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Canon USM autofocus drives - new technical article

Doug Kerr

Well-known member
Many lenses in Canon’s “EF” series have autofocus drive systems described as “Ultrasonic Motor” (USM) systems. There are actually a number of different systems in this classification.

I have recently released a new technical article, "The Canon USM (Ultrasonic Motor) Autofocus Drive System" which describes the various forms of this system in considerable detail. Both ring USM and micro USM forms are discussed with respect to both their principles of operation and their deployment in lenses.

Special attention is given to the arrangements for providing the "full-time manual focus" (FTM) feature.

Included is an extensive discussion of the unique AF drive mechanism of the Canon EF 50 mm f/1.4 USM lens, the only EF lens using a micro USM drive system that affords the FTM feature (a matter about which there has been considerable mystery over the years).

The article is directly available here:

http://dougkerr.net/Pumpkin/articles/Canon_USM.pdf

Best regards,

Doug
 

Cem_Usakligil

Well-known member
Hi Doug,

I have already read some of the individual parts which you have published here in various threads before. To be honest, I haven't yet read the whole pdf article but just skimmed it. Even then, I can see that you have yet again outdone yourself; this is an excellent article and I am sure it will become a standard reference. You are a fountain of knowledge and your work shines brightly. We all should be thankful for that, I sure am.

Cheers,
 

Doug Kerr

Well-known member
Hi, Cem,

I can see that you have yet again outdone yourself; this is an excellent article and I am sure it will become a standard reference. You are a fountain of knowledge and your work shines brightly. We all should be thankful for that, I sure am.
Thank you so much.

This was a big piece of research! But very fascinating.

Best regards,

Doug
 

StuartRae

New member
Hi Doug,

Thanks for the comprehensive article.

Just one question. On page two is the following statement:

In the micro USM type system, the motor itself is a small cylindrical
unit, 11 mm in diameter and either 26.7 mm (original micro USM) or
13.4 mm (the newer micro USM II) in diameter.


Although it confused me, it in no way detracted from my enjoyment.

Regards,

Stuart
 

Doug Kerr

Well-known member
Hi Stuart,

Hi Doug,

Thanks for the comprehensive article.

Just one question. On page two is the following statement:

In the micro USM type system, the motor itself is a small cylindrical
unit, 11 mm in diameter and either 26.7 mm (original micro USM) or
13.4 mm (the newer micro USM II) in diameter.

Oh, of course that should be:

In the micro USM type system, the motor itself is a small cylindrical
unit, 11 mm in diameter and either 26.7 mm (original micro USM) or
13.4 mm (the newer micro USM II) in length.
[/quote]​

Thanks for the catch.

In addition, it would probably be better for that statement to come a bit later, after the concept of the micro USM version had been introduced. I had done a lot of structural rearranging of the document, and that passage ended not in the ideal location.

Thanks again for the catch. I'll make a correction.

Best regards,

Doug
 
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