Re: canon-nikon AF technology- the difference

Kennedy McEwen wrote:
In article <1192391482.884706@ftpsrv1>, frederick <lost@xxxxxxx> writes
Kennedy McEwen wrote:
Secondly, the AF is specified AT THE AF APERTURE. If you read the document that I referenced right back at the start of this thread you will see that the AF only uses the outer edges of that aperture. Significant spherical aberration (SA) would throw the AF off at other apertures. Obviously lens designers attempt to keep SA to a minimum but there are limitations to what can be achieved and all lenses have residual SA. With fast glass, SA is usually the dominant aberration at wide apertures. The AF routine applies a correction for the shift in focus between its aperture and the shooting aperture for the lenses it recognises and a "default" correction for those it doesn't (newer lenses or 3rd party glass). The correction ensures that, when set up in accordance with Canon's calibration procedure) the AF remains accurate across all the working apertures of the lens.

For a lens with SA, the degree of AF error induced varies with focus distance (and or focal length). So, user "custom" calibration (and factory service calibration) can only be as good as the procedure to calibrate.

Precisely! Except that what is available as user micro-adjustment AF is nowhere near as comprehensive as a service AF calibration.

I'll wager that if the AF "user fine tuning" feature filters down to consumer level cameras, then users with fast - but doubtful quality - glass are going to cause themselves much angst and frustration.

Starting to figure it out now? Its taken long enough!

Aaaaarrrggghhh.

Can factory / service AF calibration always adjust accurately to remove ff/bf errors in a wide aperture lens at close and infinity, short and long focal length?

I expect that the answer is no.

So, if you buy a camera that is within factory specification, then can you expect that at wide aperture lens in your kit will focus to within those parameters at all times. I expect that the answer to that is also no.

So, you buy a camera factory calibrated under specific test conditions, using a specific lens and (unless QC error) then you can only be certain of getting that same degree of accuracy if you use that same lens.

So it's back to my comment that the high precision AF calibration spec is mainly a marketing tool. I'm probably more cynical than you. Sure, the camera tech can say that the camera is within factory spec, but that doesn't solve a problem for a user of another lens with which the AF is not working well enough, Sigh - I guess at least they could use it as a starting point to know that if they own a new model with user ff/bf adjustment, then they start adjustment from a know mid-point.
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