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Flektogon 35 2.4 infinity focus
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PostPosted: Mon Apr 27, 2009 12:19 am    Post subject: Flektogon 35 2.4 infinity focus Reply with quote
As some of you might know, I am currently travelling and have brought with me the Flek for shooting. I am unable to focus infinity with my Sony DSLR. Is this common to Flek? Any way to rectify this? Or is this the adapter problem? But I do not have problem with my Pancolar and my Zenith.


PostPosted: Mon Apr 27, 2009 12:28 am    Post subject: Reply with quote
You Flek seems need to adjust infinity settings. I think it was opened and wrongly rebuild or it has factory defect. I good repair man able to fix it.


PostPosted: Mon Apr 27, 2009 8:12 am    Post subject: Reply with quote
What about the 50/1.4 Takumar? Most Takumar lenses have very exact infinity focus, so it can help to prove, if the problem can be in adapter.

Do you have old SLR Praktica/Pentacon/Zenit body? Split screen viewfinder can also help.


PostPosted: Mon Apr 27, 2009 1:02 pm    Post subject: Reply with quote
no-X wrote:
What about the 50/1.4 Takumar? ....
Do you have old SLR Praktica/Pentacon/Zenit body? Split screen viewfinder can also help.


Unfortunately I did not bring my Tak 50/1.4 on this tour. I bought along the Pentacon 50 though. Was planning to use the Flek 35 as my mean lens. (It is still my mean lens as I do not have anything between 20-50mm. Well, I have bought my auto zoom lens which is not a good performer which I try not to use it).

I have tried two different adapters, one which I bought recently and one which I have used quite a long time, and I can focus infinity with either. Unfortunately I do not have any old Praktica/Pentacon/Zenit body. My film bodies also need adapters to be used.


PostPosted: Mon Apr 27, 2009 5:39 pm    Post subject: Reply with quote
Try some infinity shots with the Takumar. Takumars have usually precise infinity focus, so it can help to find out, where is the problem.


PostPosted: Tue Aug 11, 2009 10:49 am    Post subject: Reply with quote
Had the same problem with a CZJ Flektogon 35/2.4 I bought from eBay. Maybe a few lenses circle around as people sell them on to next victims. Wink

I solved it the easy way to start with - by sanding the M42-adapter down: Placed a sandpaper sheet on a table and moved the adapter against it using rotating patterns. Removed the flange shape totally and kept sanding the flat surface as well. Took several test pics along the way - with trees near and far visible in them. The starting point was, the trees at some 20 m were in focus and the ones some 600 m away were blurry (actually the ones at 50 m were blurred, too). I also compared the sharpness with the kit lens at each checkpoint.

Had to sand it down with water + P240 **a lot** first, then used P1000 to polish the surface during the last runs. Did 8 sanding sessions, taking test shots after each. The only nuisance is, the aperture scale is now rotated almost to the bottom of the lens. It was off in the beginning already, so I knew this would happen. But at least it is now possible to shoot landscape with the lens as well.


[EDIT]
Received yet another adapter today - the good one for Sony, from James Lao. Seems to have a perfect infinity focus. The cheaper eBay electric adapters WERE too thick - and the lens IS properly adjusted at inifinity focus.
Comparing a few shots now, I may have sanded the other adapter a bit too much / focuses slightly beyond infinity. Does not matter as long as I remember to focus back some .5 mm after infinity.
With perfect infinity focus, the Flektogon draws much more details for landscape. I already assumed, that it is optimal for close range only. But now the East-German lens shows impressing resolution for far-away details as well. Seems that this lens is extremely sensitive to the correct flange-to-sensor distance, both too thick and too thin adapters can blur up the distant objects.
So, you better check your adapters with a few shots where details of varying distances can be checked on pixel level - and compare those with lenses you know to be good at infinity (kit lens is OK for this, I guess.)

Last edited by mikkokam on Sun Nov 01, 2009 8:52 pm; edited 2 times in total


PostPosted: Tue Aug 11, 2009 11:17 am    Post subject: Reply with quote
Welcome! Thank you for sharing details!


PostPosted: Thu Aug 13, 2009 8:54 pm    Post subject: Reply with quote
mikkokam wrote:
Took several test pics along the way - with trees near and far visible in them. The starting point was, the trees at some 20 m were in focus and the ones some 600 m away were blurry (actually the ones at 50 m were blurred, too). I also compared the sharpness with the kit lens at each checkpoint.


You already found my old comparison of Sony M42 adapters on flickr, but in case others are interested in an illustration of how to check the infinity focus of these adapters (many of which, at least in the past, have been too thick, in my experience), here's an example of how I did it:



The focus is at “infinity”, and the building (rather distant) should be in focus with correct infinity focus. In the picture on the left, however, the branches in the foreground are sharper than the building, i.e. focus is closer than infinity. The right shot shows how it should look like with the building sharp and depth of field too narrow to make the branches sharp. (At this size its difficult to judge the focus on the building, but it is evident from the brick pattern, etc.)

