Luminous Landscape Forum
Site & Board Matters => About This Site => Topic started by: OldRoy on October 15, 2015, 05:18:16 am
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" As mentioned above, this lens is a stereographic fisheye. What this means is that unlike fisheye lenses that produce round images, the entire surface of a full-frame sensor is covered,"
The above statement is plain wrong.
I own 4 FE lenses, in FF (2 of them) Crop and M43 formats. All have an image circle which covers the entire sensor. Of course if I put a DX 10.5 mm lens on a FX body I get an (almost) circular image - should I so wish, which I seldom do.
That said I've yet to see a defished image from one of these lenses that looks any good. That shown in this article - so far as I can tell from looking at it in the browser - is by far the best I've ever seen.
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I think the point was to contrast those fisheyes which produce a circular image on the sensor, from those whose image covers the entire sensor. It's true that this has nothing to do with the type of projection, so the sentence could be clearer.
In fact there are two distinct issues: size of image (does it cover the whole sensor, or just reach the top & bottom edges); and type of projection. The Samyangs (the 8mm works the same on APC as the 12mm on FF) have a different projection than eg Canon fish-eyes, so they seem to bulge less in the middle. That's probably why the de-fish seems to work rather well.
There are other types of projection: equi-distant, equi-solid-angle, gnomic, etc etc. See under "mapping function" halfway down:
https://en.wikipedia.org/wiki/Fisheye_lens
BTW, there is an error: the article claims 180° horizontal angle of view. If that were the case, there could be no sides on the hood. I think the 180° is the diagonal angle of view, as is the standard measure for all other lenses.
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I have the Rokinon (aka Samyang) 8mm f/3.5 version for Nikon DX, and it's just as sharp. Although, the DoF is great enough that it's harder than heck to focus. (grin) It also de-fishes nicely. it also has a different kind of distortion than other fisheyes I've used, so that may have been why removing the distortion is more successful.
tom
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Yes, stereographic projection is unique to the Samyang designs for 8 and 12mm - most camera fisheye lenses use equisolid projection - see https://en.wikipedia.org/wiki/Fisheye_lens for diagrams and formulae. PanoTools also has more info, I believe.
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...That said I've yet to see a defished image from one of these lenses that looks any good. That shown in this article - so far as I can tell from looking at it in the browser - is by far the best I've ever seen.
The advantage of the method which MR used (the Photoshop warp filter) is that it allows the post processor to concentrate on anything in the image that doesn't look good, while leaving the rest more or less untouched. You can even use a selection to restrict the application of the filter. It is my favourite method for tidying up panoramas as well as defishing. Using a projection to correct a whole image sometimes creates as many visual problems as it solves.
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Yeah, that sentence could easily be misinterpreted.
But I agree with the findings. I have the Rokinon 8/3.5 for K mount and it's amazing for the price. I got it on sale for $240 and have no regrets!