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08-26-2012, 12:35 PM   #1
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What is infinity on a lens?

All my lenses seem to have a slightly different answer to that question. So what is technically supposed to happen when turned to the stop at infinity? Many go slightly past I notice which sucks when I'm doing wide angle in low light because I can't just crank it back to the stop and literally take a shot in the dark.

I mainly ask because after having my A50 1.4 apart I am resetting the fine focus adjustment and I have it set using the K1000 SE so that straight objects reasonably far away using the split prism (as far as I can clearly see straight edges though I can't see too far from here) are in focus as are a fair range of objects that are clearly beyond range of the the focus distance markings on the lens which only go out to 10 meters.

I recently had to reset the focus on my 70-210 F4 because the screws came loose, luckily its super easy on that one as they are behind on edge of the rubber grip so you don't need to take anything off to fix it, just fold up the edge of the grip a tad. The A50 1.4 is a huge pain in the ass because the plastic trim ring and another ring have to come off the front.

My K20D is unreliable for a second opinion since the lens was designed for a FF sensor, plus with the katzeye in the K20D I shimmed it to get the focus corrected but I can't guarantee its 100% perfect. Wish the Super Program was back from Eric so I could get a second opinion body wise.

08-26-2012, 02:16 PM   #2
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I have read that many lenses, especially longer focal length, allow for going past infinity so that when temperature changes cause the lens barrel to change length slightly you can still focus properly. I admit that makes manual focusing less than automatic at infinity. I would think for shorter lengths (50mm) the infinity stop should be at infinity. I have adjusted the scale on a few Takumars and I have set it so the scale is correct.
08-26-2012, 02:30 PM   #3
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It's probably also because of manufacturing tolerances on parts.
If the stop is exactly on infinity it will require higher precision on the parts and assembly of the lens, making it more expensive.
It would probably also require higher precision on the assembly of the mount on the cameras.
08-26-2012, 02:31 PM   #4
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Use live view with 10x magnification. Full moon would make a good infinity target.

08-26-2012, 03:14 PM   #5
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Ok, I just used a dark colored high voltage power line way off in the distance with the split prism, very hard to tell with a 50mm but I set it so that you can turn it ever so slightly past infinity according to the split screen but everything in the distance still looks in focus.

Setting it to match the distance scale on the lens was worthless, you can set it to be correct with one distance and you immediately lose something else, I'm not sure why they bothered.
08-26-2012, 03:57 PM   #6
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Hi PPP, I think better to examine real shots than to use the VF.

Register for infinity is much more critical on a zoom than on a fixed focal.
Some zooms are made with brass shims which can be adjusted plus or minus.

For 35 mm b/w I like to use an old Magnon 35~75mm push/pull zoom that was repaired by using parts from a donor.
The left side on the image was not as sharp at infinity as the right side.
This lens has no shims, the K mount contacts the lens mount directly.
So today I took the K mount off, using a wet-stone, hand ground 0.06 mm (2.4 thou inch) off the rear face of the lens mount,
https://www.box.com/s/edb6960a3a2a8ae738f2
I measured as I went for uneven grinding and i think I was able to do it evenly around the mount plus or minus half thou inch or so.
That tends to make the focus stop slightly past infinity. ( Register is now too short)
Then put shims of cooking parchment ( 0.1 mm, about 4 thou inch) on the mounting screws nearest the unsharp side to bring the register back with a slight tilt.

It is not perfect, but much better over the DOF on both sides than it was, this 'fence test" shot is 2 clicks off wide open at f/5.6
https://www.box.com/s/8bd668f7368a02e3dc68
08-26-2012, 08:20 PM   #7
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Using the APS-C viewfinder and stock screen is a challenge. I have an MX and split-screen which is the easiest method for me. If I only had a DSLR I'd take a few shots and look at them on a good-sized screen. A telephoto lens is easier than wide angle.

I prefer to set manual-focus prime lenses to stop right at infinity, because it's sometimes handy. I don't think there's a problem with this, and I haven't experienced one yet. I usually can see a mountain that's 55 miles away, far enough for me. The moon is a good idea for a 200mm+ lens but I'm too impatient to wait for a good moon.

I think it's a good idea to leave an AF lens room to overshoot infinity and back up, so I adjust them to allow for this. With any zoom, I check all focal lengths to see where infinity is before tightening all the screws.

A couple of Auto-Takumars that I have were apparently adjusted with factory installed shims and are still perfect.
08-27-2012, 12:25 AM   #8
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IMO going beyond infinity is required for AF lenses only because of the AF (== the AF can then distinguish between being focused to infinity and beyond) . On MF lenses this is not needed and thus they end at unfinity.

08-27-2012, 08:34 PM   #9
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QuoteOriginally posted by fuf Quote
IMO going beyond infinity is required for AF lenses only because of the AF (== the AF can then distinguish between being focused to infinity and beyond) . On MF lenses this is not needed and thus they end at unfinity.
This is not true. You may be correct about AF lenses, I have no idea. But many of my manual focus lenses go beyond infinity.
08-28-2012, 04:56 AM   #10
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jatrax: In this case I don't know as I have MF lenses which IMO all end at infinity. I've read somewhere that it might be also because of the thermal elasticity of the lens metal, but I'm inclined to doubt it.
08-28-2012, 06:55 AM   #11
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Quote from Zeiss, Infinite focus play, their response - FM Forums :

"Some play in the focus ring at infinity and beyond is possible. This avoids problems with the infinity adjustment when tolerances of the flange focal distance of the camera body are too high. 1mm sounds OK, 2mm is probably too much. But when you don`t have an application where you need a reliable stop at infinity (e.g., aerial photography) this doesn`t matter at all. If you like to have an alternate adjustment we would need camera and lens to do this kind of service. Then we could change the flange focal distance in a way that the focus ring stops at infinity."
08-28-2012, 05:10 PM   #12
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QuoteOriginally posted by fuf Quote
jatrax: In this case I don't know as I have MF lenses which IMO all end at infinity. I've read somewhere that it might be also because of the thermal elasticity of the lens metal, but I'm inclined to doubt it.
Might be brand related, most of my longer Takumars (and some of the shorter ones) go slightly beyond infinity on the scale. I also have read that at least on long lenses change in dimension due to temperature change was the reason they allowed the scale to go beyond infinity.

The change in length of a 50mm lens will likely not be enough to affect focus, the change in a 400mm lens very well could. I would think modern lenses have methods of compensating for the change more, different materials, different construction, but the old all metal 'stove pipe's really had no way except for in the focus mechanism.
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