2 Minoltas, 2 Rokkors, and Infinity

[yessammassey]

Just opened up my new-to-me Minolta 24mm f/2.8 W.ROKKOR-X lens. Another eBay crapshoot. (My first try netted me a copy that was badly decentered, I returned it.) Upon mounting it to my XD-7, loading a roll of Fomapan, and getting up onto my roof for some test shots, I find that:

• The microprism collar focusing aid in the viewfinder shows the telltale 'out of focus' shimmer on everything, when the lens is turned to infinity.
• The split-image just won't quite come together on distant objects when the lens is fully rotated toward infinity.
• Distant images cannot be brought into perfect focus by turning back (i.e. the lens is not going past infinity).
• Everything up to about 10m away can be brought into focus in the viewfinder.

Ok, so the lens is mis-calibrated, I think to myself. Maybe somebody tinkered with or 'repaired' it at some point. Lack of infinity focus notwithstanding, every 24mm MF SLR lens that I've found that's been 'worked on' or 'repaired' has had pretty crap corner performance. I'm going to box this back up and return it.

But before I do that, I gave it another shot on my SRT101. I take my old 28mm "MD" lens off the SRT, and mount the troublesome 24mm. Now it seems to focus to infinity just fine! Granted, the SRT doesn't have a split-image focusing screen, but the microprism spot doesn't shimmer at all when pointed at distant subjects with the lens turned to infinity. I'm able to bring the prism into focus on anything I want. Strange.

I put the old 28mm on the XD, focus to infinity, everything is fine... that's expected. I put the 28 back on the SRT, fine. I put the 24 back on the XD... it comes up short of infinity again.

What gives?

...I hesitate to add this final observation, because it's almost too subtle to notice and probably isn't relevant to the issue, but it almost, just barely looks like the 28mm doesn't come completely into perfect critical focus at infinity when mounted on the XD. The split image comes together fine, but there's a very, very, very faint shimmer in the microprisim collar. I've always just chalked it up to the f/2.8 max aperture. My 28mm f/3.5 Nikkor does the same thing, almost.

 

[shutterfinger]

I hate microprism focusing screens. They fooled me tooooo many times on closeups. I change the screen whenever possible or focus on the matt next the the collar.
Now if it were mine I would look up old manuals for the camera and verify that the lens is compatible.
If compatible I would sit it up on a tripod and set the lens focus scale to center of the infinity mark and shoot a target at least 5000 feet away with the lens wide open. I would then measure from the film plane mark on the camera (unsure Minolta's have that feature) and focus as best as possible on the focus screen at 50 feet, 25 feet, 15, feet, 10 feet, 5 feet, and 3 feet, make an exposure at each distance with the lens wide open. then process the film and check the target focus with a loupe. A sheet of 4x5 ISO100 film exposed to bright sunlight for each time for 1 minute, 3 minutes, 5 minutes make good general purpose neutral density filters once processed.

I have a 20 x24 inch artist canvas with several copies of Norman Koren's test target http://www.normankoren.com/Tutorials/MTF5.html , some enlarged USAF 1951 targets, and a ISO 12233 http://www.graphics.cornell.edu/~westin/misc/res-chart.html all printed at a Epson Photo printer's highest resolution.
I also have some of these targets on large sheets of foam core for testing/calibrating purposes also.

 

[yessammassey]

I should mention that the XD has been tested for focus accuracy at closer ranges with a 58mm f/1.4 lens at max. aperture. All my focus targets came back spot-on with those tests, performed about a month ago. But they were all at 2m or less.

Digging around, I see that some people have posted about inability to reach good infinity focus with the XD's viewfinder in the past, but as of now I haven't found any threads where a definite resolution was identified. Anyone with some experience with this camera model? Or a similar issue with their 35mm SLR camera lens?

I'm holding out hope that maybe the problem lies with my camera body and not this increasingly rare & expensive lens.

 

[MattKing]

Is there a surrounding matte focusing area in the finder, and does it appear in focus at infinity?

If so, it may merely be an incompatibility between the focusing aid and the lens.

Those microprism focusing aids often don't like slow lenses. Maybe slow, highly retro-focus lenses are even more unsuited to that focusing aid.

 

[naaldvoerder]

A critical focus test will tell you whether the camera is to blame, the position of the focusing screen for example. Most lenses can be adjusted to focus to infinity with a screw usually positioned under the rubber skin on the focusing collar. Maybe a previous owner made an adjustment using the lens on a Canon D5 or what have you.

 

[shutterfinger]

My test target(s).

The first is plain paper prints on canvas board, calibrated a lot of rangefinders with this one, 2 and 3 are glossy prints on foam core. These take the guesswork out of focusing problems.

 

[ic-racer]

Those microprism focusing aids often don't like slow lenses. Maybe slow, highly retro-focus lenses are even more unsuited to that focusing aid.

This^

 

[yessammassey]

I can try out comparing film plane focus vs vf focus appearance. I've got a method for that.

Pulled out my 58mm 1.4, and it focus appears spot-on at infinity with the XD, so yeah, I guess it's something wrong with the lens or something to do with the focusing screen not liking the 2.8 max aperture. But the 28mm 2.8 seems to get to infinity just fine.

I have an older XD parts body. I think the viewfinder & mirror are still intact. I'm going to test the 24mm on it, as well.

 

[yessammassey]

Update: Not reaching infinity on the other XD, either. I think the lens is not correctly calibrated.

I may take a closer look at the distance scale accuracy, just to see if it is off for nearer subjects (or if focus is good for everything but infinity), but from my point of view, this is not a lens that can be trusted to deliver sharp landscape photos. I know that for small aperture shooting on a tripod I'll probably have everything in focus anyway, but I do too much handheld and night shooting, so not having true infinity is a deal breaker.

Another 24 going back. I wonder when I'll find a good one..

 

[Sirius Glass]

The f/1.4 58mm is a wonderful lens. Do not make the mistake of getting rid of it like I did when it got a newer Minolta.

 

[yessammassey]

I got my 58mm for a true bargain price because there was a very small gnat lodged on the backside of the front element group. It's also absolutely soiled with internal dust - the kind that's only visible with application of a very strong LED light but which makes the glass look frosted when lots of light is passing through. I've never really noticed any of that having an effect on the final images, though. Maybe a little bit of contrast reduction from the dust haze (?), but certainly nothing from the gnat.

 

[yessammassey]

Before I send it back, because I'm not seeing a good replacement for this lens being available from the online sites at this time, can anyone tell me if it's likely that the lens is repairable?

The lens distance scale markings reach infinity, but 'infinity' doesn't appear in-focus in the viewfinder. Could this be fixed by simply opening up the lens and making a small adjustment, or would a tech need to pull the glass element groups out and reseat everything?

This is the cleanest example of any manual focus 24mm lens I've ever seen... probably because somebody took it apart and cleaned it? I'd like to hang on to it, if it could be made to work properly without a lot of trouble or expense.

EDIT: Just checked, and the distance scale markings are off, across the board. It's actually focusing closer than what the markings would indicate. There's also a gap between the ring fasteners around the front element, and then lens identification ring plate:

I don't know if it's supposed to be like this, but it doesn't quite look right.