...
• 12.9k

### Godox V860II Guide Number at different zoom steps

The Godox V860II and Canon 600EX both have the same 20-200mm flash head zoom range, and both have the same 60m guide number at 200mm, so I think you would be safe to use the Canon 600EX specs for a ...
• 12.3k
Accepted

### Nikon flash guide number

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 ...
• 12.9k
1 vote

### Why doesn't the distance scale on my Yongnuo flash properly change with aperture?

I have 3 of these flashes and have not experienced this problem. Only thing I can suggest is maybe downloading and installing the YN flash firmware update. Maybe try to "reset" the flash and start ...
• 1,193

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