Sigma 15mm fisheye focus problem

I'm using a Sigma 15mm F2.8 EX DG with a Canon 5D in a Nauticam housing with a 4.3" dome and am having problems getting it to focus, auto or manual. The Nauticam chart recommends a 10mm extension, which I always assumed was just to prevent the lens sunshade from hitting the dome; I cut off the sunshade, so I'm not using an extension.

I've also read of using a 1.4 extender, which I don't know anything about. Does anyone have experience with this same setup and knows exactly what's required? Thanks!

I don't have specific knowledge of this setup, but usually the extension requirements are for two reasons: to avoid hitting the lens, sure, but also to put the dome far enough forward that the lens can focus on the virtual image formed.

The Sigma 15mm has a pretty tight minimum focus distance (5.9 inches), so I'd think it would work in the small dome...but you may still need the correct extension to get the len nodal point closer to the center of curvature of the dome.

Other things you might try are stepping down a bit, which can certainly help edge and corner sharpness. You mention having difficulty with manual or auto-focus so I assume it's not an issue with not finding a good high-contrast point for the autofocus to work with.

You mention having difficulty with manual or auto-focus so I assume it's not an issue with not finding a good high-contrast point for the autofocus to work with.

I'm usually using this lens for video, and I always manually focus video since the continuous focus on most DSLRs is slow. I've got a SmallHD external monitor with focus assist that helps, too. But even when using auto-focus and getting the "green box" focus confirmation, the video will still be out of focus. So I'm thinking you're right about moving the dome out. I've never had this problem with other cameras/lenses/ports, but this is the only lens where I'm not following the manufacturer's recommendation.

Um, I feel embarrassed about even responding now. Just checked out your photostream on Flickr. I'm simply not worthy... <hangs head in shame>

One of the main requirements of using the appropriate extension ring is to ensure the centre of the dome port is aligned with the entrance pupil of the lens:

http://www.marinethemes.com/domespacer.html

http://wetpixel.com/articles/dome-theory

Um, I feel embarrassed about even responding now. Just checked out your photostream on Flickr. I'm simply not worthy... <hangs head in shame>

Ha, that's hilarious. I'm a long-time technical guy (computers), but haven't had time to really dig into the technical details of photography, and I'm pretty much still winging it. I'm hoping to have more free time this year and change that, though.

One of the main requirements of using the appropriate extension ring is to ensure the centre of the dome port is aligned with the entrance pupil of the lens:

 

http://www.marinethemes.com/domespacer.html

http://wetpixel.com/articles/dome-theory

Awesome, thanks for the links. I'll be ordering a 10mm extension ring tomorrow!

The 15mm isn't happy behind a small dome, quite apart from alignment of the dome with the nodal point or the entrance pupil and physical obstruction of the lens. That said, I've never felt the need for manual focus with a big dome or even with a smaller dome and teleconverter.

To follow up on this, I added the 10mm extension ring, and that fixed my focus problem.

The 15mm isn't happy behind a small dome, quite apart from alignment of the dome with the nodal point or the entrance pupil and physical obstruction of the lens. That said, I've never felt the need for manual focus with a big dome or even with a smaller dome and teleconverter.

The Nauticam port chart recommends either a 4.3" or 8.5" dome. I've got a 10" dome that I haven't used in years because the 4.3" creates less drag when mounted to the scooter. The video I shot after adding the ring was nice and crisp, though, so I think the 15mm works pretty well with the 4.3" dome.

In the PNW, our waters are usually murky, which necessitates getting pretty close to the subject. At those distances, I find manual focus a lot easier, especially when combined with the focus assist mode of the SmallHD monitor.

I think you need the extension if you're not getting focus. Even with it you'll have corner issues with that small a port. But for video it should be ok, especially if you stop down.

Jack

I use the Hugyfot 4,88 inch dome with the Sigma 15mm and have no focus issues.

The main problem with this lens is the very short lens body and the 180° fov leading to vignetting.

On the Hugyfot housing and no extension ring there is no vigneting just on certain distances a small stripe of the sun hood can be visible,

while on my Athena 174mm dome and a 10mm adapter ring was bad vignetting.

Sometimes it just depend that the camera is locked all foreward as a few mm can be fine or too much.

doublecheck that no vignetting occur using the 10mm extension ring.

Chris