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Odd aberations at pixel peeping level with the Tamron 150-600mm
#1
Hi guys,

             As it's quiet here so I'm posting some observations. I've been using the Tamron 150-600mm zoom for about a month now (second copy, first de-centered) and am very  pleased with it, it's easily hand holdable and for the most part a sharp lens, holding up very well till 500mm where it's very good at F-8-11. At 600mm the inevitable happens and sharpness drops a little, but with a little sharpening very decent results remain. The VC is a great bonus and works very well although it has a slight tendence to hang on and let go, which causes a little jerking if you have drunk too much coffee. The AF needs no AFMA and seems spot on. 

 I've noticed however, on looking at images @ 500-600mm at 100%,  most noticeable in the fine detail of grass, very small patches of localized blurring occurring in the frame. I've also noticed this with the Bigma maybe to a worse effect.

  I just wondered if this is caused by the lack of perfect smoothness of the lens elements which causes  these localized  patches of softening and it does suffer from onion ring specular highlights,   I don't think it's of any real concern given 100% magnification and am not suggesting a fault with the lens, just observing this effect!

 The accompanying image shows this small soft patch (there seem to be three or four of them across the frame) in an otherwise decently sharp 600mm  100% crop. I've seen this several times always on lenses of 400mm or longer.

 

  Klaus, Airydicus?

 

 

  Sorry I should have highlighted the soft area but it is in the middle of the clip!

#2
[ATTACHMENT NOT FOUND]

 

  Highlighted area.

#3
Heat blur?

#4
Quote:Heat blur?
It's true it was a fairly hot day!

 

A RAW shot @600mm F8, you can see one of the lens's minus points here, onion ring specular highlights. I think it shows good sharpness for a zoom lens!

 

https://www.dropbox.com/s/ka5r15v8c6qj8q...2.NEF?dl=0

 

 

The same image in PSD form with the background softened using the adjustment brush.

 

https://www.dropbox.com/s/n407fxdvefu11h...8.psd?dl=0

#5
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.

#6
Thanks for your autopsy, Airydiscus, strange business as the Sigma 50-500m scewdrive non OS and both the samples of the Tamron have done it, always at the long end. 

#7
It probably has to do with the optics around the OS/VR unit not being "properly" desensitized to tilts.  Theory is that the IS/VR will only translate the group in the X-Y plane with no Z movement or tilt, but in practice this is never the case so the optics there must be insensitive to tilts.  If this is not done with sufficient care, the OS/VR will impact image quality in a way that is time consuming to predict with computers and impossible to predict by eye.

#8
  I'll try a test on a tripod some time with the VR off!

 

 

 

   Thanks!

#9
  A long time later I think I now know what's going on.............

 

      I think it's Nissan bokeh that causes the effect....the spiting of OOF subjects making double edges , given the right distance apart these double edges combined with the double edges of other grasses  etc. (by chance). and produce interference patterns that  mask detail by adding and subtracting "clutter" in localized areas of the image.

 It never happens on flat areas even when there is much detail!

#10
Yes. My guess is something was in front of the soft area; very close and small so it was diffused by the time you reach the subject. If you were to go back to the location and re-shoot the image i bet the area would be gone. I do not think it is heat or atmospheric; as it is too localize.

  


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