Over the years, one criterion I’ve used to evaluate long glass is how it renders small black print on a white background. The two Canon f/11 lenses excepted somewhat, this appears to be a weak point in lenses that use diffractive optics, as well as many other mid- and consumer-level optics I’ve tried. It’s as though the reflection of the white at contrasting edges obscures the black side of the border.
Of the five or six copies of the 300PF that I’ve owned over the years, the current copy is the worst. After an outing in bright overcast with good air quality this afternoon, it appears to be unworthy of a 1.4X or 1.7X. Sample variation has long been a known issue with this lens, which is manufactured in China. This copy seems to have been finished after a four-beer lunch on a Friday afternoon. Really too bad, because the adapter from the land of cheap accessories is a very good fit.
Living in this part of the world for as long as I have, one comes to understand the excuses for varying levels of quality coming from some parts of the region, dependent largely on company structure and ownership, secondary to culture. Which is why I wouldn’t rule out trying a product of DJ Optical. If I were feeling lucky . . .
The surest bet, though, remains on Leica.