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p.1 #5 · shooting SB600 with manual settings | |
WIth color negative film the limiting factor is the range of the print paper. The negative has a much longer range with enough "headroom" 2 stops of over-exposure on the linear part of the response. Exposure in the camera just needs to be close, not perfect. When making the print the lab just uses more or less exposure.
With digital the sensor range is the limiting factor and it is less forgiving because there is little headroom for overexposure and no easy way the fix blowing highlights.
The thing to realize about flash is that regardless of how it is regulated exposure is only correct at one distance. So good exposure starts with good composition for flash, keeping what needs to be correctly exposed closest to the light. That way everything else will be darker and not blown out when the thing that is most important is exposed correctly.
I learned flash working for Monte Zucker back in '72. We used a pair of identical unmodified, single power flashes in manual mode. Exposure was controlled by shooting systematically adjusting aperture as shooting distance changed. For example for a shot taken from 11ft the off camera flash would be placed at 8ft. The difference in distance makes the off camera flash 2x brighter and because it overlaps the fill on the camera bracket, 2+1:1 = a 3:1 ratio on the face. Exposure at 11ft was determined with testing. Once the f/stop needed for 11ft was known it will always be the same because there are no variables.
It was systematic in that we shot from specific distances all the time. The 11ft distance was the benchmark and required an exposure of f/8. For a closer shot (we used TLR Rolleiflex cameras) the camera would move to 8ft, the off camera flash to 6ft (to maintain the ratio) and the aperture would be closed one f/stop (from f/8 to f/11). For a wider shot the camera would move to 16ft, the off camera light to 11ft, and the lens opened one f/stop (from f/8 to f/5.6). An entire reception was covered with a fixed focal length lens with a 3 stop range of aperture adjustment.
As mentioned the color negative process has enough latitude for over exposure to make that approach work if one errs on the side of overexposure. It can also work with digital but the margin for error is smaller and one should err on the side of underexposure which can be easily fixed in post processing.
I use Canon flash with that manual power / distance formula when shooting dual flash portraits because once set the exposure is consistent shot-to-shot. With both flashes on 1/2 power with my DIY diffusers I put the off camera flash one arm span between the center of the diffuser and the nose of the subject at a 45 degree angle to the nose, then take four steps back and shoot at f/8 @ ISO 100. My arm span is just under 6ft, I step back to 8ft from the subject. The result is a perfectly exposed file with a 3:1 ratio.
If the situation isn't static I switch to Canon's ETTL ratio mode. Any TTL metering mode is a guess by the camera regarding what needs to be exposed correctly. That's why intelligent composition is required on the part of the photographer to put what needs to be correctly exposed closest to the flash. My ideal model for flash in candid situations is stage lighting, not an overcast day. Indoor ambient light is usually hitting the faces at an unflattering angle (dark eye sockets) and is a different color so I opt to overpower the ambient, or when needed add a third flash aimed at the background. Put the star upstage in the spot light and then hide all the distractions in the receding fall off. When room ambience is important I'll either shoot a wide, ambient balanced available light shot to establish the setting in a slide show or album sequence or gell the flash to match the ambient. The solutions vary with the goals for the shot.
Finding the correct exposure is like sighting in a gun. Aim, fire the first shot, see where it lands, then you know how much to adjust. Remember with flash exposure and distance are joined at the hip. So evaluate where in the shot exposure is correct using the playback and overexposure warning, then move the point of correct exposure closer with minus flash compensation and further back with plus compensation. At weddings just key exposure of the clipping warning in the bride's dress and the white shirts.
Getting detail in the bride dress and black suit of the groom at the same time requires two flashes in an overlapping key over even fill configuration. The problem is the range of the sensor. Even in flat light if the highlights are exposed correctly some shadow detail will be lost because the scene has a reflective range of 10 stops from white to black but the camera only has a range of about 8, and only about 6 carry detail. What the fill placed over the camera on a bracket does is lift everything the camera sees to the point it can see detail in the darkest shadows. The second off axis flash then creates the highlight pattern on top of the fill, like adding white frosting to a chocolate cake. The net effect is the flash changes the range of the scene in the foreground from 10 to 6-7 stops and the camera is able to record detail everywhere.
Chuck
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