metering and AF point and AE lock
My manual says that in evaluative metering, "AE lock is applied at the AF point that achieved focus." I think I'm confused by evaluative metering and AE lock being used in the same sentence. If evaluative metering is just an average for the whole scene, then is a narrower point of AE really taking place when the focus is chosen?
Say in a portrait with some back light or strong light coming in from the side...I choose to focus on the face (and therefore lock auto exposure on the face as well)...is priority given to the exposure on the face or is everything averaged (as I would expect in evaluative) and the face may come out too dark?
I wonder if using center-weighted or partial (for portraits) would be better, and using the AE lock button when placing the center point over the face and then focusing with the appropriate focus point.
Any pointers, ideas, suggestions? Thanks!
Say in a portrait with some back light or strong light coming in from the side...I choose to focus on the face (and therefore lock auto exposure on the face as well)...is priority given to the exposure on the face or is everything averaged (as I would expect in evaluative) and the face may come out too dark?
I wonder if using center-weighted or partial (for portraits) would be better, and using the AE lock button when placing the center point over the face and then focusing with the appropriate focus point.
Any pointers, ideas, suggestions? Thanks!
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How I read this part of your manual is your camera is metering the entire time up and evaluating everything in frame. As soon as the focus function locks in on a target, the AE freezes that evaluation and uses that for exposure.
If your using evaluative metering. You can't just select the face. Your camera takes in a series of points (called zones) and comes up w/ a best average. This is why shooting subjects in the snow can be really hard. Your camera sees 8 out of the 9 zones it's metering as very bright and sees one dark area. The average for all those zones will still be bright and your camera will underexpose the subject.
I think your making this harder than it needs to be. Let me know if you understand what I posted before, then we'll discuss what the best means for portraits are.
NOTE:
I'm still a bit of a novice myself. So the pro's can feel free to correct me. I'm always learning.
SIDE NOTE:
I think you did a great job on your holiday cards! On your site. I'm completely biting of how you did your :grin
Think of it this way: if you use partial metering when focusing on someone's face, the camera will meter their face to 18% grey in the final print. Depending on their skin tone you may acutally want their face to be either brighter or darker than that. With Evaluative metering, the camera meters off an area about the size of the partial area, but it takes the rest of the frame into account to guess the proper exposure compensation for the face. Usually it is more accurate in back lit situations than partial metering is unless you are adept at exposure compensation.
Some other things I have noticed about evaluative metering:
In direct sunlight it tends to overexpose by between half and a full stop. I think this is intentional with the goal of preserving shadow detail as the expense of color saturation. Essentially it is doing a bit of expose to the right for you which can often be nice if you are prepared to adjust the exposure in post.
It is a bit shy about large areas of white and it tends to underexpose when shooting against white walls.
It rarely misses by more than one stop so in a lot of situations you can just let it do its thing and correct in post.
When working in spaces with reasonably even light, I usually shoot in manual mode using either evaluative or spot metering as assistant. With evaulative metering I just swing the camera around the room watching the meter in the viewfinder, choose a consensus setting for the camera and then chimp to fine tune it. With spot metering, I pick a target that I am sure how bright I want to be in the final print (my hand, a white wall, etc) and set the exposure based on that.
Me? Make something harder than it is? Never!!
I like the idea that it evaluates and then draws its primary exposure readings from where the focus point is (auto-locks the exposure from that selection). But evaluative to me says "over-all average" not exposure based on a particular area of the photo. Isn't that what the other metering modes are for?
I understand the shooting in the snow issue and coming up with underexposed pics. And I understand that in evaluative you can't meter off the face because it's taking in the whole scene...that's why I was confused about the manual saying that AE lock would happen at the chosen focal point. Why is any AE lock happening at all in evaluative? Maybe I'm not understanding what AE lock actually does? I guess I thought it meant that it would lock the correct exposure (according to the camera) for that location and apply it to the whole pic.
