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How good is the Minolta MD APO Rokkor 400/5.6
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PostPosted: Wed Dec 13, 2017 7:57 am    Post subject: Reply with quote
I found this
https://www.flickr.com/photos/pineauguy/21163779543/in/album-72157656876079674/


PostPosted: Wed Dec 13, 2017 9:19 am    Post subject: Reply with quote
It may be nice to compare the 400 mm with the mirror 500 mm f8 as both will have very little CA


PostPosted: Wed Dec 13, 2017 11:13 am    Post subject: Reply with quote
VLR wrote:
I acquired an APO Tele Rokkor this year.
...
Nonetheless, images shot with the lens were - to put it mildly - disappointing. Double contours everywhere, impossible to focus. See this thread for some images.

Obviously the same problem i (and others) had encountered as well. Interesting.


VLR wrote:

After fiddling with the optics, I put a 0.1 mm spacer in between the rear group and the diaphragm which fixed the double lining and returned the lens to an acceptable performance. The bokeh is still nervous, though. I really don't know what could be wrong, as I see no parts where an optical adjustment could be made and it doesn't look like the lens was dropped or even slightly bumped in its life.

I know it's probably my copy, but overall I'm not impressed with this lens.

Interesting, again. Why did you have the idea of increasing the space, and why just 0.1mm? I mean there are lots of other possibilities ...


VLR wrote:

On the plus side, it's very well corrected CA-wise and only shows some minor pink fringing in extreme backlit situations (tree branches against the bright sky). Minor means: I wouldn't even criticize modern lenses for this little bit of fringing.

Yep, that was my impression as well. At f11 (when the strange optical problem described above was corrected by stopping down) the lens had very little CAs. A modern Canon EF 5.6/400mm is even better (no CAs at all), but the Rokkor 5.6/400mm APO has as little as CAs as e. g. the Canon FD 2.8/300mm Fluorite or the Minolta AF 2.8/300mm APO. And these lenses certainly are well regarded in terms of color correction.

Stephan


PostPosted: Wed Dec 13, 2017 11:24 am    Post subject: Reply with quote
Lightshow wrote:
I found this
...


At least here in Switzerland, i would not consider moon photos as a reasonable test for a long lens. While the images are nice to look at, their resolution always is limited by air turbulence. If i put the FD 400L on my A7, and then try to focus using the magnifier, it looks as if i would see the moon surface through the waves of an ocean ... all the mountains and craters look very unstable, moving, ... all because of the moving air above me ...

I litterally have tried to compare 400mm and 500mm lenses many times, looking at the alps (about 10-15km from my home). Nearly impossible, even when the air is very clear. Movement all around that becomes easily visible in viewfinder even without focus magnifier!! The only reasonable way to compare such lenses is using the grass and trees on a hill, about 1 km away, on a calm days without sun. And that's how we did our Rokkor 5.6/400mm APO test, and the nFD 2.8/400 tests.

Stephan


PostPosted: Wed Dec 13, 2017 12:37 pm    Post subject: Reply with quote
stevemark wrote:
Interesting, again. Why did you have the idea of increasing the space, and why just 0.1mm? I mean there are lots of other possibilities ...


Well, I checked the lens for possible adjustment points and found none. Then I tested for strong decentering and couldn't detect any. As the rear-last element had some minor fungus damage in the coating, I first suspected this caused the double lining. But since the damage is random, I later came to the conclusion that it should have a different effect. Maybe globally reduced sharpness, loss of contrast and/or a diffuse glow. The double lining just looked like there was something off in the axial direction. Either another reversed element (which I ruled out), a manufacturing defect or element spacing. And the distance front to rear group was the only spacing I could adjust, which is why I tried.

I started by loosening the rear group thread by ~20° CCW at a time and did a test shot with each rotation up to about 180°. The image was best around 80-90° CCW. Since I couldn't measure the gap in the thread because it is burried inside the lens, I started testing it out by adding strips of Scotch tape (Tesa film) inbetween the rear group and the lens body. At three strips, it was just above 90°. Three strips add up to ~0.13 mm. Taking into account that the tape is slightly compressible and that 80° would be fine, too, I looked for 0.1 mm steel spacers for ball bearings and finally found some with the right inner diameter of 25 mm. Cut them down a bit, since they were too broad, and installed one. Lens performs OK now and the change is fully reversible.
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