Defishing fisheye images to rectilinear projection

This morning I vacuumed and dusted my room. After that, I installed DXO Optics Pro 5.3 to see how it work with de-fishing my Tokina 10-17mm/3.5-4.5 fisheye images. The problem I'm having with images taken with this lens is that if you tilt the lens up or down to try and get the horizon on a third line, the horizon will become quite bowed due to the fisheye effect.

Defishing the image, using a Cylindrical or Equirectangular projection, you will still get the same bowed Horizon line unless you place the image so as to correct for converging verticals. If the original image was shot with the horizon not through the center of the image, then this will mean you will need to crop off a lot of the bottom or top of the image (depending on if you placed the horizon through the top or bottom third of the image), and you'll end up with something similar to what you would have got if you had shot with the Horizon through the center, except you'll have less pixels due to the cropping.

Defishing the image using rectilinear projection, the edges (maybe about 25% of the image either side left and right, assuming a 10mm Landscape oriented shot), gets very distorted. I wanted to try DXO to see if I would still get the same problem. I did get the same problem, and also got some nasty CA that I don't see when converting the RAW in CaptureNX, and then defishing in PTGUI.

I did some googling to try and see if the Tokina fisheye image was much wider than the Sigma 10-20mm lens, and if this was why the defished Tokina 10-17mm/3.5-4.5 fisheye images were much more distorted than Sigma 10-20mm images. I found a thread on panoguide that discussed the two lenses Pentax 10-17mm fisheye vs Sigma 10-20mm, which has quite a bit of info in it.

According to that thread,

the Sigma 10-20 will give you a horisontal FOV of 76 and the Fisheye will give you around 90.

So the Tokina 10-17mm fisheye has 18.4% wider HFOV than the Sigma 10-20mm Rectilinear lens, which doesn't seem like a lot (though adds up when doing 360° panos).

When I viewed the defished images that I processed through DXO, while I didn't like the heavy distortion at the left and right edges of the image, I did like the stretched effect that the clouds got. I think that probably I will need to try taking some images at 17mm, then defishing those to a rectilinear projection, and see how they look.

After that I tried defishing a 10mm to rectilinear projection in PTGUI, to see how much info I would loose by cropping off the distortions. I found that cropping off the distortions at the edges, the defished image would actually have more pixels than the original, however, the image was quite soft at the left and right edges. Cropping a bit tighter, to the same pixel width as the original image, the softness wasn't quite so bad at the edges (especially since the image hadn't been sharpened), but the image ratio was now square rather than landscape. Of course, you could also crop off the bottom and top of the image to restore the original image's ratio, and probably not loose any resolution, however you would loose even more of the original image's field of view.

I made a Flickr set of the different methods of defishing the image, with notes: Defishing Tokina 10-17mm/3.5-4.5 images set on Flickr

Really I would like to have a Sigma 10-20mm lens so that I could take the same shot with both lenses and compare them directly, unfortunately I don't have access to this lens, and it's too expensive for me to purchase at the moment.

When reading the panoguide thread about the Sigma 10-20mm rectilinear lens versus the Tokina 10-17mm fisheye lens earlier, I noticed Hans saying that the entrance pupil on a fisheye varies, and you should try and set your pano head to avoid parallax where the stitching seams will be. I didn't know this, and have probably calibrated my head to have no parallax where the edge of the images overlap, rather than in the middle of the overlap (where I assume the stitching program would create the seam). So I will probably need to try and re-calibrate my pano head sometime before I off on holiday in October.

In the afternoon I went on Animal Crossing, then spent the rest of the day working on my pog website trying to get it ready to upload to the webserver (as I've moved hosts there are various bits on it that need changing).

Food
Breakfast: Ludlow Food Centre Celebration Jam toast sandwich; cup o' tea.
Lunch: Slice of toast; vegetable fake cup a soup; Satsuma; Timeout (single); cup o' tea.
Dinner: Breaded fish portion; peas; potatoes; salt; ground black pepper. Pudding was Rhubarb and apple pie with a bit of cream. Piece of Sainsbury's caramel chocolate; Piece of Sainsbury's Truffle chocolate; Piece of Sainsbury's Turkish Delight chocolate; coffee.