Af Focus Shift - Canon EOS 55 Service Manual

Part 3: Repair Information
4.13 AF Focus
SHIFT
PuRPosE:
To correct minute AF defocus that many result from use of a lens with very shallow
depth of focus, such as EF 50
f I
1.0 L, EF 85
f I
1.2 Land EF 135
f
/2.8SF.
AF focus shift must never be used to correct defocus caused by any lenses other
than EF 50
f /
1.0 L,EF 85
f /
1.2 Lor EF 135
f
/2.8SF.
MINuTE DEFOCUS:
EOS Series cameras and lenses are designed so that their defocus does not exceed
standard values of 0+0.03mm and 0+0.02mm respectively. EF 50
f
I
1.0 L, EF 85
f
/1.2 Land EF 135
f
/2.8SF also conform to these standards. However, these lens-
es have very shallow depth of focus. Therefore, when used together with a camera
whose defocus has the same
sign, the lenses may exceed the acceptable depth of focus even if the defocus of both
the lenses and the camera conform to the standards. In this event, the user of the
lenses may complain of unsharp focus.
Example) Unsatisfactory combination of camera and lens (Camera: 0+0.03mm
Lens: 0+0.02mm)
AF focus shift is intended to correct total AF defocus resulting from use of a par-
ticular camera in combination with a lens with a shallow depth of focus.
Therefore, AF focus shift must always be made on that camera and lens.
AF focus shift must also be made after adjusting both the camera and lens.
A lens with a shallow depth of focus may prove in focus without AF focus shift
despite is user's_complaint. This phenomenon can be attributed to the following:
(1)
Variations in range measurement that result from shooting of an object diffi-
cult to focus.
(2) Shift of an object in the period between turning on of SW2 and exposure.
EOS + EF50mmf/1.8 Lens
Depth of
-0.063mm
F cus
0.000
Defocus
+0.063mm
Out of acceptable
depth of focus
EOS + EF50mm
f
/1.0 Lens
__ Depth of
Focus
-0.035mm 0.000 +0.035mm
Defocus
Fig. 3-55 Minute Defocus
3-59
Out of acceptable
depth of focus