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Fill flash and guide number
Often when shooting outdoors we will use flash for "fill". There is often a question as to what "potency" of flash (perhaps expressed in terms of guide number) is needed to attain some desired balance between ambient and flash exposure.
It is interesting to look at this in a "homey analytical" way. I will start with a certain straightforward model.
Ambient illumination. I will assume an ambient illumination corresponding to that which is the premise of the "sunny 16" guideline for outdoor exposure. Under that guideline, if we are shooting at ISO 100, then for a scene with a "typical" distribution of reflectance, we will attain an "appropriate" distribution of photometric exposure by shooting at f/16, 1/100 s.
We get that ambient illumination here in North Texas at mid-day in mid-winter, or in summer on a lightly overcast day.Guide number. I will assume that shooting as suggested by the guide number of the flash unit will produce, on a subject surface of any given reflectance, the same photometric exposure as would be attained by a shot under "sunny 16" ambient illumination. (That is, the "guide number" scheme aims for the same distribution of photometric exposure as the "sunny 16" scheme.)
Unit of distance. I will work here with distances in meters, so that the guide number results will be in meters, perhaps the most common way to state guide numbers.
Fill balance. I will assume,. for the moment, that our objective is to have the same photometric exposure from the flash illumination as we would have from the same subject surface under "sunny 16" ambient illumination ("balanced fill"). That of course is not always our objective, but the result will give us a point of reference from which we can proceed to the needs of an individual case.
Shooting distance. I will assume a distance of 3 m (about 10 ft) from the flash unit to the subject.
Shutter speed. I was assume shooting at a speed of 1/200 sec (roughly the X-sync shutter speed limit of many cameras of interest.
If we shoot at ISO 100, with the assumed shutter speed of 1/200s, then, for "appropriate" ambient exposure under the "sunny 16" guideline, our aperture should be set to f/11. That aperture will then be assumed in reckoning the effect of flash exposure for our dually-illuminated shot.
For the situation and assumptions above, the needed flash unit guide number (ISO 100 basis) is 33m.
Effect of ISO sensitivity
This result is independent of the ISO sensitivity at which we work. The appropriate aperture would of course change.
Adaptation to other situations
If we wish that same "equal" flash-ambient balance but at a shooting distance of 5m (about 16ft), the guide number would need to be about 55m.
At a shooting distance of 2m (about 6.5ft) the guide number would need to be about 22m.
If we return to a shooting distance of 3m (10ft) but want a flash fill exposure one stop less than we would have from the "sunny 16" ambient, then the needed guide number would be about 23m
The reported maximum guide number (ISO 100 basis) for selected flash units is as follows:
Canon EOS 40D onboard flash unit: 13m
Canon Speedlite 580EX II: "50mm" beamwidth: 42m; "105mm" beamwidth, 58m*
Canon Speedlite 270EX: "28mm" beamwidth: 22m; "50mm" beamwidth: 27m*
* From which it gets its "name".
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