So, the procedure is: use the widest aperture, focus at infinity, pick a target that's “at infinity” for the focal length you are testing with, and position yourself so that some object is in the same frame at a “finite” (but relatively far-away) distance (e.g. a bit farther than the greatest distance that's marked on the distance scale of the lens). With relatively short lenses you may also be able to tell the focus from ground if you have open terrain to photograph. I suggest testing adapters both with short (e.g. 28mm) and relatively long (e.g. 135mm) lenses; the short lenses are most affected by adapter thickness and can show when it's obviously off, but when you get close to the correct thickness they may have too much depth of field to show the difference.

Once you've gotten the adapter sanded down so that infinity performance looks proper, you may fine-tune it by testing with stars, which should appear pointlike. When focus is really good, you may even be able to tell miniscule differences by carefully finding the dimmest stars you can make out in the photos on a clear night; even a slightly incorrect focus position will make them disappear. However, this is a very demanding test and many (most?) lenses may not have their infinity focus position set precisely enough, so you may actually make the adapter too thin for other lenses if you over-correct it for one. (Also things like temperature begin to affect focus accuracy, which is why many long lenses focus “beyond infinity” by design.)

(Of course, the lenses used for “calibrating” an adapter should have known accurate infinity focus themselves. =)


For the record, the adapter used in the left picture needed to be sanded down 0.25mm until it focused correctly. This much sanding causes the lens to stop almost upside down, which is most inconvenient. The sad thing is this adapter came free with a Zenitar 16mm fish-eye, which ended up having completely incorrect focus with the adapter but has so much depth of field it's not easy to see that this is the problem with it. I wonder how many people have bought that as their only M42 lens with that adapter and believed that the lens itself is uselessly soft…

Last edited by Arkku on Fri Aug 14, 2009 10:52 am; edited 4 times in total


PostPosted: Thu Aug 13, 2009 9:58 pm    Post subject: Reply with quote
Thank you Arkku!


PostPosted: Fri Aug 14, 2009 4:43 am    Post subject: Reply with quote
Nice example Arkku


PostPosted: Fri Aug 14, 2009 9:58 am    Post subject: Reply with quote
Thanks Arkku!

Your great Flickr article was an excellent source - it encouraged me to test the sanding of the adapter.
I did not check the hyperfocal distance before testing. With a 35 @ f/2.4 it apparently becomes roughly 26 m only, if I got it right. With focus at infinity, one should have a decent focus from 13 m to infinity. Anyone testing adapters with the Flektogon, should therefore keep an eye on objects not that far away?
On the other hand, this is why the Flektogon is a great lens to use for rapid shooting on the go; stopping down and setting focus to infinity, one has pretty much everything in DoF always (beyond 5 m or so).

Should've waited and tested with the 135/3.5 Sonnar I got this week - the hyperfocal distance wide open is more than 250 m(?) So, as you said in the article, one should fine tune with a tele.


PostPosted: Fri Aug 14, 2009 10:50 am    Post subject: Reply with quote
mikkokam wrote:
I did not check the hyperfocal distance before testing. With a 35 @ f/2.4 it apparently becomes roughly 26 m only, if I got it right.


Well, the hyperfocal distance depends entirely on the definition of what is in focus (i.e. what is acceptably sharp). Traditional formulas based it on a fraction of the image size, but with DSLRs one can obviously calculate it based on the pixel pitch to get the “pixel perfect” focus, and since this infinity focus test is based on pixel peeping, that's certainly the way to go about it… (Although note that many online hyperfocal calculators ignore diffraction, which begins to limit the resolution at small apertures below that of the sensor's capabilities. That's not an issue wide open, however.)

mikkokam wrote:
Anyone testing adapters with the Flektogon, should therefore keep an eye on objects not that far away?


Yes, and actually the (pixel perfect) hyperfocal distance does pose something of a problem for this infinity focus adjustment—once this distance is reached, it's no longer possible to use that lens to adjust the focus more accurately, because by definition of the hyperfocal distance things at infinity will already be in focus. Of course, it won't matter for that lens on that camera (and might actually be considered better than perfect infinity focus =), but it may still be off for longer lenses (or cameras with “smaller pixels”, if the same adapter is used on another body).

So yes, one should indeed fine-tune the focus with a long lens. The 135mm is probably best, because longer lenses often focus beyond infinity intentionally. (And everyone should own a 135mm, it's my favourite focal length. =)


PostPosted: Sat Aug 15, 2009 2:42 pm    Post subject: Reply with quote
Thanks, Arkku. Great explanation.


PostPosted: Sat Aug 15, 2009 4:08 pm    Post subject: Reply with quote
Thanks everyone for the welcome!

And thanks Arkku for the insight.
I do believe that many still get too thick adapters and thus think the lens is "soft" while focused further away. The Lao adapter works perfectly, though.
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