Metering modes.

Let me start this off by stating that I have been using Evaluative for most of the time I have been using Canon. I sometimes get results that are significantly off from what I was expecting. I am usually at +1/3 or +2/3 EC.

Yesterday I was experimenting with CWA, Spot, and Partial. I used a fixed ISO and Av mode to lock one parameter, forcing Tv to float. For most shots, all modes give an exposure within 1 stop of the others, HDR situations excepted. I noticed frequently that 2 of the 3 (Spot, Partial, CWA) would give the same exposure.

Here is my question: Even when two different metering modes give the same exposure, why do the histograms differ? They weren't hugely different, but certainly noticeable.

I can post some LR screen shots, if necessary.

Your shot is slightly different or clouds changed things slightly or whatever.

I use evaluative mostly but I've learned how the Canon sensor sees the scene so I get within 1/3 stop most of the time from where I want it to be.

If you are shooting the EXACT SAME scene on a tripod under the EXACT SAME lighting conditions using the EXACT SAME exposure and EXACT SAME camera and lens then the histograms should be EXACTLY THE SAME irrespective of metering mode.

The slightest deviation in the image can make changes to your histogram that you can't even see yourself. Drastic differences should be noticeable but they could be something as simple as a change in the light or a person with a red shirt turning more toward the camera giving a larger percentage of red in the image and decreasing any other colors that were in those pixels before.

Unless you're using a spot meter, the meter is going for some kind of average exposure. So anything with a large dark or bright area or lots of smaller shadows/highlights can push that average up or down and cause over/under exposure.

So we have several metering modes designed to help with this. Evaluative is really intelligent and evaluates the scene for a lot of different scenarios and makes the best guess. But it's still a guess. Center weighted reduces the emphasis of things outside the center when you meter but doesn't totally disregard it. I find this good for things like sports where you may have light bleachers behind a player and other such scenarios. Spot metering is the most accurate because you can meter specifically what you want but it also takes some skill to use properly. In this, you would spot meter a part of the scene that you want to be a mid-tone (as close to middle gray as you can get). If you meter on something bright it will give you a recipe for underexposure and vice versa. If you have +2/3 EC on but you meter on something that is 2 stops brighter than the intended mid-tone, then your tones will all fall about 1 1/3 stops underexposed. You can really practice this if you have a gray card.

Thanks for the responses so far. I did not shoot on a tripod. I handheld these, so there are bound to be some minor variations in composition. I did shoot with constant interior lighting and ISO 3200. I will reshoot with better controls.

I generally prefer Evaluative because it uses whatever AF point I select as the reference. The others use the center of the frame regardless of AF point. I only use spot for things like spot-lighted stage performers and such. I just thought I'd give CWA a go because that is how most (older) film systems used to work.

Will retest and repost, shortly. A day or two, please, not a minute or two.

Ok, that was a false alarm. I shot a static scene in constant light from a tripod, and got two of the metering modes (CWA and SPOT) to give me the same Tv for my chosen Av and ISO. The histograms were, as far as I could tell, identical.

My first test had slight variations in framing from being hand-held. Those tests were not valid. I bet I wasted two hours trying to figure out if the different histograms were a result of different metering modes. DOH!

Note to self: Remove head from butt, before asking goofy questions.