Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
FrancisCF Contributing Member • Posts: 985
Re: semi-closed

From what I saw during developing the AF test methodology, I noticed Canon AF has at least two stages, the lens moves fast to a point near the exact focus position, then it stops in a very short instance and then moves again and stop at the final focus position. This behaviour let me assume that the second stage is an open loop, this AF method has the advantage of faster AF speed and less hunting, the disadvantage is it is difficult to calculate exactly how much to move the lens in order to obtain accurate focus since there are too many variable and those variables are not constant or with a linear characteristics, so lens calibration with + - values is added in the data base for compensation, but it still not perfect since the compensation needed for short distance and long distance may not be the same.

I don't think Nikon AF use completely close loop, but it algorithm is not like Canon, I observe Nikon AF is somewhat one stage process and Nikon AF performance is more consistant and more accurate for still subjects, but Nikon AF will more prone to hunting in difficult AF situation than Canon.

I think Canon AF method have advantage in tracking fast moving subjects since the situation need fast response more than 100% accurate, Nikon AF method may slow down the response and need better lens like AFS and more powerful microprocessor, like that employed in the F5 or F100 series.

DavidP wrote:
From what I've seen, I tend to believe that it's really an open
loop, but that a FEW "updates" are made along the way to guide the
AF. Maybe one or two. Three max.

My understanding is that Nikon has a closed loop system.

Why Canon can't give an option to use either an open-loop system
(for raw speed) and a closed-loop system (for greater accuracy) is
beyond me. Sounds like a great idea for a custom function.

Frank Nichols wrote:

It is not known for certain how Canon AF works, they do not publish
the information, but some have researched their patents and come to
the conculsion that teh Canon design is NOT closed loop. It appears
teh reason is that the closed loop system would be slower, and
Canon opt'ed for a faster open loop system.

I think I read that Nikon was closed loop, but I am not sure.

-- hide signature --

Nikon F100, Nikon FM3A, Canon EOS 1V
Olymbus 3030, Nikon 995, Canon G2
Radio call sign VR2XEE
Handheld ham transceiver - Yaesu VX-5

Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow