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Flektogon 35mm f2.4 infinity focus query
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PostPosted: Tue Nov 22, 2016 7:33 pm    Post subject: Flektogon 35mm f2.4 infinity focus query Reply with quote

I have been looking at comments on infinity focus with M42 lenses by Forum members, including several by Arkku; especially the one from 2009 pasted below:

At infinity focus, the lens should be at its closest to the sensor/film. If the M42 adapter is too thick, it is possible that the lens cannot reach this position due to the adapter acting as a very short extension tube. Similarly, if the M42 adapter is too thin, the lens will focus beyond infinity at the marked infinity focus position (but it can still focus at infinity properly at some other position, at the cost of some, typically small, amount of close focusing distance).

Likewise the lens itself may be adjusted to go past infinity, perhaps even deliberately (but that is more typical with long lenses where the manufacturer may intentionally leave some room for thermal variations and manufacturing tolerances), or the lens may be incorrectly set so as not to reach infinity.

In your case it seems like the lens and/or the M42 adapter cause it to focus beyond infinity. The typical way to test this is to focus at some object "at infinity" (i.e. at something several times farther than the largest marked finite distance on the lens), like you did at the 400m distance. If there's still focusing range left, your set-up goes beyond infinity. Use another lens to check whether it is the lens or the adapter that is at fault. If it's the adapter (and this really bothers you), get another one. If it's the lens, I'd probably leave it alone unless you are losing a lot of close focusing distance as a result, but of course it's good exercise in lens repair to adjust the distance.


2. I have noticed unusual behaviour of the Flektogon used with a Samsung GX20 (Pentax K20D clone) camera. I am using a generic M42 to K-mount adapter without a flange and without a spring locking plate. The adapter remains on the lens.

I tested the lens by focussing on distant objects approximately 100 metres away. The focussing LED lights up at a point between the infinity mark hard stop and the last mark at 10ft on the focus indicator ring. Sometimes it lights up at the infinity lock or remains unlit there. On car number plates, there is a very slight difference in sharpness, with the pictures taken with the LED lit just before the infiinity mark appearing slightly sharper. Since the viewfinder of a DSLR is rather dim it is difficult to see if a distant object is in focus with a 35mm lens.

At the near focus end, the focussing wheel hard stop is at a point a little more than 8mm beyond the last 0.2m minimum focus mark. I am able to take the front element of the lens to abut 15 cm from the object and retain focus. I assume this means that the distance from the object to the sensor is 20 cm, i.e. the specified near focus limit. I am not worried about the quasi-macro function of the lens as I need the feature only to take pictures of fountain pens. Since the lens takes acceptable pictures I am doing nothing for the moment as i) There is no means of getting it to an Indian city where a trustworthy adjustment may be possible. ii) I do not have the necessary DIY skills or instruments.


I would appreciate comments on whether

i) the lens is focussing beyond infinity?

ii) the near focus limit achieved conforms to the lens specification of 0.2m?

Best wishes,


PostPosted: Tue Nov 22, 2016 9:48 pm    Post subject: Reply with quote

It is difficult with a 35 mm wide lens to see when sharpness at infinity is achieved, especially through a small viewfinder. I've just tried my Flektogon on a K10 and it seems to get the focus light coming on just before the focus stop - BUT....I have just tried it in the dark and haven't actually taken any pictures. I have used my K10 a great deal with manual lenses and 99% reach infinity, and a few go beyond. I don't mind that at all, and I generally find the Pentax focus confirmation to be very accurate.

I shall try the Flektogon on the K10 in the daylight, but I don't think I will get a chance for the next few days.


PostPosted: Tue Nov 22, 2016 10:52 pm    Post subject: Reply with quote

Flektogon 35/2.4 has a really steep helicoid towards infinity - a ~0.5mm shift on scale makes huge diff in inf sharpness. My adapter is a cheap Chinese one thus not made with 100% accurate thickness. It focuses way past infinity.

