Q: Does the Canon 300D have a built-in light-meter?

A: All modern cameras have built-in light meters. They are very sophisticated making it very easy to get the exposure correct in modern photography. Below is a diagram to indicate to you where the TTL (Through The Lens) light metering sensor is located.

How does the metering system work?

The screen is divided into different metering zones. The example above is divided into six zones. These zones are then divided into Primary Zone (A), Secondary Zone (B) and Peripheral Zone (C) see fig. above. The built in computer then compares the difference in brightness between the different zones and uses a set of pre-programmed instructions (an algorithm) to come to a conclusion.

The calculation works as follows: the computer subtracts the brightness value between A and B and then between B and C i.e.: B – A = 0; C – B = 0 therefore the brightness value across the entire scene is the same so the camera does not have to apply any exposure compensation. However if the values change and the equation looks as follows: B – A = 0; C – B > 0 then that means that the brightness of the primary and secondary zones are the same but the peripheral zone is brighter than the primary zone. The camera will then base its exposure on the primary and secondary zones. If the equation reads B – A > 0; C – B < 0 then that means that the secondary zone is brighter than both the primary and the peripheral zones. The camera reading is now fooled and the chances that the central object will be under exposed are very good.

There are obviously lots of different permutations of the above equation. Therefore the photographer MUST use the readings as a guide and know when to compensate and when to trust the readings. The example above was only a six metering zone with one centre focusing point. With the development of multi focusing points camera manufactures had to adapt the meter reading zones to be linked to the focusing points as the diagram below shows.

These zones have now increased to 21 Zones in the 1D series and 35 Zones in the 300D, 350D, 400D, 20D, 30D and 5D. See figures below for both.

35 Zone

21 Zone

The meter can also be used in several different metering modes. These modes are: Evaluative, Centre Weighted, Partial and Spot. Not all the camera models have all the modes. See table below:

Camera Model ------ Metering Mode	300D	350D	400D	20D	30D	5D	1D Series	1Ds Series
Evaluative
Centre Weighted
Partial
Spot

These modes work differently in the different camera models for example in the 1D series the spot metering is linked to the focussing points whilst in the 30D it can only be used with the centre point. Check your manual to see how your model uses the different modes. Below is a general description of the modes.

Evaluative Metering
The system breaks the scene into a number of parts and reads light information from each. Depending on the model of your camera, the scene can be split into anything from four to thirty five parts as we have seen earlier. In all the models mentioned above this is their default setting and all of them link the meter reading to the focusing point.

Centre Weighted Metering
This is the most common of the metering systems. It works by measuring light from all over the viewfinder, but pays extra attention to what is going on in the centre of the picture area. The image is split into two parts – the centre circle and the surrounding area. The centre circle influences the exposure slightly more than the surrounding area and the split is usually something like 60:40.

Partial Metering
This system allows you to isolate a single part of the scene to take a light reading from only that part. This metering mode is only linked to the centre spot and the size is between 8 – 9% of the metering sensor.

Spot Metering
This system allows you to isolate a single part of the scene to take a light reading from only that part. Depending on your camera model this metering mode can be linked to a focusing point and the size is between 2.4 – 3.5% of the metering sensor.