TTLS Distance measurement

When using a Canon 5d3 and an 680EXRT in a TTL mode and a lens that is capable of evaluating subject distance (which results in a flash compensation for subject distance)

Is the focus point the only determinate of the distance, which in the case of the 5D3 is the central focus point 100% of the time, and does the metering mode come into play. ( I know this answer may be different with 1Dx models).


A real life example is using TTL flash with a bride coming down the aisle. If the focus point is moved from center and the metering mode is expanded, does the center point still determine the distance and feed the information to the flash to recalibrate for a changing bride distance?

Thank you.

I'm not sure how this works, but if you require a lens that supports distance reporting, then I imagine it's going to be based on the lens being able to read the current focus position (as opposed to just being able to tell the motor to go front/back, with no idea *where* it currently is). If it works like that, then it's likely based entirely upon where the lens is focused, with whatever focus point is used, or wherever it has been manually focused to. Focus point expansion can mean using a point other than the main one selected. Metering mode shouldn't affect anything. Focus and recompose may give wrong flash powers, just as it may result in OOF shots.

Focus distance reporting lenses use an internal encoder to track the rotation of the focus adjustment barrel. This is reported back to the camera so it becomes available for use in flash power calculations. Focus points, selection modes, metering modes, etc have nothing to do with determining this value. Just the focus distance of the lens.

That is very logical.
Thank you.
I would like to investigate this whole topic in more detail.
Do you have any references or real life studies?

Thank you

Quote:

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Originally Posted by iND

Is the focus point the only determinate of the distance, which in the case of the 5D3 is the central focus point 100% of the time, and does the metering mode come into play. ( I know this answer may be different with 1Dx models).

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Why do you say it's always the center point on 5D3? There are 61 points, and you can use them in many different ways ("it slices, it dices..."). If multiple points are eligible, the camera selects the closest reading as final, and asks the lens to hit that.

How that aligns with metering mode and flash/TTL metering mode is beyond me though...

From what I understand on the 5D3 single point metering only follows the central focus point.
I believe some of the 1Dx models allow the metering to follow other focus points.

Thank you

Quote:

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Originally Posted by iND

From what I understand on the 5D3 single point metering only follows the central focus point.
I believe some of the 1Dx models allow the metering to follow other focus points.

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I believe you're correct. However, I'd assume that focus distance trumps TTL metering to some degree, as it's easy to compare the "stop-relationship" between guide number and the target-distance/ISO/aperture "product", so the disparity between focusing point and metering point would become moot.

Quote:

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Originally Posted by iND

That is very logical.
Thank you.
I would like to investigate this whole topic in more detail.
Do you have any references or real life studies?

Thank you

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I only came across the specifications for this when looking into the possibilities of creating a chipped adaptor ring to allow using Canon lenses, the MPE-65, on some high speed video camera bodies at work for a very specialized application. The amount of communication between the camera and the lens was a bit surprising to me. The breakdown on the data structure used in the packets of information used by Canon and Nikon has been sorted out by other engineering research groups and this is where my information comes from. In this case the client decided that the ability to do strain imaging from the videos of the specimens being tested was not important enough to justify the cost of development. Pity as it seemed like a very interesting project.

Several unclear items in this thread.
It's not advisable to use the term "1Dx" as a referece to various 1D models, like the 1D, the 1D Mark IIn etc, since there's also a mode actually named 1DX.
There is no such thing as a flash called 680EXRT. I presume the 600 EX-RT is the one referenced.
There is no such thing as "single point metering". There's spot metering (for light) and sigle point focusing (for distance).

Ambient light metering pattern (spot, partial, average or evaluative) is non-related to flash metering (evaluative, average or spot (when using FEL)). Data from evaluative flash meteirng is augmented by lens distance data, if such data is available and the flash is aimed straight ahead.
Evaluative flash metering is biased towards the active focus point, prvied One Shot AF focusing is used.