Intermittent lens focus issue

[Derbyshire Man]

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I've asked this elsewhere tucked away in a thread but rather than hijack that I thought I'd put this here.

I just took delivery of a used 24mm summilux in perfect looking condition.

Sometime I focus with it and the focus is spot on for each shot. I'll use it another time and every shot is considerable front focussed.

What is interesting is that the variation between front focussing seems to be reset by being on and off the camera. E.g. I took some shots yesterday and they were badly front focussed. Came home and put it back onto the body, got a ruler out to find definitive proof and it was perfect, took it off. This morning I re-mounted it, took some shots outside and front focussing again. This afternoon I put it back onto a body and it's once again spot on.

Is there mechanical reason why this might be the case? Is it a known thing or is my lens possessed?

I may add images at some point but i think you can imagine what I mean.

It's super annoying as I bought it to cover for my 28mm summilux while it is off at Leica being sorted!

Thanks!

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[Derbyshire Man]

Just noticed that there is a small bit of play between the two halves of the focus mechanism, ie distance and aperture markings. You can hear it if you tip the camera back and forwards and also if you gently flex the front portion of the lens.

I wonder if it's been dropped and someone has simply bought a brand new lens hood to make it look tidy?

Anyone else got this in their 24 summilux? Funnily enough I had the same thing int he 28mm summilux that's in Germany now.

[Flou]

I have a Summilux 24 but never had such kind of problems. Focus is spot on every time I use the lens. Hope you will be able to solve the Problem 😐

[TomB_tx]

Depending on the mount design, many lenses can loosen the optical cell from repeatedly mounting and dismounting the lens from the camera by gripping and turning the front portion of the lens instead of the back flange. That can allow the optical head to shift enough to vary the focus as you describe. That's where the "infinity lock" design really helped, as the rear flange and focus barrel locked together when mounting, so you weren't tempted to grasp the front portion of the lens.

It may just be a slightly loose retaining ring. I bought a used Summarit 35 2.5 with that problem, and just tightening the retaining ring on the rear with a lens spanner cured it quickly.