So, back to portraits where the face is most important. It seems that evaluative might not be the best metering mode. Maybe center-weighted average or partial...but you'd have to place the center of the screen over the area that you want to meter from (the face) and press the AE lock button, recompose, choose your focal point and then shoot. Right?
Thanks for discussing this with me and thanks for the
Comments and constructive critique always welcome!
Elaine Heasley Photography
Thank you very much for sharing your experiences with metering! It sounds like you tend to stick with evaluative most of the time then, even if it's just to assist you with adjusting your manual settings, is that right? Maybe I'm having a mental block about it...I mostly used center-weighted before I switched to the 40D, and I'm trying to make the most of my switch by learning and understanding even more. Evaluative just seems like a sort of band-aid mode for some reason. Maybe I need to just use it as a jumping off point and try manual even more.
Thanks again!
Comments and constructive critique always welcome!
Elaine Heasley Photography
Elaine,
Re-read what LiquidAir posted. I'm not sure it's 100% correct, but I'd say its very close to being accurate.
I think the sticking point for you understanding this is that Evaluative metering is taking an average, but --- the way Center Weighted gives more weight to the center, Evaluative gives more weight to the active focus point.
In the case of Evaluative metering, YOU can move the virtual metering "center". Evaluative by default settings, is linked to the active focus point.
That means that if you are using the center focus point, then Evaluative metering will give more weight to the center of the frame.
If you are using, say, the left most focus point, THAT is where more weight will be given in evaluating the scene.
Clear as mud?
Think of evaluative as a convenient "center weighted" metering that you can move around the frame at will with your selected focus point.
(not technically correct, but trying to give you a mental picture here)
Hope this helps.
Originally Posted by Elaine
Why is any AE lock happening at all in evaluative?
:nono
In many cases, that would be very advantageous. Let's say that you want to meter off of someones face, lock the exposure at that reading, then recompose the shot. That's one reason it's there.
Not completely sure about the 40D due to my not using one yet, but on the 30D, C.Fn 4 allows you to change up the way the metering interacts. You can un-link Evaluative metering from the active focus point if you wish.
A while back I posted an in-depth description of the C.Fn 4 choices and how they change things. I don't have that link right now, but you can do a search and find it, or, I'll edit this reply with the link when I get some time to do so.
This does help and does make sense. In fact, this was what I was hoping was happening with evaluative...a sort of center-weighted but the "center" is movable. Thanks for this!
Comments and constructive critique always welcome!
Elaine Heasley Photography
I don't think it is a straight 'average' of the zones. Each is metered independently, but then they are weighted so you can get an overall weighted average. I tried to get a link to Canon USA support page on evaluative metering but all I get is page errors.
I believe the highest weight is given to the zone that contains the active focus point - under the assumption the focus point is on the primary object of the photograph. I think rwells is saying effectively the same thing, with his "moving" center-weighted description.
Edit: (Not from Canon and I can't vouch for the authenticity of the site, but this is what I was referring to)
Excerpt from this link
http://photonotes.org/cgi-bin/entry.pl?id=Evaluativemetering
(Canon) Computerized multiple-zone light metering system, used in EOS cameras.
Essentially the image is divided into a number of zones (typically 6, 8, 21, 35) which are metered independently of one another. A computer examines the different light levels across the zones and applies complex (and unpublished) algorithms to determine probable exposure settings. It’s thus highly automated and not under the direct control of the user, but usually accurate under normal lighting conditions. This mode is indicated on cameras by an evaluative metering icon.
-Fleetwood Mac
On a related note, I often use (lock) the center AF point and use Center Weighted when I am shooting scenes where I am tracking a subject and know I want the subject center framed. I get a lot of sharp keepers using this method, especially with a shallow DOF where I want to isolate the subject. At times I'll even drop down to partial depending upon the surrounding 'clutter'.
Maybe I'm deluding myself, but the capture of subject seems faster this way than using evaluative and multi-AF point. Yeah, like I can really discern the difference (1 one-thousandth, 2 one-thousandth, 3 one-thousandth...)
-Fleetwood Mac