cgardner
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Jacob:
What I'm referring to is situation indoors in a dark room where the camera is already wide open and as slow as can be hand-held then higher ISOs are used to create more background ambience with the flash lit scenes foreground. At lower ISOs when the comparison of ambient and flash is done in the evaluative flash metering very less ambient light would be detected and more flash would be used. As the ISO rises so does the overall ambient exposure and less flash is needed. Why its an issue is because the reason to use flash indoors often isn't the lack of ambient light intensity but the fact it hits faces from a steep unflattering angle (i.e. dark eyes shaded by brow) or its a different color temp than the flash (nearly always the case indoors).
I use both ETTL and M modes as the situation dictates. ETTL when either I or the subjects are moving around, and M for things like portraits or still life where the flash/subject distance doesn't vary.
Functionally there is no difference between changing power manually or changing it via FEC adjustment in ETTL mode: IF THE SUBJECT IS STATIONARY. The difference manifests itself when shooting situations which aren't static. ETTL reacts automatically to changes in subject distance. Once FEC is set to get correctly exposed highlights if the subject moves around the exposure will stay consistent automatically. WIth manual mode you'd need to either change aperture or flash power as the subject distance changes, which just isn't as practical.
When using my Vivitar flash manually in dynamic situations I'd shoot very systematically from pre-determined distances of 16, 11, 8, 6 feet. Just like f/stop numbers those distance increments change flash power by one f/stop due to the inverse square law. Starting from a baseline of 11ft @ f/8, I'd stop down to f/11 if moving into 8ft, and f/16 if moving into 6ft for a shot. If moving back to 16ft for a wide shot, the lens would be opened to f/5.6. After doing that repeatedly your brain will start to automatically measure distance in f/stops, not feet. It works with dual flash also. As the shooting distance changes you move the off camera flash in or out to keep the ratio the same. Fill / Key distance combinations of 16/11, 11/8, 8/6, and 6/4 all produce a 3:1 highlight/shadow ratio on a face (2x brighter key + 1x fill : 1x fill = 3:1)
FWIW: The idea that flash is "fill" is incorrect in most situations outdoors. On a sunny day there are two sources of light: the direct rays of the sun, and light reflected from the sky in all directions as fill.
When a person is standing facing the direct sun with shaded eyes, or face half in sun and half in shadow adding flash lifts highlights and shadows equally. The flash "fills" the shadows in the sense they get lighter, but so do the highlights. the contrast range between shaded eyes and sun lit cheek doesn't change because both are skin of the same reflectance and both are getting hit with the same amount of light. In that scenario the sun on the face is the "key" light, the light from the sky is acting as fill for the areas not hit by the direct sun (like the eyes) and flash? Since its hitting both the highlights and shadows at the same time its neither key or fill, its acting as both which really doesn't help much in the technical sense of exposure or contrast reduction.
If we start with normally exposed skin in the sunlit cheeks then add flash the highlights become overexposed. If we lower the ambient with shutter (assuming we are below x-sync) the eyes we just lifted with the flash get darker again. On a perceptual level we will react to the "twinkle" of catchlights in the eyes the flash creates in that situation and will accept lighter looking highlights as "normal", but at some point the balance gets out of whack the the brain says "that looks fake and over-flashed". Because caucasian skin reflects more red, green and blue in that order, the red channel in skin clips first. The brightest skin highlights, when overexposed by just 1/3 stop will start to get a flat yellowish look.
Back in the days B&W fill flash wasn't needed in direct sun to put detail in shaded eyes because the "normal" dynamic range of the film was about 10 stops scene illumination and with reduced negative development or the use of less contrasty print papers scenes with ranges of 12 or 13 stops could be reproduced normally. Contrasty lighting became more of a problem when photography switched to color which only has a range of about 5 stops.
When flash was used photographers found a better strategy in sunlight: put a person's back to the sun and expose the background normally, then lift the shady side of the face to match the background INDEPENDENTLY with the flash because there is no overlap of flash and sunlit highlights.
In that scenario the shady side is filled by skylight which is very soft but -3 stops darker than the sunny side. A digital camera with a 6-8 stop range renders the shaded face quite dark before flash is added if the background is exposed normally. When flash is added to the front of the subject it creates a highlight pattern on top of the skylight fill. The areas on the face not highlighted by the flash remain "filled" by sky and unchanged in tone.
Creating highlights is what a "key" light does by definition. So in that scenario the sun changes its role to "accent / rim / hairlight", the flash becomes the frontal "key" light, and the fill for the shadows (which remain quite dark) still comes from the sky. The flash lit face will still look "fake" unless the it is raised above the head of the subject. That gets us back to the initial point I made about adding flash: the direction matters. If we want flash light to look "natural" the direct it comes from must mimic the direction of natural light sources.
In that backlight scenario outdoors if we want the shadows on the face any lighter, we need to add a secondary fill source to boost the light from the sky from the direction of the camera.
The "magic" of the photographic process is that when exposure is correct in the technical sense of having detail in both the darkest shadows and brightest solid objects at the same time, all the tones in between fall into place perfectly perceptually because the response is engineered to be linear. That's part of what makes the Master on bracket / slave on stand arrangement with hot shoe flash both a practical and technically sound solution to matching the contrast of any scene to the range the sensor can record with detail.
Chuck
Edited on Nov 08, 2009 at 02:37 AM · View previous versions
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