That's right. The word "mode" again. And the most important way to adjust our camera to get the right exposure, after exposure compensation.
First, A bunch of Icons
Right, this very advanced drawing that took me a lot of time1 to make is all of the icons for most of the different metering modes on most cameras.
First, make sure you've read the section on the histogram. Remember when I said that a 'correct' exposure was where the pixel values had nothing over or under exposed, and thus largely were clumped in the middle of the histogram?
The metering mode changes what section of the image the camera is measuring. We spoke about histograms at looking at the entire grid of pixels of an image, which it does. Yet, we don't often really care about the entire grid of pixels of an image.
Changing the metering mode changes how the camera measures the world.
First, lets consider a standard metering mode available on almost all cameras (even film cameras!). Center-Weighted metering.
Center Weighted Metering
In center-weighted metering, the camera doesn't care if the pixels at the edges of the frame are over or under exposed.
Let's say we have this image of a house and a cat on a sunny day.
Center weighted metering would look at all of the pixels that are red (in the following image) and try to get just those pixels to be middle-grey - in the middle of the histogram.
Notice how some of the red pixels are transparent. It's looking at images between the center and the edges, and trying to get them to be middle-grey, but not as much as the ones in the center.
Consider our first image, If we adjusted it so the entire image was as middle-grey as possible, it would look like the left half of this photo:
The sky is so bright that the camera would have to get a darker exposure, and in the process, it would darken the foreground (the cat, the ground, etc) much more than we want.
In real life, such an effect looks like this:
Sure, our sunset has color... but Tom, the photographer, is totally under exposed!
Center-Weighted metering could help prevent this. Enough of the ocean and Tom's face would be considered, and the bright sky would be more ignored.
What other types of metering are there? The next most important one is spot metering. Spot metering doesn't take much of an average, but looks at a really small section of the image (usually in the center or on the currently selected autofocus point) and measures just that.
The red dot is at the top left of the house
The size of this spot depends on the camera. Nicer cameras tend to have smaller (more accurate) spots.
The secret to spot metering is not hoping that that spot is the right (middle-grey) tone to base the exposure of the image off of, but pointing the spot at exactly the point we want to to be correctly exposed, measuring it, then recomposing and taking the photo, exposed correctly.
Camera's all handle that measuring process differently. With Nikon, there is an AE-L (autoexposure-lock) button to press, canon that same button looks like a star, some cameras lock the exposure when they lock focus (camera button held halfway), and this behavior can - of course - change depending on the settings of your camera. The quick answer is to find your specific camera's manual or guide and read it (try a google search first), this behavior is just a bit too different for each camera for me to cover here in practice.
One more note about spot-metering. One can combine it with exposure compensation for really powerful effects. For example, I know that I want a grey sidewalk in the shade to be a few stops darker than middle grey, and I will have a good exposure. I meter the sidewalk, and use exposure compensation to go minus one or two stops.
Shooting like this, metering spots then exposure compensating those spots to be where you want them is really powerful this way, and many photographers shoot with this method frequently.
Sometimes, a spot is too small to get an accurate or consistent exposure. Maybe measuring the grey t-shirt of somebody left you measuring the white text on the shirt, by chance.
Canon introduced partial metering to handle these cases. It's basically spot metering with a slightly larger spot, and more 'falloff' (the red dot would be a gradient).
This is just on Canon. I like partial metering a lot because I can shoot pretty lazily and don't want to spend too much time thinking of exactly where the spot should go, yet I still like to measure and recompose as a shooting method.
Matrix / Evalutive / Multi / ESP / honeycomb / Auto
The last metering mode that most cameras have goes by a million different names. I am goingf the to call it "smart" metering. Smart metering is when the camera doesn't just try to reach grey at one average of the image, but looks at different sections of the image, takes a guess as to what is more important, and then tries to get that to be exposed correctly.
This way, one super bright point doesn't throw off an average. If the camera see's a bunch of brightness towards the top and darkness towards the bottom, that's probably the sky and the ground; it will look for other parts of the image to meter off of.
It works by looking at the exposure of differnet areas on the image that it is taking a photo of, comparing it to a database of exposures for similarly measured images, and choosing the appropriate exposure. In other words: total magic. I mean, awesome engineering.
Nowadays cameras consider a whole bunch of other information, including the focus distance (what is in focus matters more), the depth of field of the current settings, and way, way more.
It all started when Nikon introduced their matrix metering mode on the Nikon FA. It was so much better than what came before it that most of the time a photographer finally didn't relaly need to worry about the exposure.
If you want a good read about the development of this technology, take it from Nikon themselves. (Seriously, it's a good read).
Wow! It's super awesome technology that has had an amazing amount of time and energy spent on it.
Yet, it still sucks sometimes.
First, it tends to value the auto-focus point too much. This makes it words when using manual focus and/or worse when you focus on something you don't want exposed well, like a sillhouette.
Second, it biases towards it's own ideas of a 'correct' exposure that makes good images "striaght out of camera". Often, we want to over or under expose different parts of our images in order to ensure that these parts have more 'data'. More contrast coming out of the camera that we can play with while editing.
Finally, it can be inconsistent between two similar shots. In fact, barely moving the camera may accidentally produce two differently exposed images, which makes editing the images a pain. This also makes cameras less pleasant to use, as there is nothing a photographer hates more in a camera than unpredictability.
"Photographers these days are too spoiled and reliant on their cameras to do the work for them, all their photos look the same" - My grandfather, in an alternate reality where he gets angry about these things. He never said this. He did shoot with a hasselblad medium format camera and almost always used manual controls. He once duct-taped a film camera to the front of his skii's and jumped out of a helicopter and skiid down a mountain back in the 60's. Take that, GoPro!
When shooting in manual, most camera's still go through the effort of metering the scene. The measured output is still visible on the exposure compensation meter (the actual numer line on the camera screen), and one can use it as a starting point or reference when dialing in an exposure.
2: In all seriousness this did take a lot of time to research. I had to dig through various manuals online. Not much time left for design work after that.