CZJ Flektogon 4/20 - Uncentered Image?

[x6h1zf65]

Greetings, I've recently acquired this lens and while testing for vignetting with some step up/down rings and circular objects, I've noticed something - the image is *very* off-centered. I can't find any information about this phenomenon anywhere, though maybe I don't have the correct search terms. Below is a screenshot of the viewfinder with a step down ring monstrosity attached, going from 77 to 55mm, with rule of thirds and 1:1 guides, as you can see, the remaining circle is shifted up and left.

My cam is a Sony a7 first generation, with a K&F M42 -> E gen IV adapter. I have 4 other M42 lenses and haven't noticed this behaviour on any of them. Any ideas?

[calvin83]

How about the image quality without the step rings? If the lens is shifted to the left, you can easily spot difference of sharpness on the two sides.

[x6h1zf65]

Hard to say, corners remain soft even at f8 but left side seems a tiny bit softer. Though my main concern is getting a lens hood that works with this thing, since all calculations have gone out the window.

[BrianSVP]

The K&F adapters hold the m42 mount portion in with set screws on the sides. These can be loosened to rotate the mount so if your lens doesn't mount upright, you can correct it. This is a common design on many adapters.

They are almost certainly not tightened evenly, which is causing the entire m42 side of the mount to be shifted to the side.

[visualopsins]

The term is de-centered.

Yes, try without step rings. Try another adapter. Try using K&F adjustment to turn lens 180 degrees -- does de-center also move 180 degrees?

[x6h1zf65]

I don't have another adapter to try, sadly.

I turned the threads on the adapter ~180 degrees, so the aperture mark is exactly upside down, the corners became a lot more even, though I'm not sure if it's because I turned it upside down or because I re-seated the threaded thingy.

For me it was very difficult to focus like this so I had to re-align it and the Flek now behaves like before. I repeated the vignetting test with a Pentacon 2.8/29 and a Mir-1v 2.8/37 and those weren't *perfectly* aligned but it was close enough that I had to hunt for it so I'm somewhat inclined to think that someone decided to "fix" it at some point and some part was left misaligned slightly. My other m42 lenses are long enough that I don't have a good way to perform the test there.

I tried to take a few corner sharpness test shots of currency, but I have to use iso 6400 @ f11 and the camera becomes noisy enough that you can't tell what's going on.

[visualopsins]

Better alignment after turning upside down indicates a faulty adapter...you are correct alighnment will show more with shorter focal length...

Camera on tripod to use lower iso at slower shutter speed.

[x6h1zf65]

Well, this keeps getting more interesting... found perfect left / right alignment but it's still shifted up. To get up and down equal, I have to tilt the lens down a bit over a milimeter and it's impossible to tighten the adapter with the thread insert in this position. The whole thing also starts to look like the leaning tower of piza. I'm open to suggestions on m42 - e-mount adapters, I suppose. The K&F stuff looks cool but I'm not sure if I'm impressed on a functional level with it.

[calvin83]

This one seems better as there is not adjustment on the m42 mount but I have not tried myself. In this case, you need a soild adapter to allow for best centering.
https://fotodioxpro.com/products/m42-sne-p-v2

[visualopsins]

[calvin83]

I have pretty bad luck with vintage Japanese SLR lenses with focal length <= 20mm. At least half of them has been slightly de-centered.

[x6h1zf65]

[BrianSVP]

My recommendation is try anotehr adapter first before you move on to the lens. If that doesn't fix it, a lot of Jena lenses actually have the rearmost group held in by side-mount set screws, similar to how it's done on the adapter mount, but much smaller. That's the first place I'd look for a centering issue.

[x6h1zf65]

Indeed, ordered a Novoflex M42 -> E-mount adapter that will hopefully have tighter tolerances than my K&F one, it was more expensive than most lenses that I'll be attaching to it, but hopefully tolerances are better. I got a lot of results for Urth adapters but I'm sceptical. While I was researching this issue I also learned that most adapters are a bit shorter than they'd ideally be and infinity focus occurs before the stop so now I have an explanation for my blurry landscape photos as well.

[x6h1zf65]

Well, novoflex adapter came in. A bit lighter than K&F and no adjustments to be made. There's a liiiitle bit of shift to the left again, but it happens with both adapters and all of my lenses so the best conclusion is the mounts being less than ideal on the first generation a7. In any case, it should be good enough.

The markings on this lens in particular are bang on with this adapter