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Guide Number

The power and distance capability of a flash

A guide number (GN) indicates the strength (power and distance capability) of the flash of a flash bulb or electronic flash. GNs are usually given for ISO 100 and used mainly for hot shoe mounted direct flash (as you need to know the exact distance from flash to subject). A higher guide number indicates a more powerful flash.

Exposure is affected by both the intensity of the flash and the distance from the flash to the subject. The correct aperture setting is therefore calculated by dividing the guide number by the measured distance from the flash to subject (read from the distance scale on the rangefinder’s focusing ring once focused).

Aperture = GN / Distance to Subject (ft)

For example, the GN of the Minolta 20 flash unit (standard hot shoe connection designed for the Hi-Matics) with ISO 100 is 64ft. So for a subject at 4ft the aperture would be f/16, for a subject at 8ft the aperture would be f/8 etc. Using the QL17 G-III with the dedicated Canolite D the aperture is set automatically according to the distance of the subject in focus.

Shutter speed is assumed to be the ‘sync speed’ (the fastest shutter speed you can use with flash) for the particular camera you are using (usually 1/60 or 1/125 with an SLR but any speed with a leaf shutter camera, the faster the better if you plan on freezing motion).

GNs are fallible as the actual range of the flash will change according to the environment and the ambient light. A small living room with white walls and ceiling will reflect and therefore intensify the light from the flash. Conversely the light from the flash will fall off dramatically in a huge hall, so the resulting exposure of a subject at the same distance and therefore with the same GN in each environment will render very different results.

  • Rangefinder Cameras
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  • Sharpest Aperture
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  • Guide Number
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About Ilott Vintage

A valuable reference for anyone wanting to learn more about rangefinder cameras, manual film photography, or both, this website is intended to serve not only as a resource to discover some amazing objects but also as a place to learn more about them, to understand exactly what they are capable of, and why. Read More

See Also
  • Minolta Hi-Matic 9 Mahogany
    Minolta Hi-Matic 9 All the quality of the 7s with an improved, faster lens
  • Original Canonet vintage rangefinder
    Canonet The original standard setter
  • Minolta Hi-Matic 7s
    Minolta Hi-Matic 7/7s A feature rich classic, manual and semi-automatic shooting down to f22
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    Wednesday 8th February 2023