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waterdog

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Does the matrix metering work on CFWA? Nikon brags that it controls the strobe better in difficult lighting situations but can you really use TTL succesfully in water? And where does everybody place their strobes? How far from the camer and what direction should they be pointed? Straight ahead? Tilted in towards subject?

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WACU's, to me, should be at or near the WA lens's closest focus distance. A Nikonos 15 mm will focus down to 1' but it's DOF begins at 6" at F/22. When I do this, I pull the strobes in almost as tight as macro, pointed slightly more forward.

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As long as the close subject fills enough of the image area, TTL can work very well, though I'm not convinced matrix metering works any better than center weighting in these situations.

 

As ScottyB says, you definitely need to pull the strobes in tight for CFWA, but at wider apertures using powerful w/a strobes, the TTL logic may not be capable of shutting down the strobes in time to avoid over-exposure of the close subject. If this is the case, try moving the strobes back behind the plane of the camera. For really close subjects, you may need to elevate one or both strobes to get the necessary distance while at the same time avoiding shadowing the close subject by the housing and port.

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Thanks for the info, however I think my question wasn't the right one. I re-read the manual for the N90S. I am confused about how the flash metering system works. The matrix meter, I now realize is for ambient light. But there is some sort of evaluative interaction going on, at least when you use the camera with a SB-26 for land. Does anybody know if their is anything like computer controled evaluation going on when the SB series of flashes are used in water?

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Well, that depends which camera you are using. Are you referring to the N90S or to a digital SLR camera like the D100 or Canon 10D?

 

Cheers

James

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I think this is a big difference between digi and film. On film I use TTL for CFWA (so long as the subject fills a good proportion of the frame), but on digital with the instant feedback of the LCD screen I always use manual - because of the extra control it offers.

 

To get back to the point - none of the Nikon UW guns (e.g. SB104/SB105) can do the preflash evaluation that more advanced land guns (e.g. SB26/SB28) do with the N90S. But the underwater guns still use a multizone TTL flash metering system during the exposure - which works well.

 

Alex

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Here we go. The camera meters and the strobe meters (when used in TTL mode or D-TTL mode for that matter). These are two completely different meterings.

 

So, let's say you are shooting macro. The flash will be the light source for the entire frame. It doesn't matter whether your camera is in matrix metering of spot metering. The resulting exposure reading will be meaningless. The TTL strobe will measure and fire the strobes based on 1) inperceptable pre-flash and 2) the measurement of light on the film plane (for the S2 only it is the sensor chip) taken from 5 spots.

 

That is how Nikon TTL works.

 

Now, for wide angle, just think about what the camera is doing. The matrix metering is giving you the suggested exposure setting for the entire frame. It doesn't know anything about your strobes, only whether they are attached and on. Nikons default to a minimum 1/60 shutter speed in normal sync mode non-manual mode. In slow sync mode in aperture priority or manual, it will allow the "correct" shutter speed to be assigned.

 

Now, the matrix meter is trying to give you an ambient exposure that will proplerly expose the entire scene. Is this what you want? Probably not, but sometimes it's darn close. The close in subject or anything within strobe distance is part of the calculation, but shouldn't be since it will have artificial light hitting it. There's a good chance that it will try to overexpose the scene.

 

This is also true of TTL strobe control when doing wide angle. If 75% of the scene is beyond the strobes' range and fairly dark, the strobes will keep firing longer than you want, trying to light the background that it can't. It is trying to get the whole scene lit to about "18% gray".

 

One approach is to spot meter part of the scene that you want as neutral, set exposure, then set strobe power based on guide numbers for foreground.

 

Others simply use matrix metering, negative exposure compensation, manual strobes or even strobes in TTL with slightly more negative EV dialed in. I could never figure out how much - EV to dial in. I was once told, "...between .7 and 2.1.....". Where between that range was kind of fuzzy.

 

For strobe placement, I have mine straight out and wide, but strobes should be no further than 3-3.5ft from each other. Point straight out. Strobes should be back at or behind film/sensor plane.

 

Now, when something is close up on one side, turn strobe so that it is hitting the subject from a direct angle. The strobe to subject distance might be quite different from one end of the subject to the other. I usually forget this. If I do remember, then I forget to turn it back for the next shot. At least with digital, I might actually see my mistake.

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