MF lenses at infinity

Started Oct 26, 2007 | Discussions
Alek79 Regular Member • Posts: 434
MF lenses at infinity

I've noticed that on my two mf lenses (sigma zoom and helios M42 58/2), the infinity position of the lens ring results in out of focus images. The camera (K100D,) indicates focus only when the ring is twisted a bit from the extreme. Is it normal? or could it be lens or camera miscalibration? All your input would be much appreciated.

Take care,
Al

LesMizzell
LesMizzell Contributing Member • Posts: 517
Re: MF lenses at infinity

While I can't give you any useful information here, I can share your pain!

I've got a old Sears 135mm 2.8 and a Prospec 35-70mm 3.5-4.5 and both seem a bit soft at full-out infinity. I've not had a chance to play with them as much as I'd like yet, but the seem to take decent pics closer up and seem just a bit sharper when I back off just a bit from full infinity.

When I get a bit of spare time, I'll put them through some additional (unscientific) test and see what I get.

Would be interesting to see how many other folks have noticed this.

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soldbear Contributing Member • Posts: 804
Re: MF lenses at infinity

Alek79 wrote:

I've noticed that on my two mf lenses (sigma zoom and helios M42
58/2), the infinity position of the lens ring results in out of focus
images.

The camera (K100D) indicates focus only when the ring is
twisted a bit from the extreme.

Twisted which way? If twisted closer and just a bit, I think it is normal. You know - lens wear and tear, old age, manufacturing tolerance, ..... I would just leave it a lone.

If it completely prevents you from having focus at infinity (doesn't sound like this is the case), I'd investigate to fix the problem.

You mentioned M42 lens, what adapter are you using? Is it flush with the lens mount (the adapter is completely inside the camera body)?

Dave C T Regular Member • Posts: 426
Re: MF lenses at infinity
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Hi Al, I've noticed the same thing, most of my manual focus lenses seem to focus past infinity and I have to back off a little to get a sharp image in the viewfinder.
That's both K mount and M42 ...Dave...
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Just1moreDave Regular Member • Posts: 421
Re: MF lenses at infinity

Alek79 wrote:

I've noticed that on my two mf lenses (sigma zoom and helios M42
58/2), the infinity position of the lens ring results in out of focus
images. The camera (K100D,) indicates focus only when the ring is
twisted a bit from the extreme. Is it normal? or could it be lens or
camera miscalibration? All your input would be much appreciated.

Take care,
Al

If all your M42 lenses behave the same, it could be an adapter problem. Check to see whether it fits flush with the K-mount surface or maybe a little deeper. Deeper would cause your problem.

Infinity focus can be adjusted on the typical Takumar lens if you have a tool to remove the "name" ring and some small screwdrivers. Once you gain access, you simply rotate the front element a bit and see if you have infinity focus. If you have a film camera with a good viewfinder, this is not hard to do by eye. If you only have the pentamirror DSLR models, download the images and view them at 100%.

Other lens brands range from "I can't figure out how to get it apart" to "wow, that was easy."

PCD1 Regular Member • Posts: 190
Re: MF lenses at infinity

I have a couple of MF lenses and both of them have to be a little "off" of the actual infinity mark in order to be in sharp focus on my *ist DL. One is a Sigma KA mount, the other is "Brand X" T-mount with KA adaptor, so your problem may not be due to the M42 mount/adaptor. The focus rings on both of mine turn well past the infinity marking, and are totally OOF by that point. If I back up to just a tiny fraction before the mark lines up with the center of the infinity symbol on the barrel, focus is perfect.

Mike Goltsman Contributing Member • Posts: 580
I see the same behavior

All of my lenses (bunch of M42 and K-mount bottle-glass off-brands from eBay) seem to be slightly misfocused at their mechanical stop, and I see something resembling infinity focus when I back off just a bit. I checked and the M42 adapter is flush with the camera mount surface, plus I think I saw the same behavior with K-mount lenses as well (can't be sure just now though).

Anastigmat Forum Pro • Posts: 12,686
Re: MF lenses at infinity

On manual focus lenses, the lens ring does not focus past infinity. Notable exceptions include some mirror lenses. On some autofocus lenses, the ring does move past infinity.

On M42 lenses, the lens is designed so that it will mount securely even when the threads have become worn with age. But when the threads are worn, the lens, when twisted further to secure it, will go closer to the focal plane than is normal. In that case, infinity focus as marked on the lens is no longer infinity. The lens needs to be extended slightly away from the body, using the focusing ring, to achieve infinity in such cases. So it appears to me to be wear in the thread mount adapter and or the screm mount lens itself.

Alek79 wrote:

I've noticed that on my two mf lenses (sigma zoom and helios M42
58/2), the infinity position of the lens ring results in out of focus
images. The camera (K100D,) indicates focus only when the ring is
twisted a bit from the extreme. Is it normal? or could it be lens or
camera miscalibration? All your input would be much appreciated.

