Recommended Exposure compensation for the A2/E/EOS-5 used with TCs and telephoto lenses
The reasons why compensation is only necessary for the EOS 5/A2E/A2 are fairly complicated. It boils down to the following:
1. The EOS 5/A2/A2E focusing screens are optimized for brightness. As a result, they pass disproportionately more light to the metering system at moderate apertures than other screens that are not optimized for brightness.
2. All EOS cameras with bright screens incorporate a program that automatically compensates for these anomalous light transmission characteristics. This program was originally designed to work properly at all apertures up to f/5.6. (EOS cameras introduced after the 5/A2E/A2 contain additional programming that negates the need for user-set exposure compensation when EF Extenders are used with any compatible EF lens.)
3. All EOS cameras with multi-point AF systems must incorporate an additional exposure compensation program that accounts for variations in the amount of illumination received by off-center metering segments compared to central metering segments; in older models such as the EOS 5/A2E/A2, this compensation program is also tied in to maximum apertures of f/5.6 or greater.
4. The use of extenders with lenses having nominal maximum apertures of f/4 or smaller results in effective maximum apertures smaller than f/5.6, which exceed the parameters of the 5/A2E/A2's built-in compensation programs, thus requiring user-set exposure compensation.
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