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 p.1 #10 · Using Strobes Outdoors - Need Help | |
Your starting baseline for normal exposure in sunlight is "Sunny 16" or f/16 @ 1/ISO.
For a camera with an x-sync of 1/250th with manual flash you'll be maxed on the shutter at f/11 at ISO 100 for a normally exposed background. If you were to put the subject's back to the sun and meter the flash at same f/11 setting you'd get a seamless match on the front of the subject.
The shady side of the subject is about three stops less than the sunny side, or f/4 when the sunny side meters f/11 @ 1/125th @ ISO 100. So to get to f//11 from the baseline of f/4 sky fill on the shaded face you need to add 3 stops more light with the flash. The flash you add creates highlights over the sky fill so technically its a key light not fill. The parts of the face the flash doesn't hit will still be sky fill only, about 3 stops darker than the highlights you create on the face with the flash. To make the shadows lighter you'd need to add a second flash, near the camera, for fill just like indoors.
To get the background darker than normal (day as night) you need to cut ambient exposure. If using conventional flash at x-sync you'd have no option except closing the aperture more or adding .30 ND for every stop darker you want to make the background.
Let's say you want the background 3 stops darker than normal. First add .90 ND filter to the lens on top of the f/11 @ 1/250th normal exposure exposure. The .90 ND will also cut the flash exposure by 3 stops also so you'll need to raise flash power until it meters f/32 (3 stops more than the original f/11 matching reading) get back to a normally exposed foreground against the - 3 stop background.
- or -
Set the shutter at 1/250th and the aperture you want for DOF and then keep adding ND until high noon looks like 10PM or as dark as you want it. Next you hand your subject a white towel. Crank up the flash power in front until you see the towel clip in the camera warning, then back off the flash a bit until the clipping disappears.
Just about everything is a value judgement so you will need to rely more on how it looks than trying to paint by numbers. After you get the look you want by eye then break out the meter and record the readings to give you a blue print for repeating the set up next time.
Hot shoe flash:
With hot shoe flash in high speed mode select the desired aperture for DOF and then adjust shutter speed to get the background as dark as you want. But the subject's back to the sun hand them a white towel and turn on the flash in ETTL mode. Increase FEC until the towel clips then back it off one click. Since flash power is limited in high speed mode it works better at wide apertures where the flash power goes further. Click the WWW button and look at the end of the Canon section for a high speed flash tutorial.
Chuck
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