Canon FTb Stopped Down Metering

The Canon FTb has a CdS sensor within the camera body, which is in the path of light entering from the lens to measure the light level entering the chamber. When the aperture is fully open, the maximum amount of light enters the camera body. However, when the aperture is set to any other position, then less light enters the camera body. Less light results in the viewfinder image being dark and you will have difficulty framing the shot properly. In addition, the aperture diaphragm would hinder the "field of view" within the viewfinder. To solve this problem manufacturers designed the aperture to remain fully open all the time, and close for a fraction of a second just before the shutter releases. This method solves the immediate problem, but leads to another problem, because as the photographer, you need to have a light level reading of the light entering the camera body whilst the aperture is set to your desired value.

On the Canon FTb you can set any aperture value on the barrel, however the aperture itself will always remains fully open and only actuates a fraction of a second before the shutter operates. Since the aperture is always fully open, how does the light meter give you the true light reading for the particular aperture you have set? To solve this problem the manufacturers use electronic scaling and calibration methods. Since each aperture setting allows a constant percentage of light through, it becomes a simple matter of calibrating the light meter, so it would display a percentage of the full light level for any particular aperture setting, without the aperture actually being in that position. The meter needle is therefore always displaying the "calculated" value of light entering for any particular aperture setting. Hence, the term "full aperture metering" because the meter displays the "calculated" value of the light level at any aperture settings, whilst the aperture itself remains fully open.

With stopped-down metering, you are disabling the "full aperture metering" mode, and going back to the old-fashioned method of camera operation. In this mode, the aperture remains set in the desired position all the time, before, during, and after the shutter release. With the calibrating circuit disabled, the aperture remains set in the required position and the light sensor meter displays the actual value of the light level entering through the aperture. Hence, stopped-down metering is taking a light level reading from the meter whilst the aperture is in position also known as stopped-down.

This Article Continues...