Originally posted by Timothy has a less advanced optical design, and is significantly slower than a comparably priced third party option
So far you are making the assumption that
- because it's a telescope lens design it will be not as good as complexe optical design
- it will be more expensive than competitor
- it will be slower than competitor.
And all this are merely false assumption
Good telescope lens are very good opticaly, if not better than "usual" lens design sometimes.
The main difference is that telescope don't need to be very "transportable" : you deploy it on a place, and you don't move it again until you decide to go home.
The opposite is that telephoto lens are suppose to be easily movable.
Originally posted by Timothy -lacking or limiting exotic elements...
Those exotics elements are usualy made to keep the lens quite mobile and good opticaly. Telescopes don't need those as much as telephoto because they don't need to be light.
Pentax here, decided to build a lens that will be collapsible to reduce the space it takes during transportation.
Thus, making it f5.6 to keep it light, and somehow cheap.
Fact is when the sky is bright but with cloud, there is usualy enought light to use a a f5.6 or f8 lens with 1/500.
Pentax already produce a telescope that is a 560mm f5.6 so i guess they adapted the formula with some tweaks to fit it to DSLR.
Annoncement is not far so time will tell.