Doug Kerr
Well-known member
We are often intersted in shots in which ambient light (perhaps sunlight) is present but flash is to be used to fill shadows or to get a "luminance distinction" between the principal subject and the background. (In the latter regard, Will Thompson has called attention to some of Frank Doorhof's work seen on this forum.)
Often the concern is in getting sufficient flash exposure to match the ambient exposure in the way we wish, especially given that, using normal flash technique, we are limited as to how short a shutter time we can use.
I thought it might be worthwhile to review some of the math involved here.
We will use as our "reference result" the situation where the photometric exposure due to the ambient illumination on the main subject is equal to the photometric exposure due to the flash illumination.
In that situation, if the camera exposure is adjusted to produce our "standard" exposure result on the main subject, then on the background, we would have an exposure result one stop down from "standard".
That of course may not correspond to the photographic effect we want, but it is easily understood, and we can work up or down from there.
We will assume operation at ISO 100. The ISO sensitivity doesn't affect the ratios we will discuss here, and the assumption of ISO 100 saves a step here and there when dealing with guide numbers.
We will assume that the ambient sunlight is such that "appropriate" exposure is given by the infamous "sunny 16" rule.
We will also assume that flash-only exposure following the "guide number" doctrine produces a correspondingly-appropriate exposure result.
We will also assume operation with a shutter speed of 1/200 sec (perhaps the fastest at which we can safely assume successful "X-sync" flash operation with a range of flash equipment and typical "35-mm family" dSLR cameras.
So, lets get started.
First, we will consider ambient-only exposure. Thus, for operation at ISO 100, we would expect an exposure of f/16 at 1/100 sec to be "appropriate". For our adopted shutter speed of 1/200 sec, that would require an aperture of f/11.
Next, we will consider a flash-only exposure (however we might make that happen - big black tent, maybe).
With the flash at a distance D from the main subject, and our f/11 aperture, we would want a flash guide number (GN) of 11D (ISO 100 basis). (GN and D need to be in the same units, feet or meters as we might choose.)
So, if we assume the flash unit to be 10 feet from the main subject, we would want a guide number of 110 feet.
For reference, a Canon Speedlite 580EX II flash unit, with its beamwidth set to "50 mm" (intended to cover the field of view of a full-frame 35-mm camera with a 50 mm lens), has a rated maximum guide number (ISO 100) of 138 ft; at the "80 mm" setting, 174 feet.
Now, we don't necessarily know the guide number of our flash setups. Probably all we know is the rated energy storage, expressed in joules (watt-seconds). That is not a measure of any photometric output property of the setup.
The relationship between the energy storage and the factor we really are intersted in, the on-axis luminous intensity-time product (which in fact the guide number tells us, in a special way, based on some assumptions about exposure strategy) is affected by these factors:
• The discharge fraction (the fraction of the stored energy that actually goes into the flash tube before it extinguishes).
• The electric-photometric conversion efficiency of the flash tube.
• The photometric efficiency of the overall setup (involving how much of the light emitted by the tube is not absorbed in the rig).
• The effective beamwidth of the rig (which depends of course on the reflector, light modifier, etc, in use).
The relationship can be expressed as:
GN = k sqrt(E)
where GN is the guide number (ISO 100 basis, assume feet), E is the energy storage, in joules (watt-seconds), and k is the conversion factor (for GN in feet).
The overall conversion factor varies widely.
Manufacturers who state a GN for their monolight units, with the typical small "included" reflector, generally give numbers implying k in the range of about 5 to 6.5. (They of course don't state it that way.)
When light modifiers such as softboxes are involved, k can be much less.
Some rough data from Will Thompson suggests for a setup with softboxes, a k value as small as 1.5 may obtain.
Just as a matter of interest, the Speedlite 580EX II, at its "80 mm" beamwidth setting, would thus broadly correspond to a studio-type monolight, with a small reflector, with an energy storage rating of about 530 J (530 W-s).
Best regards,
Doug
Often the concern is in getting sufficient flash exposure to match the ambient exposure in the way we wish, especially given that, using normal flash technique, we are limited as to how short a shutter time we can use.
I thought it might be worthwhile to review some of the math involved here.
We will use as our "reference result" the situation where the photometric exposure due to the ambient illumination on the main subject is equal to the photometric exposure due to the flash illumination.
In that situation, if the camera exposure is adjusted to produce our "standard" exposure result on the main subject, then on the background, we would have an exposure result one stop down from "standard".
That of course may not correspond to the photographic effect we want, but it is easily understood, and we can work up or down from there.
We will assume operation at ISO 100. The ISO sensitivity doesn't affect the ratios we will discuss here, and the assumption of ISO 100 saves a step here and there when dealing with guide numbers.
We will assume that the ambient sunlight is such that "appropriate" exposure is given by the infamous "sunny 16" rule.
We will also assume that flash-only exposure following the "guide number" doctrine produces a correspondingly-appropriate exposure result.
We will also assume operation with a shutter speed of 1/200 sec (perhaps the fastest at which we can safely assume successful "X-sync" flash operation with a range of flash equipment and typical "35-mm family" dSLR cameras.
So, lets get started.
First, we will consider ambient-only exposure. Thus, for operation at ISO 100, we would expect an exposure of f/16 at 1/100 sec to be "appropriate". For our adopted shutter speed of 1/200 sec, that would require an aperture of f/11.
Next, we will consider a flash-only exposure (however we might make that happen - big black tent, maybe).
With the flash at a distance D from the main subject, and our f/11 aperture, we would want a flash guide number (GN) of 11D (ISO 100 basis). (GN and D need to be in the same units, feet or meters as we might choose.)
So, if we assume the flash unit to be 10 feet from the main subject, we would want a guide number of 110 feet.
For reference, a Canon Speedlite 580EX II flash unit, with its beamwidth set to "50 mm" (intended to cover the field of view of a full-frame 35-mm camera with a 50 mm lens), has a rated maximum guide number (ISO 100) of 138 ft; at the "80 mm" setting, 174 feet.
Now, we don't necessarily know the guide number of our flash setups. Probably all we know is the rated energy storage, expressed in joules (watt-seconds). That is not a measure of any photometric output property of the setup.
The relationship between the energy storage and the factor we really are intersted in, the on-axis luminous intensity-time product (which in fact the guide number tells us, in a special way, based on some assumptions about exposure strategy) is affected by these factors:
• The discharge fraction (the fraction of the stored energy that actually goes into the flash tube before it extinguishes).
• The electric-photometric conversion efficiency of the flash tube.
• The photometric efficiency of the overall setup (involving how much of the light emitted by the tube is not absorbed in the rig).
• The effective beamwidth of the rig (which depends of course on the reflector, light modifier, etc, in use).
The relationship can be expressed as:
GN = k sqrt(E)
where GN is the guide number (ISO 100 basis, assume feet), E is the energy storage, in joules (watt-seconds), and k is the conversion factor (for GN in feet).
The overall conversion factor varies widely.
Manufacturers who state a GN for their monolight units, with the typical small "included" reflector, generally give numbers implying k in the range of about 5 to 6.5. (They of course don't state it that way.)
When light modifiers such as softboxes are involved, k can be much less.
Some rough data from Will Thompson suggests for a setup with softboxes, a k value as small as 1.5 may obtain.
Just as a matter of interest, the Speedlite 580EX II, at its "80 mm" beamwidth setting, would thus broadly correspond to a studio-type monolight, with a small reflector, with an energy storage rating of about 530 J (530 W-s).
Best regards,
Doug
