Problem with Manual Focus on 300mm f/2.8 Lens

[steve_byland]

I recently purchased a used 300mm f2.8 (non IS) lens from a friend. He never

used the manual focus, but I am finding it to be finicky. Sometimes, when

focusing in one direction, the darn thing actually changes direction (i.e. as I

am focusing closer, all of the sudden it begins focusing towards infinity, even

though I am turning the ring in the same direction). If I ?roll? the ring back

and forth, it moves only in one direction (towards infinity). I have tried

switching all of the various switches on and off, with limited success. Am I

just doing something fundamentally wrong, or is something broken? It isn?t so

bad that I would send it in to be fixed, but it can be annoying.

 

Any help would be GREATLY appreciated

[zackojones]

Perhaps there's not good contact between the lens and the camera body? Did your friend ever have any issues with focusing?

[steve_byland]

Zack - My friend never used the manual focus on this lens.

[keith_lubow]

Steve,

 

Are you experiencing this problem with the switch on the lens in the manual position?

 

Keith

[steve_byland]

Keith,

 

Yes. I occurs when I switch to the manual position (AF is off). The problem is the least pronounced when the autofocus speed is set to "3" which is the fasted position. Using the manual focus when the lens is set to AF and depressing the shutter button half-way seems to work OK, but isn't a good solution when you really want to just use manual focus.

[dave_holland]

Just a couple of thoughts. With new equipment, sometimes you do things that you don't mean to do. Are you sure that you aren't moving that large, narrow, spring-loaded outer ring, that tells the camera to focus on a preset point? If you accidentally moved that while you were manually focusing, it might give confusing, paradoxical focus movement. Be sure you understand how that prefocus ring works. It's a strange design, in my opinion. You could change the prefocus point to something close, and see if the problem continues.

 

If autofocus is off, the lens shouldn't be doing anything, unless it is trying to achieve a preset focus. Does this happen while the IS is activated? The other thing you might do is to set your camera CF4 to focus with your right thumb. That way when you press the shutter down half-way, autofocus will not be activated unless you press with the thumb as well. I always use my camera that way, and in your case it might offer diagnostic information.

 

Dave

[steve_byland]

Dave

Thanks for the info. I am aware of the preset focus ring and have carefully avoided that (and have turned the prefocus feature off). This is an older, non-IS model. I tried the custom functions of CF4 and there was no change (good thought, though). Something else I noticed is that, when moving the manual focus ring, there is a slight "ticking" in the lens as if something is going on inside. Also, unlike my other L-series lens, the manual focus ring only works on on this lens when the camera is in the "On" position, whereas my other lenses will move the focus even off of the camera.

[dave_holland]

Hmmm. Not much more I can suggest. I would be inclined to send it in to Canon for a quote as to how much it would cost to repair. Of course the other option is just to wait and work around the problem. However, parts for older lenses become difficult to find after several years and it may be more difficult to find parts and expertise later. That said, there is a chance you could drop it or have it stolen in that time as well. Good luck.

 

Dave

[alan_myers]

Hi Steve,

 

Do you have an instruction book for the lens? If not, perhaps you can get one from Canon or download one somewhere online?

 

I have the I.S. version of that lens and the instruction book that cam with it, but am not familiar with the non-I.S. version, so can't really make more than a few suggestions.

 

You mention setting AF on the lens to the "3" position, but is that even relevant when you are turning AF off entirely? Is that a range limiter switch, like on my lens (where the settings are: 2.5M to infinity, 2.5 to 6.4M, and 6.4M to infinity)? If so, I would think it wouldn't have any effect with AF turned off (it doesn't say one way or the other in the instr. manual for my lens... and I've never tested it). Or, perhaps it still tries to limit the range, even in manual focus mode, and reverses the focus if you exceed the range with the manual control. I.e., it keeps trying to stay within the range.

 

Yes, the focus recall ring Dave mentions is another possibility, if you are inadvertently activating it while manually focusing. But, on my lens that function can be turned completely off by setting the Focus Preset switch to "O" (the "I" position activates it, and the third position, if the same as my lens, activates it along with an electronic tone that gives an audible confirmation).

 

Also, I think in this lens the manual focus ring itself is not mechanically linked to the focus mechanism, but serves to activate motors that drive the focus mechanism (the same as AF does). So, some slight noise like you mention when activating manual focusing wouldn't surprise me.

 

Electronic connections of the focus ring may be faulty, perhaps just have some oxidization on their surfaces from disuse. It could just need a cleaning. Or may even become more reliable with regular use, as wiping electronic contacts tends to clear them.

 

Other possibilities that occur to me:

 

The AF off/on switch might be faulty. Perhaps it's not truly off when you think it is.

 

The focusing motor(s) might be faulty. Or, whatever is controlling the motor(s) could be faulty, sending incorrect signals.

 

Finally, there may be little or no reason to turn off AF, if the lens is FT-M/USM the same as my I.S. version. You are probably aware, you can override focus on FT-M lenses any time, without actually turning off AF.

 

Also, the AF of this lens - at least in the I.S. version - is often cited as one of the fastest and best of any Canon EF lens.

 

Perhaps these ideas will help, or the lens just needs to go to Canon Service for a thorough checkup.

[steve_byland]

Alan

 

Thanks for the detailed information. BTW, when I mentioned speed #3, what I should have said was it is the switch that controls the speed of the MANUAL focusing. Your information about the way the MF works electronically rather than mechanically was very helpful. I think that will help me narrow down the problem. I tried putting it on MF and ran the focus ring back and forth a number of times and it seems to lessen the problem, so you may be right that it is the electric connections (especially since my friend never used the MF ring).

 

Also, the main reason I use the MF is if I'm trying to take a picture of a bird or animal in deep cover where it is difficult to get the AF to pick up the right spot. Also, I often use Extension Tubes when I come across a particularly choice subject that is very close and cooperative and MF helps keeping the lens from hunting too much under very poor lighting conditions.

 

Many thanks to all - any further info would also be much appreciated!