Rebel XT: Diagram
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Canon EOS Rebel XT (Canon 350D) Digital camera (D-SLR) diagram for partial metering
and Manual Exposure / Operational cheat sheet

Partial Meter area covers about 9% of the viewfinder area
Partial angle with a 27mm normal lens is about 12.5
° (degrees)


What is a Partial Meter? 
A Partial Meter is a less accurate type of spot meter with an approximate partial angle (acceptance) 
of about 7° to 13°. The angle of some Partial Meters (like Canon A2/E,  Digital 10D, and 20D that 100% of the light measured passes through the spot circle), can be reduced to fall within the spot metering angles using a telephoto zoom lens (the higher the mm of the lens, the smaller/narrower the angle) . In this case they can be used as a spot meter.

Some other Partial Meters (like low-to-medium-end Canon SLRs/D-SLRs such as Rebels, and Elans) lack a distinct frame and are difficult to use their Partial Metering function easily and accurately. Others with a frame (like low-end Nikons such as N50, N55, N60, and N65) that do not measure 100% of the light going through them (it is close to 80%) cannot act as an accurate spot meter even with a long telephoto lens. 

When does a Partial Meter becomes a Spot Meter?
When a Partial Meter (such as Canon Rebel XT)  used with a telephoto lens, it could  be used as a narrow-angled Spot Meter.
For example, when a Canon Rebel XT camera is used with a 70mm lens, then, the Partial angle (acceptance) is decreased from 9° (degrees) to about 4° (degrees) matching Nikon D70's spot angle (D70 lacks a distinct spot frame) with a 31mm lens.
Please note that only 100% partial meters (where 100% of the light passing through the partial/spot circle is measured) with a distinct spot frame (Rebel XT lacks this) can become a true spot meters when used with a longer / telephoto lens.


Actual Dimensions: 15mm x 22.5mm

Partial Metering Example for 
Canon 20D
D-SLR (Digital SLR)
Aspect Ratio: 15/22.5 = 2/3

Diagram illustrating the spot frame for Canon EOS 20D (Digital SLR) viewfinder. The angle of measure (acceptance) for this camera with a 27mm (approx) normal lens is about 12.5° (degrees).

The percentage area occupied by this 30 sq. mm Partial Metering frame is about 9%.

 

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