I have found myself using spot metering on several occasions in order to get the right exposure, such as sunsets and on bright days with a subject in the shade. What other situations are well suited for spot metering instead of the standard matrix mode?
With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area).
This means you could get a light reading for a very specific area in the frame as opposed to a general measurement for the overall picture. Using this built in meter you can tell specifically how your subject will be exposed with your current camera settings and whether you need to adjust them to get the exposure you're looking for. Using the spot meter, you're telling the meter that the subject you're pointing at is at an 18% gray level in the Zone System. If the subject is indeed in the mid tonal range, you would leave your exposure as is but if it's brighter or darker, you would have to adjust your exposure accordingly. More on 18% gray and the Zone System here.
Situations this is useful for:
- Photographing the moon
- Photographing someone indoors in front of a window
- Sunset behind the subject
- Subjects who are lit like the rest of the scene, but appear darker/lighter due to their colour. ie Black horse in a bright field
- Anytime there is a high Subject Brightness Range
How to use Spot Metering
Simply point the Spot, the dead centre of the frame on most cameras, to the area you want to meter and adjust your shutter speed, aperture, ISO etc. as needed to get the scale in your viewfinder to 0. (Or the exposure you're looking for.)
Shooting in P
Point at the area you want properly exposed, lock the exposure and from there compose your shot as necessary.
Using Spot Metering is generally more time consuming and takes a little more practice to use effectively but if you're trying to get a certain look for your photograph it usually yields better results.
See How Spot Metering Affects Your Settings
A good experiment to really see the affects of Spot Metering is to put you camera in semi automatic shooting, P, select spot metering and frame a picture of a light bulb lighting up a dark room. Point directly at the bulb and notice the settings your camera changes to try and get the correct exposure. As your move around, you'll notice even slight movements can drastically effect the settings because the metering is so specific. Try the same experiment in average mode and you'll notice you can move your camera to almost any point in the frame and get only 1/2 stop changes in shutter speed and/or aperture.
Any time there is something in the frame that you want to be white or black (and where the subject is still, or at least slow) works well. For white, you can spot meter on the white surface and then overexpose the given reading around 1.5 - 2 stops. For black you do the opposite, measure and then underexpose as much (test with your own camera to figure out exactly how much to adjust the exposure). I often find that this works best with the camera in manual mode.
The basic principle of spot metering (when compared to matrix metering) is that it loses the comfort of having camera guess how should different parts of scene contribute towards exposure settings and gives that control to you.
Therefore, situations favoring spot metering are when you want precise control over what part of the scene exposure is measured by. The primary reason why you might want to have such control is that you have a clear vision on how should some part of your scene be exposed (especially when shooting JPEG or film, where your options to tweak exposure afterwards are more limited than with RAW). Usually that "area" would be the subject, but it could also be background, or a key object of your composition.
In other words, spot metering is your best friend when you're trying to use zone system. You aim the metering point at the desired area and use exposure compensation to select whether its brightness on image should be at the neutral 18% grey level (no compensation), darker than that (negative compensation) or lighter than that (positive compensation). Note that unlike focusing, metering does not suffer any error from recomposing afterwards - so you can use spot metering for any part of scene even if it's only available for center point.
Another kind of situation favoring spot metering is when experimenting is not an option and you don't trust the matrix metering - because you're new to the camera, or you haven't figured out how it would perform in a complex lighting situation you're facing.
The important difference is that matrix metering tries to guess how should things look like; you know how they should and spot metering lets you communicate that knowledge to camera, still sparing you from the dirty work (measuring the light).
To summarize the other answers (thus far): the primary reason for spot metering is situations with extreme contrast. The high contrast means that getting a "reasonable" overall exposure isn't likely to give (even close to) the correct exposure for the parts of the picture you really care about. That being the case, you need to meter those specific parts you care about, and expose specifically for them.
I'd disagree with one answer though: a spot meter is rarely useful for the moon. To shoot pictures of the moon, you're generally better off exposing manually. Even a spot meter will usually overexpose the moon quite badly unless you use an extremely long lens. Instead, you usually want to just open up about one stop from a normal daylight exposure (aka the "loony 11 rule").
I find it very useful to us spot metering when taking photos of birds.
In a zoo, this might be different, but when you want to capture them in their natural habitat, they often fly quickly and the background and surrounding areas change a lot from dark (such as a branch or tree in the shadow) to bright (such a branch or tree in direct sunlight).
I tend to get a lot more keepers when I use spot metering in these situations.
I've used spot metering in situations where there's a high range of light in the frame, and evaluative metering might get confused. I meter off of something either relatively bright (so I don't overblow things too much) or whatever I want to focus on. It might take some trial and error to find the right spot to get the effect you want.
If you're using spot metering and then recomposing the shot, you'll notice that the metering changes as you recompose even if you've got the shutter halfway depressed. To solve this, use the exposure lock button; on my Canon DSLR, it's on the top right of the back of the camera and has a symbol like *. Point where you want, press the shutter halfway down, press the exposure lock button, and it locks the exposure data it currently has, so you can recompose and shoot at will.
I ask myself two simple questions:
- What is my subject?
- Is my subject bouncing off more light than the background?
If the answer to the second question is "no" and I am outside without external lights, I'll use the spot metering.
Also, for forcing a silhouette, I'll use the AE lock after spot metering on the brightest object. That way I am guaranteed a silhouette.
I will always tend to use spot metering at an Airshow or when shooting birds/wildlife, as usually you will have a bright sky filling 85% of the frame with a dark blob in the middle!!
The same would be true for the opposite - say you were shooting a music gig at night, or indoors. Overall the scene will be very dark, but you need to expose for the face of the lead singer in a spotlight, or something like that...
Just a comment about those who have mentioned using spot metering for birds/wildlife. I do a lot of wildlife photography - primarily birds. Using spot metering for wildlife as some have suggested is far too simple an answer and largely incorrect IMHO. If your bird (or animal) is primarily white or black, and you spot meter on it, the camera will try to make the subject 18% gray. White birds should not be 18% gray. Black birds should not be 18% gray. Wildlife photography requires a bit more finesse/subtlety in choosing your exposure. I'm afraid that I would have to disagree with those who use it for this. While there are potentially some isolated circumstances where you could use spot metering, other techniques for getting your exposure correct for wildlife will be far more useful overall.