So now we know about aperture, shutter speed and ISO… how do we put them all together?
Welcome to the world of Metering!
Ah, metering… easy concept… difficult to master.
Each picture is like a measuring cup. You need exactly the right amount of light to expose a picture correctly. If you shoot on automatic, the camera does this for you. But it takes away creative control. If you shoot on aperture or shutter priority, you decide only a portion, and the camera decides the rest. And as I will show you later on, the camera is a just a box and is a tad on the dumb side. You don’t want to cede control to it.
So what is metering?
Most cameras these days have in camera metering systems. But before we get into that, let me introduce the all mighty grey card.
18% Grey
When a camera meters, it is looking to find 18% grey. Ansel Adams measured his black and white photographs way back in the day, and found that 18% grey reflective was the perfect middle ground to expose pictures at. This is why it is also called middle grey (half way between black and white on a value scale). It is highly technical and you really don’t need to know the specifics. What you need to know is that no matter what meter system you use, you are exposing for 18% grey. The camera wants all tones to read as close to 18% grey as possible. This is why on bright sunny days a point and shoot camera will take a dark picture. Basically, 18% grey is “technically” correct. It is what you camera does when it set to anything other than manual.
Set your camera to manual and look through the view finder. You should see a little line at the bottom with a vertical line in the middle. That is your meter. It should look something like this:
———-|———-
That line in the middle? That is middle grey. Depending on what metering system you are using, the camera takes in the part of the scene that you are metering and figures out what will make most of the image 18% reflective or middle grey (don’t worry, the colours stay as they are).
First there is center weight metering (check your manual to find these settings). Most point and shoot cameras are center weight. What this means is that the camera takes in to account the center circle of the photo and disregards the outside. The center will be correctly exposed but the outsides won’t be.
My advice? Don’t use center weight.
Matrix or Evaluative is a much better system. It takes the entire image and divides it into five sections. It then takes readings of all five quadrants separately and then averages them to get a correct reading. It is a remarkably accurate system.
Spot metering is used by most professionals. Instead of reading a huge area, spot metering reads just a small square of the image. With spot metering, you read the highlight and the shadow areas and YOU decide what part of the image you want correctly exposed. Want a face to be correctly exposed but don’t care if the sky highlights blow out? That’s fine. Same with dark shadows… you are in control.
So how do you use the meters?
First decide what ISO you want to use. Then decide if either shutter speed or aperture are a priority for this picture.
For example, I decide I want to use ISO100 for this shot. I want to have a shutter speed of 1/125 to eliminate camera shake. I would then change my aperture until the needle is in the middle of the meter (or in other words, set to middle grey). Sounds easy enough but it takes a lot of practice to get good at it. It is a recipe. ISO + Aperture + Shutter Speed = needle in the middle. You want to get it so the picture is balanced… ie. in the middle.
So, what does the grey card do? If you put a grey card into the scene, you automatically know that that is middle grey. So if you are using evaluative metering, fill the frame with the grey card, find your exposure, remove the grey card and take the picture. You will get an accurate exposure. The camera isn’t being tricked by highlights and shadows but instead is taking a picture of the light available. This is a great tool to learn. Don’t have a grey card? Other things that are known to be 18% grey are the pavement, and the blue sky (away from the sun and not a cloudy day). The trick is to put the grey card into the scene with exactly the same lighting. There is no point taking a reading in the shade if the picture you want is in the direct sun.
Here are some examples of the metering patterns. Not the greatest pictures but maybe you will get an idea.
Center Weight
This is actually not a bad job. The frame is blow out but the majority of the picture is correct. Center weight does a decent job. It just has a tough time when you have highlights or shadows that you want that are in the corners of the picture. You can see at he top of the picture the wall is getting blown out because it is outside of the metering section and the camera didn’t take in to account the light there.
Evaluative or Matrix
This has done a better job overall. See how the whites are going a tad grey. That is how the camera meters them (we will get to how to outsmart the camera another day). This metering pattern has toned down the picture so the frame isn’t as blown out.
Spot
I metered the TV in this one. See how it is nice an evenly exposed.
I metered for the picture frame in this one. The whole picture is dark but the picture frame is better exposed. It is still blown out because it is directly under my light. If I wanted an even better exposure of the frame, I would put something between the light and the frame, so you don’t get that glare.
In this one I metered for the planter. It is correctly exposed but the highlights are getting blown out and some of the shadow areas down below are getting dark.
In this shot, the crate was exposed for. See how everything is starting to blow out but the shadows down below are nice and visible.
So you see… metering is just another way for you to control the shot. Play with metering and get to know your camera. Then you will be in control and not your expensive toy. 
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S.
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