Canon EOS 55 - Page 50

Part 2: Technical Information
1) Focusing System Components
The basic autofocusing optical system, focusing unit construction and multi-BASIS are
the same as the EOS 500/Rebel X's. The BASIS is designed to work with f /5.6 and
faster lenses.
As shown in Fig. 2-5, part of the light passing through the camera lens and beam split-
ting mirror (40% transmittance) is reflected by the piggy-back sub mirror (100%
reflectance) downward at 83°. The light subsequently passes through the field lens,
reflects off the AF mirror, and goes through the infrared fl.lter, fJXed aperture, and sec-
ondacy image-forming lens before reaching the multi-BASIS.
Multi-BASIS
Fixed aperture
Main mirror
Sub-mirror
Primary focusing
plane (equivalent
to film plane)
Field Lens
AF mirror
Secondary image-forming lens
Fig. 2-5 AF optical system
Fig. 2-6 shows the focusing points in the viewfinder and the respective type.
2-6