Take care,
Al

PvH Regular Member • Posts: 465
Re: MF lenses at infinity

Anastigmat wrote:

On M42 lenses, the lens is designed so that it will mount securely
even when the threads have become worn with age.

Correct.

But when the
threads are worn, the lens, when twisted further to secure it, will
go closer to the focal plane than is normal.

Not correct. The lens is screwed on the body until the flange of the lens reaches the flange of the body. That is the end position, the glas cannot come closer to the film/sensor.

The K mount ( as all mounts) works the same way. The lens always gets mounted at the right position/distance.

As for the infinity point of a lens: The infinity mark on the focus ring is on a calculated/designd point of the lens. The real position can be slightly off (production tolerances/lighting conditions).

There will always be some more travel after the infinity mark before the end stop.

If you have a lens with IR (Infra Red) marks, and know about IR photography / light breaking, you will understand. (Only a small part of the light spectrum is visible to the human eye). When shooting IR photos, you can focus normaly, then mount the IR-filter and then need to correct the focus for IR to get a correctly focussed photo...
(The end stop position is never the infinity point of a lens).

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BJL Veteran Member • Posts: 9,342
A bit beyond infinity is normal, for IR , thermal expansion?

I believe that it is normal for a lens to be able to focus a bit beyond infinity, for a couple of possible reasons:

  • to allow infra-red light to be focussed at infinity, with IR focus being a bit different than visible light focus

  • to allow for temperature-based variation in where the focus is.

If your lens has an IR focus mark, is the normal focus mark "beyond infinity" when the IR focus mark is at infinity?

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PvH Regular Member • Posts: 465
Re: A bit beyond infinity is normal, for IR , thermal expansion?

From a pentax lens manual (do a google for a pdf of the 'Pentax D FA lenses 112205' manual):

...If you use infrared film and attach an R2 or O2 filter to the lens to take infrared pictures, the point of focus will be different from when you shoot normally (with visible light). A focusing correction will be necessary. (As shown above left,) memorize the focusing point on the lens distance scale when viewed through the viewfinder and turn the focusing ring until the point you memorized aligns with the infrared index...

No pics here, but the IR point will be slightly before the memorized focus point: not closer to the end stop at infinity.

If your lens has an IR focus mark, is the normal focus mark "beyond infinity"
when the IR focus mark is at infinity?

Yes, you turn back the focusing ring to get focus right for IR...

(looked at the manual carefully now: i always thought that the IR point came after the focus mark... so, it's before the mark...)

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shane gerrish
shane gerrish Veteran Member • Posts: 5,980
Re: A bit beyond infinity is normal, for IR , thermal expansion?

out of 60 to 70 lenses i have there are only a handful that focus exactly at infinity!

the two AF lenses that do [DA12-24 and DA18-55] get "stuck" at the infinity mark when using AF "sometimes" depending on the temp of the day.

AF lenses seem to need to be able to go past infinity before they can turn back and refocus the other way but this is rarely a problem for me as i usually manually focus anyway.
--

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OP Alek79 Regular Member • Posts: 434
Recap

Many thanks for your replies. Looks like it's a common behavior of lenses. Just to briefly address your questions: The M42 adapter I have is genuine Pentax and it seems to sit flush in the body, plus I am able to focus at infinity. The problem occurs both with K and M42 mount lenses. I don't have the sigma with me but on Helios, the infinity mark seems to be perfectly alligned, this lens has been used very few times so I wouldn't expect any wear. Nevertheless, I have to back off the focusing ring a tiny bit to get correct focus.
Guess it's something we have to live with. Thanks again and happy shooting!

Al

Sean Nelson
Sean Nelson Forum Pro • Posts: 14,808
Re: MF lenses at infinity

PvH wrote:

There will always be some more travel after the infinity mark before
the end stop.

Incorrect, for at least the SMC Pentax-M manual lenses that I own. They all focus to exactly the infiinty mark on the lens (where "exactly" means that the intersection in the middle of the "sideways-8" mark is aligned with the focus index). There is a "hard stop" to the focusing mechanism at the infinity point, and the lenses are correctly focused at infinity when mounted on my K100D Super. This is true of my 15mm f/3.5, 28mm f/2.8, 50mm f/1.4 and 100mm f/2.8 lenses. The only manual lens I have which allows focus "beyond" infinity is my 600mm f/8 Sigma reflex telephoto.

If you have a lens with IR (Infra Red) marks, and know about IR
photography / light breaking, you will understand.

The IR infinity focus point occurs with the lens set to slightly CLOSER than the visible-light infinity position, so there is no need for the lens to focus past visible-light infinity to take IR photos.

Here's a photo of my SMC Pentax 15mm lens which illustrates this point:

The IR focusing index is the red line between the f/4 mark and the visible-light focus index. If you align the infinity mark with this line, the lens is focused for approximately 7 metres in visible light.

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