I’m investigating some occasional and intermittent focusing problems, and in this particular stage, I’m looking for information on the nature of USM focusing. It's my understanding that there is no mechanical connection between the focusing ring and the focusing action on a USM lens.
Background: 1DsIII with EF50mm 1.4 USM used in aerial photosurvey work with computer controlled mount, controlled by custom software written using the SDK. The camera has been back to Canon Service, and has the latest firmware installed.
We had been having out of focus images using AF, so we went to manual focus with the lens set at the hyperfocal distance, the focus ring taped in place.
We are still getting OOF images.
My question: does that lens focus when the focusing scale is set, or when each shot is taken? Is it possible that the focus can change between shots? When does the lens actually focus?
Our next step is to use a manual focus, mechanically focusing lens to eliminate the possible electronic focusing problem. What's out there? The new Zeiss EF mount lens?
__________________ Dennis
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
Last edited by Dennis_Vied; 03-26-2009 at 12:35 PM.
Reason: delete occasional
Hi Denis; I'm having the same problem with my 1DSIII also. It occurs with all my lens, but seems worse with my 16-35II. It appears to happen when the lens is focused on infinity when shooting landscapes or seascapes with no objects in the forground. I have sent the camera to VA. for the service letter fix and should have it back soon.
WE are close enough that we could get together and compare. Too bad the Red Baron burned down. It was a nice flyin lunch stop. You might like to see my ride on the Airship Venture over SF. sjprg.us
I’m investigating some occasional and intermittent focusing problems, and in this particular stage, I’m looking for information on the nature of USM focusing. It's my understanding that there is no mechanical connection between the focusing ring and the focusing action on a USM lens.
Not so (except for the EF 85mm f/1.2L and -II, which manually focuses electronically). Let me tell a long story to put everything into context.
In ring-type USM lenses (with full-time manual focus) [this does not apply to the 50 f/1.4], the position of the focus drive motor, and the position of the manual focus ring, are added together by a differential, and the sum is fed to the focusing mechanism proper. (he concept is just as if we took the familiar rear-wheel drive auto differential - out of the car - and found that the position of the input flange was the (scaled) sum of the position of the right axle and the position of the left axle.
Quote:
Background: 1DsIII with EF50mm 1.4 USM used in aerial photosurvey work with computer controlled mount, controlled by custom software written using the SDK.
But the 50 f/1.4 is a unique beast. It is the only micro-motor (not ring) USM lens with full time manual focusing. It appears that a different mechanism is involved than the "differential" I described above..
My conjecture is that with this lens, when the manual focusing ring is moved, its initial travel (which of course to us just seems like "play") operates a clutch which disconnects the drive motor from the focusing mechanism proper and instead connects the ring to it.
The play that ensues is very irritating.
Quote:
We are still getting the occasional OOF image.
My question: does that lens focus when the focusing scale is set, or when each shot is taken?
First lets think of the lens set to AF. If you move the focusing ring, the focus mechanism is moved.
Then when we half-press, the AF system comes into play, and operates the focusing motor to move the focusing mechanism to the position the system has determined will produce proper focus.
But you have set the lens to MF. In that case, when you move the focusing ring, the focusing mechanism is moved (in the sloppy way I described above). Period.
Now, with the lens set to MF, and nobody touching the focus ring, can the focus change? There is no reason it should. But I have no confidence at all in the focusing mechanism of this lens, and I cannot rule out the possibility that vibration could move the focusing mechanism from shot to shot.
Taping the manual focus ring would not alleviate that, since (as I mentioned above), with "hands off", the manual ring is not (yet) connected to the focusing mechanism (it only connects when you start to move it, even with the lens set to MF).
I hope that helps you to further investigate what you are experiencing here.
In any event, the 50 mm f/1.4 has essentially the worst manual focus system of any EF lens.
Any other EF lens with full-time manual focusing (of necessity, a ring-type USM lens) has a continuous (but not direct ) connection between the focusing ring and the focus mechanism. (Not direct in that it passes through the differential mechanism I spoke of earlier.)
If the lens does not have full time manual focusing (the non-USM and non-ring USM lenses), the connection between the ring and the focusing mechanism is continuous and direct. Curiously enough, it is those lenses that, in my opinion. have the most reliable manual focusing action!
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
Thanks, Doug. Very helpful. It was my suspicion that taping the ring does nothing to insure accuracy.
I learned in talking to the Canon tech rep that our camera was serviced at Irvine, but the actual mod which our camera needed is only done in Virginia, so we'll be sending it back there.
In any case, I think we need a different lens for this work. I've seen at least one online opinion that this lens is famous for inaccurate focusing. I think I can confirm that.
We have two options: get the 1DsIII modified and go back to AF, or use a mechanically focusing lens set to hyperfocal distance. For our purposes, I'm favoring the latter.
__________________ Dennis
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
In any case, I think we need a different lens for this work. I've seen at least one online opinion that this lens is famous for inaccurate focusing. I think I can confirm that.
We have two options: get the 1DsIII modified and go back to AF, or use a mechanically focusing lens set to hyperfocal distance. For our purposes, I'm favoring the latter.
If 50 mm is what you need, you might even consider the EF 50 mm f/1.8 II. It has a very primitive (and likely reliable) focusing mechanism! ($90 at B&H!)
Another choice might be the EF 50 mm f/2.5 compact macro ($250).
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
When we first started, we were using a 24-105, and quickly found that 50mm was the ideal focal length for our purposes.
I don't mean to disparage the 50 1.4. It's a great lens for its purpose. I used it before as a normal lens, and it works ok for general photography, but for the aerial survey work, it isn't satisfactory. This, of course, assumes that the camera is not at fault, which may not be a valid assumption.
The camera is mounted on a mount remote from the operator, so we're not looking through the viewfinder to confirm focus. (Hence, the need for the hyperfocal distance). We set it before takeoff, so it needs to be accurate in it's mechanism and markings. (You know, like the old days). This is brought about by the unreliability of AF, which may of course be solved by the mod offered by Canon. We'll see.
We'll take a look at the 1.8, and the new Zeiss offering.
__________________ Dennis
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
Last edited by Dennis_Vied; 03-26-2009 at 12:12 AM.
Hi Denis; I'm having the same problem with my 1DSIII also. It occurs with all my lens, but seems worse with my 16-35II. It appears to happen when the lens is focused on infinity when shooting landscapes or seascapes with no objects in the forground. I have sent the camera to VA. for the service letter fix and should have it back soon.
WE are close enough that we could get together and compare. Too bad the Red Baron burned down. It was a nice flyin lunch stop. You might like to see my ride on the Airship Venture over SF. sjprg.us
Yes, very similar to what we experienced. We had it on the 24-105, and again on the 50, so naturally we suspected the camera. We thought it was fixed on the camera, but discovered that the particular mod advertised by Canon was only available in the Virginia location, so we'll still need to do that before we eliminate the camera as the source of the problem.
Well, we have Beebe's, where the Red Baron used to be. It isn't bad, but the OAF club, (the Old Airport Farts club), prefer a narrow selection of restaurants in downtown Livermore, all of whom offer senior discounts, (with one offering free pie on Wednesdays, and to an old cheap international captain, that's irresistible).
Come over sometime. We'll do lunch.
__________________ Dennis
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
Last edited by Dennis_Vied; 03-26-2009 at 12:23 AM.
Reason: spelling
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