When in Auto mode, the ZOOM function on your Speedlite was intended to make flash photography simple for people who don’t know what they are doing. When paired with a compatible lens, your Speedlite will reposition the flashtube so that the its angle of coverage is matched to what the lens sees. The range, on a full-sensor camera is 24mm to 105mm. Then, if you pull out the flip-down Wide-Angle Diffusion Panel on a 580EX/EXII, the angle of coverage expands to 14mm (with a 3-stop loss of light, see the end for details).
The idea is that if you are shooting a 50mm lens, then there is no point in lighting the area covered by a 28mm lens. For on-the-fly photojournalism, Auto Zoom is a good feature—even if you know what you are doing. From my perspective as a creative Speedliter, the ZOOM button is a built-in modifier that I can use to change the look of the flash to fit to my vision.
The effect of the Speedlite zooming to a longer focal length, it is it concentrates the light. So, at the center of the frame, the Speedlite is brighter when zoomed to 105mm that it is when zoomed out to 24mm. You can see this effect in the frames below. They are all shot with the Speedlite’s power in Manual at 1/1.
As you can see, the is a distinct difference in the light on Tony’s face between the top frame (zoom = 24mm) and the last frame (zoom = 105mm). I think the first is slightly underexposed and the last is definitly overexposed. To my eye, the optimal exposure happened when the head was zoomed to 50mm. Again, for these demo pix, the only thing that is changing from frame to frame is the zoom setting. In a real shoot, I would have first found the vignette that I wanted and then dialed the power up or down.
There is nothing that says you should not take the Speedlite out of Auto Zoom (by pressing the ZOOM so that the zoom display blinks and then turning the dial). More often than not I will use a Speedlite in Manual Zoom and zoom it much tighter than the lens I am shooting. I do this intentionally to create a vignette and, by doing so, send the viewer’s eye to what I want him to concentrate on.
ZOOM’s Power Hit
After shooting the above series and seeing the difference in light on Tony’s face and hand at each zoom setting, I set up a test to measure the amount of the light loss. In doing so, I resurrected my trusty Minolta Flashmeter IVF from the dark recesses of my camera bag (where I’ve kept it for years, largely for sentimental reasons).
I ran two series of measurements—one with the flashhead 24″ from the dome of the flashmeter and one with the flashead 48″ from the dome of the flashmeter. (There was, at most, a 0.1-stop difference.)
To keep the everything constant (other than the Zoom setting), I mounted the 580EX to a an off-camera E-TTL cord and then to a light stand and adjusted the height to the distances indicated. The flashmeter was set on the floor. I fired the Speedlite in Manual mode with the power dialed to 1/1. The flashmeter was set to ‘Non-cord Flash’. In this mode, it waits for a burst of light to take a measurement. The ambient light was not a factor.
So, using the output of 105mm as a baseline, here is the power lost at each ZOOM increment. The results are expressed as EV (exposure value). One EV is one-stop. I have separated them into the most relevant divisions (meaning that there is essentially no difference between 80mm and 70mm or between 28mm and 24mm).
105mm = Baseline
80mm = -0.1EV
70mm = -0.2EV
50mm = -0.6EV
35mm = -1.0EV
28mm = -1.5EV
24mm = -1.5EV
14mm = -2.9EV*
*14mm is achieved by pulling out the flip-down Wide-Angle Panel on a 580EX/EXII
Re-thinking the Wide-Angle Panel As A Viable Mod
I have to confess that I never considered the light lost by pulling out that little plastic Wide-Angle Panel at the top of a 580EX/EX II… until I shot these demo pix. Then, to measure it at nearly 3-stops, well I thought about breaking that sucker right out of my Speedlite. A 3-stop loss is even more than the power hit that comes from switching into High-Speed Sync (which I recently calc’d to be 2.5 stops—details to come soon). Just to be fair, after measuring the Wide-Angle Panel, I parked it back in it’s garage and popped a StoFen Omni-Bounce on the front. The light lost through this translucent white plastic cap was 2.6 stops—still a sizable hit, but less than the Wide-Angle Panel .
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