Canon Wireless Flash Ratios

So, on the 1D2, 17 metering points are used and on the 20D all 35 points are used...interesting..

John

Yes, on the 1D2, only the sensors in the center eliptical area are used for ETTL average metering. But it's really not as bad as it sounds. What isn't used are the sensors on the edges of the frame which probably aren't really that important.

Although the 20D has 35 sensor points, 20 of those are along the edge of the frame (the area ignored by the 1D2) with the remaining 15 being in the middle area where the subject would probably be.

So the question regarding the difference between the two cameras is how important really is the data collected by the sensors along the edge?

Thank you, Leo.

I always thought the 17 was for both evaluative and average....must have misread the 1Ds2 white paper.

I can see why the 1D would only want to use the central 17 in ETTL-II average mode. This kind of results in a form of CWA. It would be interesting to know how/if the meters are averaged...on the 1D and on the 20D.

It might help to explain why some people find CF14=1 provide better flash exposure....though I have been pretty much happy with =0...though +FEC is sometimes needed.

John

The averaging is a simple "add them all up and divide by the total number of observations" average. No fancy weighting is involved here. (I'm assuming you are asking how the averaging is done in ETTL-II Average mode)

Yes...thanks again, Leo

Therefore, assuming you were trying to get specific lighting ratios, on, say a portrait, you would definitely want to use ETTL-II mode...i.e. assuming the pre-flash evaluation and ratio setting in averaging mode was weighted the same way.

I am assuming that ETTL-II will be able to better meter and evaluate light intensity changes on relatively small area of a scene, particularly if there are large differences in the area under illumination.

Flash averaging mode should work fine when large or equal areas of the scene are illuminated by each group.

John