Asymmetric loss in resolution when stopping down

[jng]

Hi everyone,

I have what may be a strange question that hopefully someone here can help me answer...

I recently acquired what appears to be a very nice copy of a Hassy Zeiss 250mm Superachromat that I'm using on a V system body(ies) with a 60 MP IQ160 back. The lens is incredibly sharp and from what I can tell thus far, the name "Superachromat" is well earned. With that said, over the course of putting the lens through its paces, I picked up a weird effect. Shot wide open at f/5.6, the image appears to be remarkably sharp from edge to edge and corner to corner, with perhaps just a touch of softness (maybe) at the left edge/lower left corner of the image field. So far, so good. When I stop down to f/8, however, the image quality at the left edge actually *degrades* significantly whereas the center and right edge hold up fine (no discernible loss in resolution due to diffraction).

Below are some 100% crops from frames I shot of the moon (true infinity focus!), focusing the lens with the moon at the center, taking a shot and then swiveling the camera to place the moon at the left edge or right edge of the frame. The top row shows images shot at f/5.6 and the bottom row shows images shot at f/8.

So, my question is: what might be out of alignment to account for one part of the frame being sharp wide open but blurry when the lens is stopped down (in separate tests, things get progressively worse when stopping down to f/11)? Testing on a star chart suggests that the lens itself is properly centered and the camera body was just calibrated and aligned. Could the diaphragm be decentered or otherwise mis-aligned?

Thanks in advance!

- John

 

[Alkibiades]

I would make the test with another lens. maybe the interface adapterplate on your back is not OK.

 

[jng]

Thanks. I should have mentioned that my 180mm does not show this issue. Also a misalignment of the back seems unlikely to me given that the loss of sharpness is seen only when stopping down.

- John

 

[ErikKaffehr]

Hi,

Try to put the lens in front of a projector or a flashlight and look for fogged areas. No better idea…

Best regards
Erik

 

[Jack]

One guess would be fungus on an internal element in that area of the light path. The fungus could be either active or cleaned at some point, but it can leave a pitted spot.

 

[D&A]

I've seen something like this with a couple of different lenses in years past. They were lenses that contained a machine made aspherical element. It would result in a region of blur as though vasoline was smeared on a very tiny portion of a lens element. My guess at the time was that it was a defect of the machine poured aspherical element which I believe has some sort of liquid resin as part of its makeup and then it ultimately hardens.

To hypothesize why it becomes prominent when the lens is stopped down...maybe as the rays of light are more well focused as the apature is closed, the light is more concentrated as it passes this opaque region on one of the lenses elements. Again just conjecture. The region of blue that I detected was not in a corner of an image, but more inward about 1/3rd the way into the frame.

Dave (D&A)

 

[jng]

Thanks to all for your ideas. The glass and aperture blades look clean, but I doubt that I'd be able to distinguish any subtle defects in the lens elements. I thought perhaps that the diaphragm was mis-aligned, but centering using a star chart appears to be fine and stopping down doesn't seem to make any difference. For now my work-arounds are 1) shoot the lens wide open for far-distant landscapes (fortunately, resolution is near-optimal wide open) and 2) use the lens on my Flexbody, whose lens mount is rotated 90 degrees compared to the 500 series bodies, placing the soft part of the image field at the bottom and therefore outside of the sensor area when shooting in landscape orientation. I suppose a fellow can have worse problems, but still, this is all a bit of a mystery to me.

- John

 

[jng]

Quick update on this weird issue with my 250mm Superachromat: I spent a little more time down the rabbit hole by checking the lens again on my 501CM body (recently aligned and calibrated) and Flexbody. I was mistaken - the same part of the image field gets soft when stopping down on both cameras, which I think rules out any misalignment or other defect in the lens itself since the lens mount of the Flexbody is rotated 90 degrees compared to the 501. I also tested my 180mm and 350mm and found that although the image sharpness at the edges wasn't degraded upon stopping down, there was purple fringing on the right (good) side and green fringing on the left (bad) side, which I take to indicate longitudinal CA owing to front and back focusing, respectively. Of course, there's no fringing with the Superachromat - Zeiss wasn't exaggerating when they affixed this name on the lens! This suggests to me that the sensor and/or body is not aligned square with the lens. There's still the puzzle of why the lens is sharp wide open but blurry stopped down on the one side. I'm guessing that some combination of focus shift plus a sharper roll-off in focus with the SA compared to other lenses could explain things. Stopping down would push the left side of the image away from original focus plane whereas it would push it closer on the right. In any case, adding around 1 degree of tilt brought the bad side of the image back in focus at all apertures (but at the expense of sharpness on the other side). So, I guess I'll need to contact my Phase One dealer to see about getting the back checked out.

- John