We know how our camera tries to meter for a middle grey, and we know that we can tell the camera which area we want it to give most weight to when taking an image by using different metering modes. The final part is how to then adjust your exposure based on the area of the scene you are metering for, if this is needed (it may well not be)
Again, I'm a visual learner (as I suspect most photographers are!) so I've going to try to explain again with some pictures. (I'm using my trusty flowers again as they are good for demonstration purposes mainly because they don't move off in mid photo!) First, I'm going to show how different modes can affect the final outcome, and then talk about how we can then change it.
Here is my first picture taken on evaluative mode with the exposure bar set to meter at 0. As you can see the picture is well-exposed. Why? This is because of the mix of light and dark tones in the the image it has evened out to a nice middle gray, so evaluative gets it spot on. In this instance there is no need to use change the exposure at all.
Now look at the difference in the picture when I change to spot metering, and meter off the flower - it's now under-exposed. That's because the small area that the camera was metering for was white, so it tried to darken it down to a middle grey.
Most pictures won't be so clear cut, but it's a good demonstration on how the different modes can affect the final results. Now, let's move on to how I could have changed the result given by the spot metering so that it had a proper exposure. If I know (or I look at my histogram or image playback and see) that my image is going to be under or over-exposed due to the tones of the image, then I can compensate for this by adjusting my exposure. So, it this particular case, the flower was white so I know that I would need to over-expose (bearing in mind the camera is going to darken the white to the middle gray) by at least one stop. Here'e the image after in spot metering but with exposure compensation.
Most DSLR cameras will offer exposure compensation, which is shown in a scale like the one below. (You will see this scale in your viewfinder and your LCD) The numbers indicate the exposure in stops, increased exposure is to the + side, and reduced exposure to the - side. The little bar shows where you are currently exposing to.
Photo Credit Digital Camera Review
So, with a light toned scene (or spot metering off a light subject) tell the camera to expose a little more to compensate, and if it is a dark toned scene (or again spot metering from something that is dark) then adjust to expose less to compensate. Here's how you do that.
In Priority Mode
In priority mode you use Exposure Compensation. Here's how I do it with my Canon, but once again, you will need to check your camera manual for your particular make or model.
> Half press the shutter button to activate the camera and take a meter reading
> Press the Quick Control Button (Q)
> Select the meter reading area
> Turn the dial to set compensation - to the right or to left, depending on whether you want to tell your camera to adjust to over or under expose.
> Take your pictures! Your camera will automatically adjust the exposure so that it is metering to your compensation. It won't change until you go in and change the setting again.
In Manual Mode
In Manual Mode it's very simple: Instead of metering to 0, you set the bar to over or under to compensate for the tones in the scene - in other words to +1 or -1 (or whichever setting suits)
The metering modes still work in exactly the same way, it's just that you are compensating for the mistakes that the camera will make in certain situations by manually adjusting the exposure.
Unfortunately there is no guide to tell you exactly how much exposure compensation you should dial in, it will vary from scene to scene, depending on the light and the tones that are in the frame, and even on your particular camera model! Once you get to know your camera a little better you will start to be able to do this automatically.
I intend to do one last post on metering which hopefully ties everything together, with a couple of tips on how to use this information in the real world, and answers any questions anyone may have - so if you have any, fire away and I'll try to cover them!
Other Posts You Might Like:
Metering Part 1: How your Light Meter Works
Metering Part 2: Metering Modes
Metering Part 3: Exposure Compensation
Metering Part 4: Putting It All Together
How To Use a Gray Card