[JensE]

Don't forget the Voigtländer 20mm f3.5. You should stop it down though to achieve this freaking sharpness you want.

As my contributions to the 20mm thread show, I do like my Color Scopar a lot for its rendering and compactness, but it never gets fully sharp in the corners before diffraction hits when stopping it down. I think it would be corner-sharp with a very slight crop (maybe ~30MP) at the shorter edges (4:5 ratio) though.

[torashi]

I use a Rokinon 20mm 1.8 (Samyang) and the Cokin "P" holder on my K1. When you get to 20mm and wider on full frame your options are limited.

Can you post any work you've done with that lens? I'm really interested in it. A good copy on FF is the closest thing there seems to be to the SMC DA15/4 on APS-C.

[JensE]

A good copy on FF is the closest thing there seems to be to the SMC DA15/4 on APS-C.

For this, you could really look at the Voigtländer. They have fairly similar characteristics - I own and use both. The Voigtländer, while very good, isn't quite as flare immune as the HD-DA 15/4, but otherwise I can use them interchangeably on their respective formats with consistent results.

[torashi]

For stitching, it helps to have a gimbal rig where you can rotate the camera and lens around the entrance pupil (or was it the exit one?) to avoid any parallax shifts. This is definitely a method for landscape and architecture without moving subjects. And taking that in mind, if you take enough overlapping pictures a few minutes apart, you can even edit any people or cars out if it's a crowded place.
If you look around, there are descriptions on how to position the camera and lens correctly on a gimbal for this. As a warning, an adjustable, rigid gimbal don't come in cheap.

[torashi]

For this, you could really look at the Voigtländer. They have fairly similar characteristics - I own and use both. The Voigtländer, while very good, isn't quite as flare immune as the HD-DA 15/4, but otherwise I can use them interchangeably on their respective formats with consistent results.

Well, the size of the Voigtländer is clearly the more portable option, while the Rok is better for wide field astro. I wish I could get my hands on one copy for a fair price.

[digitaldaydreams]

Thanks to all for all the advice. I'll be revisiting this thread as I have time. For now, I'll use a single position shot on a tripod, no panning no stitching. I'll do 2 shots from that position. One shot at a high f stop (whatever it takes) for corner sharpness and one shot (pixel shifted) at a stop for best sharpness overall, other than the corners. Then I'll replace the unsharp corners with the sharp corners in post. Seems like a quick enough solution that still uses the whole sensor rather than cropping out the corners. So now I just need to pick and purchase the lens.
At some point in future, I'll probably wind up doing automated panning cropping and stitching for massive tack sharp prints, since that's really going to be best when I have more money and time for the learning curve.

[bdery]

Show us the resulting picture!

[Dan Paris]

I tried a Samyang 20mm and returned it after few days. Image were soft on the borders and in the corners wide open, as I more or less expected, what I didn't expect is that it didn't improve that much while stopping down. At f/5.6 image quality was still unsatisfactory near the borders - even downsampled to 2048 pix. wide.