Originally posted by northcoastgreg The only thing Pentax about this lens are the HD coatings, the mount, and the Pentax label.
I'm not sure about that, from looking at the marketing blurbs both for the 24-70mm f/2.8 lenses from DPR.
From Tamron:
" ...It includes three Low Dispersion (LD) glass elements, two Extra Refractive Index (XR) glass elements, three glass molded aspheric elements"
and From Pentax:
"By incorporating three Extra-low Dispersion (ED) optical elements and four aspherical optical elements, the lens assures sufficient light levels even at the edges of the image field"
There is an extra Aspherical in the Pentax lens, and no mention of the XR lens elements by another name. Which suggests that there have been some alterations made to the Tamron design. One aspherical lens can make a surprising difference when it comes to controlling focus shift caused by residual spherical aberration ( which is a known issue with the Tamron 24-70mm f/2.8)
P.S:
The optical formula for the current Tamron 24-70 f/2.8:
Last edited by Digitalis; 09-25-2015 at 04:35 AM.