Originally posted by Blue That was the only thing I could see that would hold it back. It has 3 ed elements in it.
The number of ED* elements alone cannot determine the performance of the lens, it is the grade of ED glass used and how they are employed in the optical design that makes a difference. It appears to me that Pentax is using High grade ED glass as it is expensive and use of exotic glass types is a requisite for apochromatic lens design - but the upshot is with high grade ED glass that you can get away with using less of it to obtain a high degree of optical correction. I will point out that the Sigma 100-300mm f/4 APO EX DG has only four SLD** elements in it and while it is a superb lens, it still suffers from a small amount of chromatic aberration.
*
Extra-low
Dispersion - was historically made using regular borosilicate glass with thorium dioxide added to it, this glass type was discovered to be potently radioactive and was phased out. Modern ED glass is
typically composed of a small percentage of Lanthanum along with other elements such as titanium dioxide added to stabilize the glass structure and improve durability - however with high amounts of lanthanum, even with these additives ED glass can become very soft compared to standard optical grade borosilicate glass.
**
Super-
Low
Dispersion - the differences in chemistry between this and standard ED glass is an increase in the percentage of lanthanum dioxide, along with zirconium dioxide which adds to the expense of production as this glass requires the use of Platinum crucibles in order to make it.
Last edited by Digitalis; 02-14-2015 at 01:51 AM.