cgardner
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 p.2 #9 · Shooting a person w/ sun behind them. | |
The basic technical problem outdoors is that the ambient light contrast usually exceeds the range of the sensor. That means if we expose for highlight detail the shadows are rendered darker than seen by eye; the pupil eye adapts when looking into the shadows, the camera aperture doesn't.
Angle of the ambient light affects contrast and content affects perception. For example here is a side lit test object with a full range of tone...
http://super.nova.org/TP/DR_FlatLight.jpg
Exposure was based on highlight detail the white towel. Here's the same target exposed the same way for detail in the towel in the same light, but from the opposite direction and further away...
http://super.nova.org/TP/DR_Backlight.jpg
Same light, same exposure criteria (detail in textured highlights) yet the two shots look different perceptually. Why? Content. In the first shot the highlights and middle tones are large and mostly in the sunlight. In the second shot they are small and mostly in shadow.
http://super.nova.org/TP/DR_FlashFill.jpg
In the shot above I just started with the ambient shot and reached up and turned on my flash in ETTL mode, set at FEC = 0. The evaluative 35-zone metering in the camera was able to detect the "hole" in the ambient pattern by first making an ambient "map" of the zones, followed immediately by one created with ambient + pre-flash:
http://super.nova.org/TP/Eval_Ambient.jpg
http://super.nova.org/TP/Eval_PreFlash.jpg
Subtract the ambient map from the ambient+pre-flash and you get an map of the "hole" in the ambient the flash needs to fill to match foreground and background. That's the technical part of how flash allows the scene contrast in the foreground to match the background and fit the sensor. What the histogram represents is the range of the sensor and a selection of just the foreground shows that shot came about as close as possible with a single light source:
http://super.nova.org/TP/DR_FillHisto.jpg
In the original RAW file the sunlit parts still have detail and the shaded side is still perceptually darker, but not quite as dark as perceived by eye. In person we'd expect the shadow side to look a bit darker than an exact match. In fact an exact match would destroy the optical illusion of 3D the rim light creates in a 2D image.
That's the technical apart. Now lets consider what happens when a face is lit. Starting with just the ambient light a person standing back to the sun, exposed for the sunny highlights on their white shirt, will have a very dark face. It not because there is no fill. There is lots of fill from the sky the person is facing. The problem is the camera sensor can't handle the contrast. We can make the face normal, but that would blow the highlights. That's the technical dilemma.
So we add one flash. What does that flash do? It creates highlights on the face. So its that flash fill? No. The fill still comes from the sky, the flash is acting as the "key" light. Like any key light the flash used in that way outdoors with sun to the back should be raised over the head of the subject if a flattering, natural lighting pattern on the face is desired. The same key light positions that flatter indoors will flatter outdoors: centered butterfly for a full face pose, short lighting for an oblique view. Both patterns involve placing the key light about 45 degrees from the nose; vertically for butterfly, horizontally for short lighting.
When the key flash is moved off axis the parts of the face not hit with the flash remain lit by the sky just as they were before flash was added, except to the extent the "key" light wraps around the face. The problem with the sky fill is that its 3 stops darker than the sunny side, an 8:1 incident ratio. The character of shadows results from two variables: how parallel the light rays are that create the shadow, and how much fill from the ambient light or a secondary fill source there is. The illusion of "hard" and "soft" light is a result of the tone of the shadows. Lighter shadows = softer looking lighting. 8:1 is very hard looking lighting.
There are two basic strategies to make shadows lighter:
One way is to use a huge modifier on the key light. People who learn with one light and a reflector will tend to use this approach. When the source begins to dwarf the object it is illuminating the light wraps around the object making the edges of the shadows (penumbra) lighter than the center core (umbra). The net result is a two-tone shadow: dark core, lighter edges. The bigger the apparent sizes of the modifier (how big it looks from the point of view of the object casting the shadow) the fuzzier the edge will be.
The other strategy to make shadows lighter and look softer is to add a secondary fill source. It can be a reflector or a second flash. When the fill comes from the direction of the camera it will mimic the direction of the natural fill from the sky and reach all the nooks and crannies on the face the camera captures. Logistically that is easier to do with flash than a reflector for a simple reason; the reflector gets in the way and can be a handful on a windy day if working alone.
Ideally a fill source should not create shadows of its own for a very simple reason: where there are shadows there is no fill and there there is no fill the shadows will be darker. Next time you shoot outdoors and put a person's back to the sun pay careful attention to the eyes. You'll notice that the eyes will be darker in open shade in the same way they are in direct sun, for the same reason: the brow is shading the light which hits the face from too high an angle to reach the eyes. If we don't start by making the sky fill even on cheek and eyes when fill flash is added the eyes will remain darker than the cheeks because the combined ambient +flash is still darker than on the cheeks. The solution to that dilemma? Simply raise the chin of the subject so the light reaches the eyes, the use a step ladder to raise the camera position to maintain the same flattering angle. Bring a ladder on your outdoor shoots.
The goal of the lighting exercise is to blend the artificial light into the natural as seamlessly as possible. Fill flash above eye level will create shadows, but they they will fall in the same places natural fill from the sky does and will perceived as being more natural than fill placed low or to the side. Not a rule, just cause and effect. If we want to mimic natural fill light, observe where is comes from and match the direction, being mindful of having it fill the eyes. Just try it different ways and compare the results.
What is different about flash outdoors vs indoors is that is isn't working alone. Behind it is all that light from God's own softbox the sky which wraps completely around the face. So when a second flash is added for fill by placing it above the lens of the camera, or just under it, all its is really doing is kicking up the 8:1 sky fill a stop or two, depending on how light we want the shadows, which controls how soft the overall lighting will look. Since the second flash isn't creating many visible shadows other than those that occur naturally there is no compelling need for a huge modifier on the fill source.
The wrap-around effect of the sky also contributes to the off camera "key flash" creating the highlights. What it winds up doing is overlapping the combined wrap-around ambient fill + secondary flash fill and kicking it up about a stop or so to create the highlight pattern. Because of the softening effect of the sky on the frontal lighting you may find that a smaller modifier than used indoors is needed.
That's the cause and effect of the lighting. Not rules, just how the light actually works together. Adding one flash in front doesn't add fill if it is moved off axis, it becomes the key light. As with lighting indoors do lighten the shadows beyond what the sky provides one of two strategies are needed: a big key light to wrap the face, or a secondary source of fill from a reflector or second flash.
With the sun at the back and two flashes in from in a overlapping key over fill pattern the net effect is the same as a four light studio configuration indoors. The difference is that the ambient light contributes to all four:
Background: sky light and/or direct sun
Hairlight: direct sun
Fill: skylight + flash #1
Key: skylight + flash #2
Try it sometime. Start from the baseline of just using direct flash and you may be surprised how soft the light on the face looks due to the contribution of the skylight when the flash complements rather than fights it. Add gradually larger modifiers and observe how the character of the lighting changes. Since most of the light comes from the huge sky the effect of larger modifiers will be seen mostly in the highlights. As the modifiers become larger the highlights on the skin will will become less specular. At some point you'll find a balance between the logistics of wrangling larger and larger modifiers and the difference they make in the lighting. I don't presume to know what will work for you, but I get by with a either direct flash or small diffusers outdoors.
Heres a link to a PDF tutorial on outdoor lighting: LINK
Chuck
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Oct 15, 2009 at 10:22 AM