Subject:	I was looking for answers to similar questions
Author:	Javier
Date:	4/4/2002 6:29:33 AM
Message:

for Canon eos camera. Although I think you are using a Nikon ("Matrix Metering"), perhaps it behaves in a similar way.
When a Canon eos camera detects a strobe, it OVERRIDES whatever light-metering pattern you had slected to the following:
#1. Ambient (or background) light is measured from all the segments that are far away from the focusing point that you are using. According to this measured light it will define the EV value. This implies that even if you were using spot metering, matrix, centered-evaluated etc. the camera now changes the rules of the game and selects a special "background light measurement mode" that tries to isolate the "Background" from the "Subject". So you don't have to actually measure the background light using spot-metering and then AE locking (the camera does it by herself without asking for your permission). In this case actually trying to do the "Background spot-measuring and AE lock" technique gives wrong results. There is a strict warning about this in all canon references that I've seen not to do that since it will fool the camera making it think that the place where you performed the spot-measurement is your subject (which is not).
Now it gets even more complicated:

#2a. If the camera measured an EV value above a certain threshold, it knows there's enough light for backgorund exposure and stops down the strobe by -1.5 EV as a fill flash. It will choose speed and aperture according to the background measurement to correctly exposure the background.

#2b. If the camera measured an EV value below the threshold, it knows there is not enough light, assumes it is very dark (or night) and says something like "OK, forget the background since you'll be needing a very long exposure. I'll choose a 'steady speed' of 1/60-1/250 depending on whatever aperture you chose. You'll get a dark background but a well exposed subject". You can always switch to manual or other modes that will allow you to shoot at the appropriate background speed but many times (especially at night) you'll be forced to use speeds such as 1/10 or several seconds...

#3. TTL (in case you want to use it) measurement will be from the segments that are closer to the focusing point.

I have written all the above although I'm not sure it applies to Nikon users, it may help clarify the things for Canon users who perhaps didn't even know that the camera takes this type of decisions. I will not be surprised if you get similar answers for Nikon's behaviour with an external strobe. BTW, IT IS NOT EVEN MENTIONED IN THE USER'S MANUAL...

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