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Mr Kris wrote:
While that info is good, I'm not sure it's completely relevant here. The comments in the thread have been more geared toward using flash as a fill, as opposed to your mention of using it as key.
Along the same lines though, I guess my thought would be that on or near axis fill flash doesn't look unnatural as long as it's not intense enough to create its own strong shadows.
It is relevant in ways you just don't grasp yet.
Point 1: The sun is a lousy key light for portraits. Sunlit faces work great in sports / action / candid shots where things like shaded eyes and dark harsh shadows look natural. But in posed portrait where you do want light in the eye putting the sun in them results in at best squinting and at worse a blinded subject. That's one reason backlighting is used for portraits, facing the subject to the skylight not the sun.
Point 2: In open shade and backlight the skylight models the face with "key" and "fill" components. The sun changes role from "key" to "accent" in lighting parlance.
Point 3: If you add one flash near the camera axis to balance exposure on the shaded front to match the sunny back side, by the time you add enough flash (2-3 stops) match front-to-back you will have killed all the natural modeling on the front the skylight was creating: Here's an example from a HSS test I did. I used a target because I didn't have a model available.
Ambient only exposed for highlights below clipping
Single near axis full power HHS flash moved progressively closer....
In the ambient only shot the front of the 3D towel is underexposed, but the downward "key" component of the skylight is creating modeling of shape you can see. But note how as more and more near axis flash is added that natural modeling disappears. The background will still have all the natural modeling clues in a "filled" flash shot but the foreground subject will wind up with the same flat "deer in headlights" look of a near-axis indoor flat shot.
NEAR AXIS FILL WILL KILL NATURAL MODELING
The solution? The same thing you do with flash indoors: move the flash off axis.
Below are two single flash shots, the first taken indoors the second outdoors with the same equipment — 580ex flash on bracket with diffuser. Moving the flash vertically off axis makes it a "butterfly" pattern key light.
In the case of the indoor shot I was standing on a chair with the top of the diffuser on the ceiling for maximum "spill fill" off it and the walls. The direct flash bouncing forward was stronger than the bounced fill creating the "mask" pattern of highlights that define the higher parts of the faces creating the illusion of 3D..
Is the flash there key or fill? Both because I used the diffuser and bounce to spilt the one source into both those components. Both originate from overhead, just like skylight, and the white walls of my office also bounced the light sideways for "skylight" wrap-around fill effect. Had the office been painted black the highlight pattern would be the same from the downward light coming from the diffuser but there would be no fill component and the shadows wouldn't have detail — it would have been all key flash, no fill flash (bounced).
Now look at the blurred version of the outdoor shot...
The same mask pattern is created by the flash coming out of the diffuser above the camera. I was also standing several feet above her on the river bank at about the same angle as on the chair in the indoor shot. The shadow here are filled not by bounce flash — there's nothing to bounce it off — the fill source is the skylight.
Is the flash in the second shot automatically "fill flash" because I moved outside and it's hitting the shaded side of the head? That's how photographers generically refer to flash added outdoors, but in terms of role and cause and effect it is "key" light because it is modeling the face with highlights. Fill doesn't model a face the "key" light does. The fill in the outdoor shot came from the natural ambient light.
Indoors and out raising the flash off axis vertically on the bracket in changed it from flat "fill" into a directional "key" light.
Point 4: When a single flash is moved off axis and it's functional role changes from "fill" to "key" it no longer cancels the the natural modeling of the skylight. In the case of a flash raised on a bracket the downward angle of the flash closely matches the dominant downward "key" angle of the natural skylight already modeling the object or face. The flash becomes a second smaller artificial source placed in line with a larger natural one, complementing its modeling not canceling it.
Point 5: When you move your single flash off axis and make it the "key" light it will not hit the shadows. What is left to fill the shadows outdoors? The skylight. Is the skylight bright enough to fill the shadows? That depends how dark you like them.
Lets imagine you and your subject get up before dawn and you set flash on a light stand 45° to the right of the subject's nose and take a shot. Will there be any fill in the shadows? No.
Now six hours later the sun is up, your subject's back is to the sun and you expose to keep the sun hitting their white shirt below clipping. The face is 3 stops darker. Now you turn on the flash 45° to the face and raise it's power until the highlights it creates on the front of shirt are similar to the sunny side, just about 1/3 stop darker. Will there be any more fill on the shaded side if the face than there was before you added the "key" flash? No. The shadows will be as dark as they were before you added the flash off axis.
What many will do when seeing how dark the shadows are is to change the overall exposure. But adding more ambient exposure to the shadows will blow the sunny highlights. That's why you typically see blown out hair and skin on many single flash shots where a single flash is moved off axis outdoors.
Point 6: If you move a single flash off axis and you want lighter shadows than the skylight creates you need a second fill source. It can be a reflector or a second flash. Here let's just consider flash. Where do you need to place the fill flash to lift the skylit shadows? Centered just under the lens where it will be shadowless.
Point 7: In terms of cause and effect what happens when two flashes are used in a flat fill, off-axis key light configuration outdoors in backlight is:
A) The fill flash kills all the natural modeling from the skylight
B) The key flash placed on the same angle as the natural light recreates the modeling
That's what I did below. I was just testing flash range so it's not the best example of modeling but what I did was first move the flash on the camera bracket (as near axis fill) until I saw shadow detail at 1/1 power in HSS mode, then I moved my second flash at 45° in until the front side if the white towel nearly matched the sunny rim lit back side...
The "fill" flash killed the natural modeling on the target, the "key" flash added it back from the 45°V /45°H angle to the right recreating with flash "faked" natural modeling.
The take away?
You never want to use a single flash for "fill" you instead want to make the sun your "hair light", the skylight your fill, and raise the flash above the head as your frontal "key" light.
Logistically the simplest way to do that with speedlights is to use a camera-flip flash bracket. If using a light on a stand the cause and effect is the came so you'd also want it centered and above the camera creating the same "butterfly" full face pattern. With a full face butterfly pattern there aren't many shadows seen on the face except under the chin so you can often get away with just one flash on a bracket for less formal candid flash assisted shots...
When you move a flash off axis to the side it will create shadows on the face. The skylight alone will usually not fill them light enough to look natural or flattering. You can alter shadow exposure with shutter or aperture but that will blow the highlights.
A "nuclear halo" effect of blown out hair and skin as become the accepted norm in outdoor shots but if you want highlight and shadow detail at the same time you need do what you'd do indoors to get it: control the lighting ratio at will by adding a second fill flash, centered about chin level so it is shadowless as possible.
When using two flashes outdoors add only enough fill to put detail in the shadow the key light creates on the darker objects, then overlap your off axis flash and adjust its power until the highlights it creates over the sky + flash fill are just below the sunny highlights. That application of flash outdoors is the same thing you'd do indoors to light a face, The only difference? The ambient light outdoors is brighter and has a natural downward direction you need to take into account when posing the subject before adding the flash. If you go outside with two flashes and do what I suggest — fill for shadows, add key light just below the sunny side — this cause and effect should be obvious.
If you can keep the subject in shade and don't mind an overexposed background you don't need flash. If it is an overcast day you don't need flash. But if you are faced with a clear sunny day and want a normal looking background with a normal looking face in the foreground you will need to use two flashes because the scene range exceeds the sensor's.
Will the net result of a face lit that way be as natural looking as natural light? Is an indoor flash shot ever natural looking? If you think yes, then why would you think the same tools can't be used outdoors to get the same natural results when the natural light there is doing most of the work?
Edited on Mar 08, 2012 at 04:40 AM · View previous versions