Through the lens light metering

We've spoken about aperture and shutter speed but we haven't spoke about how we gather the information needed on which to base the combination of both on. To help and aid with this, its a good idea to find out the definition of what 'Through the lens light metering' means.

Definition -

Through the lens light metering, often just said as 'TTL', is a feature cameras have whereby light levels are measured through the lens which captures the picture. This is done opposed to using a separate metering window.

So now we know what is meant by TTL, how do we complete this operation? 

How we do this is quite straight forward and easy to grasp. With the light meter switched on, you should compose and get ready to take a photograph as normal. It'll then start to take a reading from the scene. Information about the correct aperture and shutter speed will that be sent back to you as the lens automatically adjusts to this, ready for you to take your shot.

How are these readings calculated, what is the process in how the meter finds the correct settings?

The readings are based on the amount of light reflected back from the scene you've set and on the sensitivity of the film you're using. In order to do this correctly, you must inform the meter of the right film speed manually or using a DX coded film. Only do the second option if the camera you are using supports it though.

Depending on your selected mode on your camera, you will be given information about the shutter speed and/or aperture.

Extra information - 

When your camera is in manual mode, you can have control of both shutter speed and aperture. You can adjust either one or both to reach the correct exposure but you can also prioritise over aperture or shutter speed as well.

When aperture is prioritised, the meter will show its chosen shutter speed based on the aperture you have set personally. This can be shown on a scale or in the viewfinder and changing the aperture will also make the shutter speed adjust accordingly.

The same thing happens when you prioritise the shutter speed over aperture, the aperture will adjust to the shutter speed you input.