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
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