IMPORTANT: all I will be saying applies to EVALUATIVE MODE only.
FACT 1: Canon says that evaluative mode is a general-purpose metering mode suited even for backlit subjects, doesn't really say if there's a bias towards point(s) in focus, although many photographers say it does. By experience I can indeed say it does: if I put a person or a chair against a bright background, focusing on the perosn/chair gives me a quite difefrent exposure than focusing on the background. This is also prooved by the fact that Canon also says that when AE Lock is engaged the exposure centered on the AF point in focus is locked.
FACT 2: There is difference between POINT(S)/AREA in focus (from now on POINT in focus, for simplicitly) and SUBJECT in focus. As confirmed to me by a Canon product owner, If you have subject detection on but there's no detected subject and camera focuses on a chair, the chair is not a subject. This is important to understand how "Detect priority AE while AF" works (see next). Said otherwise, if subject detection is on and we have something in focus and this something is detected to be a persion/aniaml/vehicle, then we have a subject in focus, any other "something" in focus is not a subject in focus. This also means that a SUBJECT in focus is a special case of POINT in focus. This laso means that if subject detection is off, whatever is in focus is just a POINT in focus, even a person/vehicle/anial in focus is not a SUBJECT in focus but simply a POINT in focus: a person /vehicle/animal is a SUBJECT in focus if and only if subject detection is on and that person/vehicle/animal is in focus.
If we all agree on the above two facts, let's talk about the new option that Canon has introduces for R5Mk2: "Detect priority AE while AF", also known as "AE for Priority Subjects During AF". Canon says that in evaluative mode if "Detect priority AE while AF" is on and subject detection is on then metering is based on the AF point(s)/area where the subject was detected: said otherwise, if "Detect priority AE while AF" is on and subject detection is on and there is a SUBJECT in focus then metering is based on this SUBJECT in focus.
BUT... if we agree that in evaluative mode the exposure is already biased towards the POINT in focus, and a SUBJECT in focus is a special case of a POINT in focus, why do we need then this new "Detect priority AE while AF" option?
I am asking because I honestly do not understanbd at this point the difference between
A. Evaluative + "Detect priority AE while AF" OFF + "Subject Detection" OFF or ON + a chair or a person in focus against a bright white backgroud
> I expect the camera to consider the whole scene with a bias on the chair or the person that are considered a either a POINT or a SUBJECT IN FOCUS (depending if subject detection is off or on) but in any case "Detect priority AE while AF" is off.
B. Evaluative + "Detect priority AE while AF" ON + "Subject Detection" OFF + a chair or a person in focus against a bright white backgroud
> I expect the camera to consider the whole scene with a bias on the chair or the person that are considered a POINT IN FOCUS, being subject detection off (even if "Detect priority AE while AF" is on).
C. Evaluative + "Detect priority AE while AF" ON + "Subject Detection" ON+ a chair in focus against a bright white backgroud
> I expect the camera to consider the whole scene with a bias on the chair that is considered a POINT IN FOCUS (even if subject detection is on a chair is not a SUBJECT not being a person/vehicle/animal).
D. Evaluative + "Detect priority AE while AF" ON + "Subject Detection" ON + a person in focus against a bright white backgroud
> I expect the camera to consider the whole scene with a bias on the person that is considered a SUBJECT IN FOCUS
