Which lens did I buy and why does it do some funny focusing

[Karhallarn]

Hi guys,

yesterday I purchased my first M42 lens on a garagesale. I don't know what it ist. Maybe you can help!

It's a 28-70 3.5-4.5 Zoom-Lens.

Hre are some pictures:






It looks prety fine quality wise but there is one flaw. It does focuse to infinity set 0.8 m at the wide end and just gets blurry when turning the ring towars infinity. At the 70mm end where the macro is located, everything seems to work fine!

At 55mm it focuses to infinity at 3m setting! But seems to do the closer focusing without severe problems!

Any idea about the focusind isue?

I'm using a glas element adapter for my nikon, which I just shot at eBay for 8€.

Thanks in advance,

Karhallarn

[LucisPictor]

I don't really understand your problem, but some lenses have a special "macro"-ring, if this one is "activated" you cannot focus on infinity.
Don't think that this is the case here, though...

[Karhallarn]

I can focuse to infinity actually. The problem is, that the infinity distance doesn't match the markings on the lens!

So If I want to focuse to infinity at 28mm, I have t turn the focusing ring to 0.8m, which is actually the shortest focusing distance! If I turn the focusing ring to say 2m the whole picture gets out of focuse again, instead of having 2m in focuse!

Was that more clear? I don't really know how to explain!

[casualcollector]

Something has gone wrong inside your lens. The spacing between front and rear lens cells has been changed or the connection between the zoom/focus grip and the internal mechanism has slipped.

[Karhallarn]

So do I have to take it apart and try to adjust it?

If so, what to expect and what to be careful about?

[LucisPictor]

The reason might be the adaptation.
You also use a lens adapter which completely makes the distance scale obsolete.

[Karhallarn]

So the distance change is related to the register distance? Is this also why I can focus much more preciesely with longer focal lengths?

[LucisPictor]

[Karhallarn]

@Lucis Pictor: I meant that there is more spacing in the distances at the longer end, e.g a longer focus path.

[casualcollector]

Lucis is correct in saying your problem may be related to your adapter. I didn't read your posting carefully enough. Best thing to do is first check it on a screw mount camera body. If it doesn't work correctly on the screw mount body, then my comments would apply.

[Minolfan]

I agree with casualcollector. Must be a M42 body somewhere around?
I have tried my adapter (Minolta MD to Sony, principle is identical) and the distance scale of the mounted lens is still correct. So I think that something is wrong in the mechanics of the zoom or the lenselement in the adapter has not the correct negative strenght.

[vilva]

The adapter is probably wrongly dimensioned, i.e., either too thin or with an incorrect negative element. I had a similar problem with a cheap M42 adapter and 20mm Flek combination, which focused to infinity at 2m setting. This problem is always more pronounced with short lenses. Try fixing the lens at infinity with a piece of tape and unscrewing the mounted lens a couple of turns simultaneously observing the actual focus distance - a thin spacer between the lens and the adapter may be enough to remedy the problem.

Veijo

[kansalliskala]

Does the rubber of the focusing ring peel off easy? If so, are there some screws under the rubber?

By loosening them little you might be able to move the ring itself without changing the focus and see if different positions of the ring make more sense?

[Karhallarn]

No, the rubber is really tight! As is the rest of the build. I took of the Bajonett yesterday but everything looks fine! I think I will just use it as a 70mm Macro lens!

Thank you all for those tips!

[dof]

Are you sure you're using an M42 adapter and not a T-mount adapter?

Both accept 42mm screw mount lenses (sort of) but mounting an actual
M42 lens in the T-mount adapter will cause all sorts of focusing problems.

[Karhallarn]

The Adapter says M42 to Nikon!

[peterqd]

I don't think it's wise to start dismantling the lens just yet, not until you know for certain there's something wrong with it.

Normally, with a plain adapter, M42 lenses on a Nikon camera cannot quite reach infinity focus because the lens cannot get close enough to the film/sensor plane. If you can focus well past infinity with your optical adapter I think the first thing to suspect is the adapter, not the lens.

You could do a rough check of the lens quite easily. Remove the adapter, set the focus on infinity and aperture wide open and hold the lens mount against the Nikon camera mount. Looking through the viewfinder, objects in focus should be around 10-20m from the camera and objects further away should be out of focus. Now move the lens slowly away from the camera and the focus point should move closer to you and the distant objects should become even more out of focus. If something different happens with your lens, for instance if distant objects come into focus, then you know the lens needs adjustment.

[Karhallarn]

@peterq: today I tried to take of the class element of the adapter and see if the lens acts different. Without the adapter the lens doesn't get sharp focus at all! So it seems to totally screwed!

[peterqd]

[Karhallarn]

Was a try! So then I need to try another lens! Get back to ebay than!

[vilva]

[Karhallarn]

I'll try this! Thanks!

[Karhallarn]

Tried it but no changes!

[vilva]

[Karhallarn]

Tried it already! Doesn't make a difference!