During my 300D days I was very current in these matters, but the details for the newer cameras (and for the 300D, actually!) have gotten away from me (and I'm not sure I ever knew the answer to this particular issue, anyway).
If we have an EOS 40D set for "all" AF points (automatic AF point selection), AI Servo AF, and Evaluative metering, what AF points does the Evaluative metering system consider "active" to apply its "emphasis " (whatever that is anyway)?
I could imagine possible answers could be:
1. The center one.
2. Which ever one was controlling focus at the time of full-press (as a result of "lateral" tracking.
3. All the ones that would qualify as "active" if One Shot AF were in effect (that is, essentially all those considered to embrace the "subject", based largely on consistency of observed distance at those points in the scene.
The EOS 40D is consistent with other EOS Digital SLRs in terms of its evaluative metering algorithms, so what I'm about to say here applies just as well to other models like the 50D and the 5d Mark II, etc.
First, Evaluative metering is always linked to a single focusing point. This is true even when multiple focusing points light up simultaneously, as might be the case with a large subject when the camera is set for the combination of One-Shot AF or AI Focus AF and Automatic Focusing Point Selection. The active focusing points do not light up when the camera is set for the combination of AI Servo AF and Automatic Focusing Point Selection and the camera is focusing.
Speaking about One-Shot AF and AI Focus AF with stationary subjects: If only one focusing point lights up, then that one is the active focusing point. If more than one light up, it means that those points are all within the depth of focus as determined by the AF system, but the camera's AF detection algorithm selects the metering segment surrounding one of those points as the primary segment for the evaluative exposure measurement. In such a case, the user cannot be certain which focusing point the camera has chosen to meter for exposure. However, in many cases, it's possible to take a preliminary test shot and review the histogram on the LCD screen to determine if there is any need for exposure compensation.
Speaking about AI Servo AF with Automatic Focusing Point Selection: In this case, the user is never certain about which focusing point is active because the focusing points do not light up while the camera is focusing. As in the case of One-Shot AF, it's sometimes possible to take a test shot and review the histogram on the LCD screen to determine if there is any need for exposure compensation.
Evaluative metering is at its best when photographing people, but it also works very well with a wide range of non-human subject matter. It may need some help from time to time in terms of exposure compensation, especially in situations where the primary metering segment is centered on subject matter that is particularly bright or dark.
I would also say that there are some situations where it might be best to take an exposure reading ahead of time and set the camera for manual exposure. This can help improve the consistency of exposure in burst mode shooting, compared to using an AE mode like Program, Shutter Priority or Aperture Priority.
Thank you for the wonderful and very thorough exposition on this entire area. You continue to be the most valuable "extra" that is in every Canon carton!
Quote:
Originally Posted by ChuckWestfall
First, Evaluative metering is always linked to a single focusing point . . .This is true even when multiple focusing points light up simultaneously.
Wow! I never understood that. I have been since my first EOS days laboring under the misconception that evaluative metering placed "emphasis" (whatever that is) on all the AF points that "lit up" (to use a homey way of describing it). I suspect many of my colleagues here are similarly afflicted!
Thanks again so much for this thorough response.
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
One point you did not mention. If I remember correctly, when manual focusing, evaluative metering assumes the center focal point was used....is that correct?
Do your comments on evaluative metering hold true for 1-series bodies?
Also, anything you can share on the relative merits and/or idiosyncracies of evaluative vs. orther modes such as center weighted? Particularly if using the center AF point (I guess I am a dinosaur, too)
John
__________________ John Schwaller
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
Linear Mode
