Why checking infinity (and servicing lenses) is worth it

[Esox lucius]

These are all exposed at f/1.2 and 1/8000s with lens focused turned to infinity stop, shot with three different copies of the same lens, using a sturdy tripod with exposure delay (to rule out mirror slap) and triggered in self timer mode.

1978 Nikkor 50mm f/1.2 Ai
1982 Nikkor 50mm f/1.2 Ai-S
2009 Nikkor 50mm f/1.2 Ai-S

Images shown here are 100% crops, same WB and moderate sharpening.



The centre Nikkor 50/1.2 Ai-S had a loose focusing helicoid, and it was serviced by a serviceman I consider one of the most skilled in Finland. For a measily 40 EUR he transformed it into the sharpest 50mm f/1.2 that I have. Needless to say, the other two copies of this lens will be dropped off at http://www.kamera-apu.fi first thing Monday morning, I'm going to let him calibrate infinity on them as well. First class service well worth the price.

[Excalibur]

erm I don't understand (maybe it's because I use film cameras ) in that I focus when subject looks sharp, and sometimes it's at infinity (i.e. at focus lens stop) and sometimes it's not

[Esox lucius]

Many AF lens designs allow manual focusing past infinity point. This lens is not such, focus turn stops at infinity.

The Nikkor 50/1.2 Ai/Ai-S are manual focus lenses, designed so that the lens focuses by moving the whole lens assembly inside the lens. They are easy to calibrate for infinity thanks to this design, and they should be given this service about every 10 years, because with time wear and dried lube can change the infinity calibration.

[torbod]

Is it a true infinity stop design? I mean if calibrated at normal room temperature, will it still be correct at freezing temperature wintertime or in the sunny desert?

Will not some mechanical assemblies needs the little extra past infinity possibility for this reason and perhaps also to cope with tolerances in camera house (mount to sensor plane distance)?

/T

[berraneck]

it´s useful to have infinity point at the end of the ring rotation - that´s the thing I understand, you cannot focus below the infinity. but I don´t think it´s a thing you cannot live without - how often you focus by marks on the ring? and how often to infinity? i can imagine situations, where it´s useful to have the focusing ring marks in right position (f.e. setting the focus to hyperfocal distance) but these are possible to solve.

I agree that servicing is worth it - rotating is smoother and not stiff or too loose, lenses are in right positions etc. but I don´t think that setting focusing ring in precise position with focus marks is the most important thing. imho:)

[Esox lucius]

With my shooting style and methods, I find it very useful that lenses stop correctly at infinity, for example stitched panoramas. I've also memorized depth of field values for most focal lengths and apertures that I use regularly, and sample values are built on infinity.

[symphonic]

[kds315*]

Long teles are usually deliberately adjusted by lens makera to allow a tiny bit beyond infinity focusing due to thermal effects (very hot vs very cold). Take that into account please...

[visualopsins]

Not an easy solution here. Depends on relatives of lens speed, focal length, and value of the convenience, whether to calibrate for infinity focus at end of focus ring travel.

Temperature coefficients of expansion of lens mechanics and lens elements, both add to change exact focus point with changes in temperature. Mechanics expansion and contraction move positions of lens elements. Lens elements change specifications with expansion and contraction. There is greater effect in lenses with larger lens elements, lenses with more lens elements, and larger lenses i.e. fast lens, large/long lens, zooms.

Calibration movement effect is greater in shorter focal length lenses than in longer focal length lenses. Temperature changes in some wide lenses can move the calibration point for more effect than in some long lenses, i.e. a fast wide angle may be more out of focus than a fast telephoto with changes from calibration temperature.

Or, maybe it is an easy solution: depend on the viewfinder to focus, calibrate infinity stop with the lens at the temperature where ring must be fully turned against stop, so at other temperatures the lens can be focused to infinity with ring turned somewhat less than against stop.

For many lenses, changes in focus with temperature changes is probably indiscernible. I would propose an experiment. Calibrate the lens in cold temperature, maybe at outside at night, then check calibration in warm temperature, maybe next sunny day. I think the 50/1.2 will show any significant differences. Maybe there is no problem!

[poilu]

I calibrate my lenses, it is helpful if the lens cannot reach infinite
http://forum.mflenses.com/infinity-tune-for-contax-lens-t7011.html
but I don't often use the infinite mark, even a 40m target is less than infinite

[trifox]

Vilhelm - superb examples

Tuning infinity on lenses such as 50 f1.2 is extremely important

I have done several conversions of FD xx with initial aperture f1.2 and I always do infinity in 100 per cent..

High speed lenses really behave differently when infinity is NOT SPOT ON even at shorter distances.

All checks are being done via LiveView on 5DMkII (21 mpx) and then RAW developed ...

50 f1.2 ? Playing with infinity is absolutely essential

tf

[Excalibur]

****High speed lenses really behave differently when infinity is NOT SPOT ON even at shorter distances.****

If I see the subject sharp at 5' away (and it could also show appx 5' on lens barrel), why does it matter if the lens doesn't/does stop at infinity.

[Esox lucius]

Thank you Stan

[trifox]

[Excalibur]

[Esox lucius]

This is not a film vs. digital issue. It's a focusing issue.

None of us is so skilled that we can every time nail exact correct focus with an f/1.2 lens at close distances, DoF is only a few millimeters max. Viewfinder focusing is not accurate enough method to produce academically validated and comparable sample photo series, when study goal is to detect and define differences in three different copies of the same lens. Which is the very reason I quadruple-verified focus using live view.

The sample shot before calibration service evidently also revealed to the repairman exactly how to adjust the lens, speeding up the process of adjusting it (ie. making price lower). I'm guessing he could see from the sample photo's abnormal amount of vignetting and CA and loss of detail that the lens brought for service actually focused past infinity (as opposed to the two others which in those photos revealed they don't reach infinity). He is a very experienced Nikkor repairman, now retired and running a small photo business. Likely the most qualified person in Finland who fixes older Nikkor lenses, I trust him due to many good experiences.

[visualopsins]

Oh! I see now...for some lenses, setting infinity focus when focus ring stops is essential.

I imagine for these lenses the focus ring turns multiple helicoids, that by setting infinity focus with ring at stop, the differential movements between helicoids begin at a common point. Setting infinity focus before the focus ring stops causes the helicoid differential at closer focus distances to be off, hence the CA and increase in other aberrations.

[Ignas]

[Esox lucius]

http://www.kamera-apu.fi/yhteystiedot.php

tel is +358 9 685 3813

I honestly don't know if the gentleman speaks English but I will be happy to help you should you need translation. Just let me know what should be fixed (pm me for skype details) and I can relay the info to him, his shop is 5 blocks from my studio and I often walk past it.