Ok, the plot thickens that we are seeing a 4" telescope design here. The length (very long for a tele lens but normal for a scope) being the fifth hint or so.
So, I'd like to contribute a bit more insight about what to expect of such designs.
Let me assume for the best which seems to be an optical 4 element
Petzval design plus maybe an additional LFF (Large Field Flattener). A Pentax scope of similiar specification is the
Pentax 100 SDUF or
Pentax 105 SDP. In this
PDF, you'll find a scientific measurement of the resolution of this Pentax scope (
German!). The Petzval design dates back to physicist Jozef Maximilián Petzval and was first used in 1841.
The best direct comparative tests of such a scope design and an advanced 10+ element photo tele lens design I could find was conducted by Samir Kharusi from Oman who has an amazing pool of devices
In the first test, he compares 140mm optics (as opposed to 100mm) using what appears to me being the 21 MP Canon 1DsmkIII FF camera.
In the second test, he compares 60mm optics (as opposed to 100mm) using the 10 MP Canon 40D APSC camera.
Note that in the first test, he uses a Petzval design plus additional Large Field Flattener. Also, he clearly misses exact focus sometimes with the tele lens.
Direct sources are here:
->
televue_canon
->
TV-60is
From this and other sources I conclude that a
good scope design has the same excellent optical performance
in the center and at infinity focus compared to a more complex photo tele lens design. However, it's performance drops off when approaching the edges or a closer focus. OTOH, contrast and clarity in the center seems to even outperform the more expensive 10+ element tele lens designs.
So, I think I can summarize the pro and cons of the forthcoming 560/5.6 already:
SUMMARY
IMHO (based on peliminary information as of today) the Pentax DA 560/5.6 is a tele prime lens at a good price point (sub $2000) which rivals the optical performance of much more expensive glass, near the center and at infinity focus, good enough to be used with a tele converter and with good clarity and contrast. OTOH, the lens is very long (and possibly heavy) and the performance will degrade towards the edges of the image field which makes it better suited for APS-C than full frame although it most likely would cover a medium format image circle. Or all in one: Be Different