Image shifts during focus shift

https://www.youtube.com/watch?v=PybTIWOnQX8

 

Hello, in this video I am shifting the focus on a 46-year-old 55mm 1.2 AI lens. At second 4, you see how the image suddenly nudges to the left? That's not the camera moving. Something inside the lens shifts as I turn the focus.

 

The lens is damaged, correct? Does anyone have any idea if it is easily repairable?

 

It gives a gorgeous bokeh effect at wide open so I'm wondering if worth it to repair.

Could be a decentered element, but are you sure it's not the camera shifting? It doesn't seem to be moving very far, and there's a lot of play in every support system I've ever used - enough for uneven pressure on the focus ring to have this effect. Tripod legs don't bend much, but they bend; every ball head I've used (including expensive ones I've tried in stores) has significant sag when you stop supporting the camera during adjustment unless everything lines up - not because the ball is moving but because of combined sag between the tripod neck, tripod apex, QR clamp, etc.; floors bend a little under enough pressure too, and most tripod feet squash a bit. A good support system won't move much, and certainly doesn't move once the tension has been taken up, but they don't magically stop materials bending when you put pressure on them. (This is why I have a gear head - for precise positioning, it's not that it's any more solid than my ball heads, it's that it's already holding its own weight when you're positioning it so you're allowing for any flex).

If you shake the lens GENTLY can you hear anything at-all?

 

I suppose a notch or piece of grot in the helicoid could make it jump a fraction. Can you feel anything when you turn the focus ring?

 

My 50mm 1.2's focus ring is pretty loose, but I've never noticed image jump.

I had a Nikkor 50mm 1.8 lens that did this. If I recall correctly the front lens group was loose enough to move, as was the retaining ring (the one with Nikkor printed on it). Obviously someone had tampered with it. I simply tightened everything up and all was well. YMMV, however.

Judging from the relatively smooth transition, the lens helix is probably off-center. The issue is somewhat confused by the change in magnification. While not "breathing" in the traditional sense, the distance the lens moves during focusing is more significant the closer the subject. I would expect a jerky motion if something were loose, and you might feel lost motion in the focusing ring. Strictly speaking, the lens is "centered" if the elements are concentric and normal to the film plane, even if the lens barrel is somewhat eccentric.

 

The difference is so slight you probably wouldn't notice it, except for tightly composed closeups, and then you would simply make an adjustment. Everything becomes more important with video, because the transitions are as important as the end points. That is why dedicated video lenses are so outrageously expensive. Try zooming with the video running! Nasty. I'm using conventional zooms with a focus motor, which eliminates the disturbances your hand introduces. Even so, long or quick zooming causes the image to be displaced, due to looseness in the extension (if any) and torque applied to the gear ring. Slow and easy is the game plan.

 

It would ease your mind, and probably improve the lens' operation, if you had a CLA performed (~$200) by a factory approved service agency.

As Ed has written, your lens could use a good CLA job, which should include, at no added cost, an adjustment/tightening of the helicoid key.

In Nikon manual focus lenses this side-to-side image judder is usually caused by a loose or worn-out helicoid key. The key is a a metal rod that the lens or lens subgroups ride against while you turn the focus. It is easily serviced by a pro if loose (usually the case) and is replaceable if worn (much less common). In cases where the replacement or used donor part is not available, a new key can be made by the shop IF they can fabricate parts., which most shops today cannot.

In Nikon manual focus lenses this side-to-side image judder is usually caused by a loose or worn-out helicoid key.

I entirely agree that it's most likely due to slop in the helicoid. Not sure about the key being to blame, or even adjustable though.

 

A loss of grease can easily cause enough slack in the helicoid to allow some shift or tilt of the entire lens assembly, and it only needs a fraction of a degree offset to throw the image about by a noticeable amount.

 

Early lenses like the 55mm f/1.2 (probably pre-AI and later AI converted) might have been made with a split helicoid key that could be opened up to take up wear. Later designs had no such refinement and can't be adjusted. However, a re-greasing of the helicoid is usually sufficient to prevent any sudden tipping of the lens assembly inside its barrel.

 

Whatever. The lens is definitely due for a CLA. It probably has no other fault than a sloppy helicoid.

I entirely agree that it's most likely due to slop in the helicoid. Not sure about the key being to blame, or even adjustable though.

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Early lenses like the 55mm f/1.2 (probably pre-AI and later AI converted) might have been made with a split helicoid key that could be opened up to take up wear. Later designs had no such refinement and can't be adjusted.

 

Agree with Rodeo Joe that slop in the focusing helicoid is the likely source of your problem. While I have not serviced the 55/1.2, I have CLA'd many early Ai lenses from 24mm to 200mm, and a few pre-Ai models. All had helicoid keys that allowed the backlash in the focusing ring to be adjusted (the later AiS series have no such adjustment) . Either the key itself is adjustable (split key shown below) or if the key is solid the groove it rides in can be adjusted. If the backlash is really sloppy, or more likely the very tiny screws holding that small split key are loose, it will slightly bind in the groove when focusing and when it breaks loose the focus will jump.

 

These early lenses have a very robust internal construction, so I would expect that a simple CLA will cure the problem.

 

It gives a gorgeous bokeh effect at wide open so I'm wondering if worth it to repair.

 

Absolutely. ;)