View Full Version : EF 50mm f/1.4 USM lens - focus drive mechanism

Doug Kerr
January 3rd, 2011, 11:59 AM
The Canon EF 50mm f/1.4 USM lens is unique in that it is the only EF lens using the micro USM drive that offers the full time manual focusing (FTM) feature. Thus feature allows the user to set or refine the focus with the manual focus ring even when the lens is in the AF mode, without interference from, or the risk of abuse to, the motor drive.

Over the years, aficionados have conjectured as to the nature of the mechanism used for this, generally espousing one of two possibilities:

It is a differential, functionally equivalent to the one used in ring USM lens for the same purpose, although obviously different in construction.

It is a clutch mechanism, which shifts the drive of the focusing cam from the motor to the manual focusing ring, actuated by preliminary motion of the manual ring.

Canon has chosen not to enlighten us in that regard (although their EF Lens Work III book does speak glancingly of a differential in this connection).

An idiosyncrasy of this lens is that it typically exhibits a substantial amount of "lost motion" when the manual focus ring is first moved in a direction opposite from its last motion - a phenomenon that makes critical manual focusing very difficult.

We here at laboratories dak have acquired a new complete AF drive mechanism assembly for this lens and are in the process of thoroughly examining and analyzing it (noninvasively). We will be issuing a full report shortly.

So far, our observations are:

It's a differential

The basic mechanism is indeed a differential. We do not know exactly how it is made - its guts are enclosed. We can of course find out for $75.00 by dissecting the assembly rather than returning the it, undisturbed, back to the supplier. I think we will not do that.

However, if any of you would like to underwrite such a discovery, please come forward. Our PayPal address is:


You will be given full credit in our final report.

We suspect that the differential is what is sometimes called a "ball-bearing" differential. Imagine a conventional ball bearing (not just a bearing ball, which is often but inappropriately called a "ball bearing").

We turn the outer race as one input to the differential action, and turn the inner race as the other input. The output is taken from the ball cage. The output motion is the sum of the two input motions, each scaled by a factor (the sum of the two factors being 1.00).

The lost motion

In the overall focusing mechanism, in place, there are 7 sites of contact in which there is a small amount of lost motion (most of them contact between two adjacent gears), plus the possibility of lost motion in the differential itself (although preliminary examination suggests that this is negligible). These all add up (with various scaling factors) to produce a lost motion that is manifest at the manual focusing ring.

The magnitude of the lost motion at two of those sites cannot be evaluated from the assembly ex situ; they are between the manual focusing ring's ring gear and the mechanism input gear, and between the mechanism output gear and the focus cam barrel's ring gear sector.

At first examination, it appears that the accumulated amount of play resulting from the other sites is on the general order of the amount of play observed in the complete lens.

Thus, the lost motion is not an artifact of some tricky mechanism action, but just an accumulation of the play inherent in simple gear trains and the like.

In any case, we hope to have a complete report, with photographs and drawings, available soon.

Best regards,


January 3rd, 2011, 02:36 PM
Hi Doug,

Very interesting report, thanks so much for taking your time to do this. I was kind of wondering what has caused that lost motion from your other thread, this clarifies things.

...In any case, we hope to have a complete report, with photographs and drawings, available soon.
Looking forward to it.