I wanted to see just how far I could push this lens, to see if it would be possible to capture any features of Jupiter, so I did some testing indoors before attempting the challenge of field photography with such a tiny angle of view. While I have the Pentax L-converters, neither one fits on the DA*300, so I used the cheap 4-element TCs I have available. I wasn't really expecting this to work, so I was surprised and pleased by the results. One key is of course the extraordinary resolution of the lens. The other, I think, is that even cheap TCs do well at the image center, and with the Q that's all we see.
My test subject is a printed box shot from about 6 meters away. I put the box at an angle so that I could then select the best-focused part in PP, and hence to take focusing variation out of the comparison. I used flash to also take motion blur out of the comparison, and didn't adjust the power, other than putting a diffuser on for shots where I was getting overexposure, so I've adjusted exposure (and white balance) in PP. I did sharpening subjectively, trying to maximize detail in each shot without creating sharpening artifacts. For each configuration I took one shot wide open and another down one stop; in each case the stopped-down shot was better. (I also did two stops down with the lens alone; it was no better than one stop down.)
All 100% crops.
1. Lens alone. In the thick gray lines I can see some texture, but not detail per se.
2. With 1.4x. Now the texture of the thick gray lines resolves into nearly vertical rows of dots, but it's hard to discern separation between the dots.
3. With 2x. Now I can clearly see the separation between the dots in the thick gray lines.
4. With stacked 1.4x and 2x. I see no loss of detail compared to the 2x alone.
To my eye, each addition in magnification adds additional detail, or at worst does not lose any detail, while making the image larger. I'm surprised for several reasons. One reason is that while the lens has proven itself on the Q, magnifying it by an additional 2.8x is asking a lot. Another reason is that these are cheap TCs, and while I knew that both perform well at the image center, at least on APS-C, again this is really pushing things. A third reason is diffraction. With both TCs and the lens at 5.6, this is effectively f/16.