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SU800 and autofocus illumination activation


Boise, US
211 posts

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mwmbjones Registered since 30th May 2006
Sat 03-Jan-09 12:07 AM

Now that the weather is no longer cooperating for location sessions I'm shooting a lot more studio portraits, multitasking up to 4 SB800s controlled wirelessly by an SU800 Commander unit. Since the SBs have no modeling lights, the ambient light in the studio is uncomfortably low (particularly so for a new model in for a first glamour portfolio session), so I've been working out the compromises in adding ambient light up to the level where it keeps the studio from feeling like a windowless dungeon but doesn't contribute to the exposure. So far, so good in improving everyone's psychological comfort level, but unfortunately, the best compromise still doesn't seem to be enough to provide for consistent autofocus.

After a frustrating weekend shoot during which the only way I could find to reliably activate the SU800's autofocus assist light was to set the D3's AF-area mode selector to "Auto area AF" (the mode where the camera overrides manual input and auto-selects focus based on the closest point--usually, of course, the model's elbow or the tip of her nose), I sat down this evening and bullied both manuals into coughing up their typically Nikon-guarded secrets.

I guess I've figured out why my preferred single-point AF setting wasn't working. According to the SU800 manual, there shouldn't have been any difference between the two camera settings as regards AF-assist. I finally noticed in the D3 manual (page 195), however, that the active focus points that will trigger AF-assist illumination on the SU800 (and, presumably, on a hotshoe-mounted SB800) vary according to the focal length of the lens. Annoyingly, since portraiture usually implies a focal length longer than 50mm, lenses in the 50-105 range only have 9 of the D3's 51 points available, arranged in a loose cross. Five of those are the stretched center vertical column. Then it skips two columns in either direction, makes one point available on the center horizontal row, then skips one more column and makes one more point available on each of the outer columns. In short, other than the center vertical column, there are holes all over the focus-point grid, and those holes change each time the focal length changes. Frankly, I don't even begin to anticipate being able (or even wanting) to memorize these floating patterns. For the sake of my dwindling sanity, I guess I'll just remember that the center column is all active, and bypass trying to set a focus point anywhere else. Nuts.

Just a cranky gripe, mostly--now that I know what it is, I can work around it--but it does beg a small, weary question. Anyone know a technical reason why Nikon would set the AF-assist illumination system up in this manner? The red assist light is displayed in a cross-shaped grid itself, so I'm assuming that the system might be restricting focus points to something resembling the assist-light pattern (or, conversely, that the assist light's pattern is set to emulate the focus grid). But that still doesn't explain why 15 points are available to a 24-34mm lens and 17 points are available to a 35-49mm lens. Seems completely arbitrary to me. Anyone with more understanding of this particular tech have any ideas what's going on?

Is somebody at Nikon just messin' with me or is some higher function being offered that I'm too dense to figure out and utilize?


Bruce Jones
Writer, rider, shooter