| Re: Chuck - question on 1.4XII converter and D60
Hi, Andy:
Are you sure you really want to know? [img]images/icons/smile.gif[/img]
SLR autofocus performance is affected by many factors including overall light level. That's why you'll usually see a specification for AF Working Range, such as "EV 0.5 ~ 18 @ ISO 100" in the case of the EOS D60. This specification applies no matter what the maximum aperture of the lens may be, and therefore is not usually the decisive factor in whether or not AF works with a lens that has a relatively small maximum aperture.
Assuming a light level that meets or exceeds the threshold, any EF lens should be able to autofocus with the EOS D60 as long as the effective maximum aperture is f/5.6 or larger.
The next thing to look at is the design of the AF sensors themselves. In the case of the D60, there are 3 focusing points. They are represented as boxes on the focusing screen, but in reality the two outer points are vertical sensor arrays whereas the center point is a cross-type sensor array.
As light enters the lens, about 35% of it passes through the main reflex mirror and is reflected downward by a sub mirror into the autofocus sensor mechanism located in the bottom of the camera's mirror box. As the reflected light passes through the optics of the AF sensor mechanism, it is reflected once again to change its direction. It eventually reaches a special optical assembly called a secondary image registration lens, which is actually a collection of multiple miniature lenses. The plane occupied by the SIR lens is geometrically equivalent to the camera's focal plane. Each of the miniature lenses built into the SIR directs a portion of the light at a row of pixels on the AF sensor chip. There are two rows of pixels for each of the outer focusing points on the D60, and 4 rows of pixels for the central cross-type sensor. Each row of pixels has a discrete physical length. In order for the sensor to function properly, all of the pixels in a given row must be illuminated by the incoming light. Therefore, the diameter of the incoming light beam must be wide enough to cover the entire row of pixels. The maximum aperture of the lens has a direct effect on the diameter of the light beam, and when the effective maximum aperture becomes smaller than f/5.6 on a D60 or D30, then the light beam no longer covers the entire row of pixels. That is why the camera automatically shuts off the AF system if the maximum aperture reported by the lens is smaller than f/5.6.
If you "fool" the camera into thinking it has an f/5.6 or faster lens on it when in fact it does not, the AF system tries to work, and in certain contrasty conditions it may succeed. But if the subject contrast is too low, then it is likely to fail. This is what often happens when you tape over the 3 extra contacts on a Canon EF Extender, or when you use a 3rd-party extender that doesn't have the extra contacts to begin with.
The moral of the story: Assuming that the overall light level exceeds the low-light threshold (which in the case of the EOS D60 is actually so low that you would be hard pressed to read a newspaper at normal viewing distances), it is the effective maximum aperture of the lens that counts.
Hope this helps!
Best Regards,
Chuck Westfall
Assistant Director/Technical Information Dept.
Camera Division/Canon U.S.A., Inc.
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