The D40 with its three focus points makes it easier to illustrate . These pictures were taken without flash and the focus point was selected manually in " Dynamic Area " focus mode where the user moves the focus point to the subject . This can give confusing results the first time until you learn what is happening .
With the centre focus point selected it behaves like the 'old' matrix metering and normal TTL flash by over-reacting to the black in the centre of the frame .
Then I moved the focus point onto the subject for completely different results ......
All of a sudden it is behaving like TTL/BL flash [ remember there was no flash used here , this is just to illustrate the 'focus point' concept ] , it is doing a much better job of exposing for the subject regardless of the centre of the frame .
Again , here is a comparison between ''normal'' matrix metering and TTL flash .....
And then again with the focus point moved to the subject compared to TTL/BL flash .
The similarity is that TTL/BL automatically metered off the brightest part of the frame even though I had the centre focus point selected in that last shot , but you can still get varied results without flash depending on which is the active focus point which is what annoys many people about the metering on the newer Nikons .
Basically this means that when you are not using flash results can vary quite greatly depending on where your focus point is . When you are using flash TTL will meter strongly for the centre of the frame while TTL/BL will automatically meter off the brightest object under the focus point diamond regardless of which focus point you have selected .
Today I did some outdoor tests in bright sunlight to test the focus point theory in a real-life situation . When the subject is mostly front lit the focus points are picking up a lot of light and backing off the flash . here is an extreme example of what could happen .
We introduce a bright white object to the bottom focus point and the flash backs right off ....
I suppose this is a real-life possibility , we have a subject with a strip of white under three focus points and the flash backs off in response . We replace that with a darker strip which is no longer the brightest object under the focus points and it takes the next brightest object - the face in this example , and gives what it considers the right amount of flash .....
As I've learned more , and of course considering the D40 only has the three focus points in the centre , but reacts to objects near the top of the frame , it has become more obvious that TTL/BL uses the entire 'focus point diamond' and has the ability to select an area based on colour to meter off . This sound very much like Nikons " 3D colour matrix metering " concept in that it can decide , based on colour , what to meter off within the important area that you normally find the focus points in .
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