05-12-2015, 02:39 PM
Aspheres cause there to be funky hotspots in the image, the more you add the worse things get. E.g this happens:
http://i.imgur.com/9w7v9PV.jpg
where a lens may have little to no coma in the corner, but "a lot" of coma in the center.
The tamron does not have an asphere, so that can't be the case. The first few elements need to be made on a large diameter grinder and likewise for the polisher, which is expensive. Lower quality machines exist, but they leave surface defects - this is probably the case. You can actually fairly clearly see the steps in the grind/polish process as the tool or workpiece is moved and then spun (the rings in the bokeh).
Such defects will hurt contrast and typically do not result in a "clear aberration" in the image, a very narrow faced edge would have to be made which is unlikely.
It most likely has to do with the VC, atmospheric conditions, or subject motion.
Looking at the image more closely there appears to be a circle centered around that spot which is also fuzzy, so it may be something to do with a filter of some kind, a bad element inside the lens, or some other defect.
http://i.imgur.com/9w7v9PV.jpg
where a lens may have little to no coma in the corner, but "a lot" of coma in the center.
The tamron does not have an asphere, so that can't be the case. The first few elements need to be made on a large diameter grinder and likewise for the polisher, which is expensive. Lower quality machines exist, but they leave surface defects - this is probably the case. You can actually fairly clearly see the steps in the grind/polish process as the tool or workpiece is moved and then spun (the rings in the bokeh).
Such defects will hurt contrast and typically do not result in a "clear aberration" in the image, a very narrow faced edge would have to be made which is unlikely.
It most likely has to do with the VC, atmospheric conditions, or subject motion.
Looking at the image more closely there appears to be a circle centered around that spot which is also fuzzy, so it may be something to do with a filter of some kind, a bad element inside the lens, or some other defect.