The simplest solution with Canon EOS-M42 adapter for me was to mark the real infinity with a marker pen. I mean, it was really close to infinity at the scale but not in the pictures.

Anyway, during my infinity testing, I've confirmed myself that the Flektogon has really noticeable field curvature even on f/11 - resulting in soft corners. You can compensate this by "focusing on corners" (a 0.3mm or so offset) but ofc you lose center sharpness. It's really not an infinity suitable lens. Near focus fits the lens way better Smile

1) Yes, your adapter is too thick. The lens focuses past infinity. LED confirm/beeping accuracy is useless in this case.

2) My copy (older version with no rim around its front lens) can also go past 0.2m mark.


PostPosted: Wed Nov 23, 2016 12:04 am    Post subject: Reply with quote

SOunds as people have indicated that it is focusing past infinity. Here is a way to test for and establish an accurate infinity focus. Using the camera on a tripod focus to the stop. Mark this on the lens in fine point pencil. (ie a line where the focus ring marker is at that time) then moving ever so slightly repeat and add another line. repeat several more times. Then view your photos on a monitor. Count the number of exposures until you get the sharpest available image. the number of lines back from your infinity stop that matches the number of exposures should be the infinity focus.


PostPosted: Wed Nov 23, 2016 7:00 am    Post subject: Reply with quote

Thank you!

Lloydy, please revert with your result after trying in the daytime, either in this thread or PM.

Y, my Flektogon is the later type with the raised rim around the first element, all markings in white including MC and 5 digit serial number. I seem to be able get near focus at the specified Minimum Focussing Distance of 0.2m.

Jamaeolus, I am going to try out your suggestion on marking the actual infinity position of the lens.


PostPosted: Wed Nov 23, 2016 11:44 am    Post subject: Reply with quote

OK, I quickly tried the Flektogon on the K10 and the Sony A6000, and my lens goes beyond infinity on both cameras. The lens mark is maybe 1mm out, it's the difference of the red line being at the centre of the infinity symbol ( sideways 8 ) and the end of the symbol, and it's exactly the same on both cameras.

The K10 is accurate to the focus confirmation in the viewfinder, and seems to be better when I turn the focus ring to the hard stop then go back.

Then....I tried the Flektogon on a Pentax SP1000 body with a very good prismatic viewfinder and it reached infinity when on the hard stop.


PostPosted: Wed Nov 23, 2016 12:16 pm    Post subject: Reply with quote

Lloydy wrote:
OK, I quickly tried the Flektogon on the K10 and the Sony A6000, and my lens goes beyond infinity on both cameras. The lens mark is maybe 1mm out, it's the difference of the red line being at the centre of the infinity symbol ( sideways 8 ) and the end of the symbol, and it's exactly the same on both cameras.

The K10 is accurate to the focus confirmation in the viewfinder, and seems to be better when I turn the focus ring to the hard stop then go back.

Then....I tried the Flektogon on a Pentax SP1000 body with a very good prismatic viewfinder and it reached infinity when on the hard stop.


Thanks. In my case on the Samsung GX20, the sharpest focus seems to be when the red line is a hair to the left of the left loop in the infiinity symbol. Never thought much of the DSLR viewfinders of any brand. I have two Pentax MX bodies and yesterday I found a box of old 35mm cassettes. After reading about your result with the Flektogon on the SP1000 body, I think I will have a go on the film camera with the Flektogon.

Do you think the adapter is in any way responsible for the lens going beyond infinity?


PostPosted: Wed Nov 23, 2016 12:46 pm    Post subject: Reply with quote

PK M42 adapters should not change the flange focal distance of the lens, they sit entirely within the bayonet mount & the lens comes flush to the camera body.
It's very common for longer lenses to be designed to have the focusing travel move beyond the infinity mark (to cope with thermal expansion) but this is rare on wide angle lenses.