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PostPosted: Wed Oct 15, 2008 6:44 am 
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Joined: Mon May 05, 2008 10:58 am
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Location: Brunswick Heads, Australia
The perfect lens imho is the Sigma 12-24mm EX DG.

Full frame 12mm, if you ever upgrade to a 5D or higher, or shoot on a EOS film camera.

equivalent of 20mm on a 1.6x crop.

Lovely and sharp, straight lines stay straight.

I have fantastic success with it on my 30D and NN5L.

I also use a 50mm f/1.4 which i got for $50 AUD off ebay last year (Olympus G.Zuiko) which is adapted, fantastically sharp @ f/4.


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PostPosted: Thu Oct 16, 2008 3:30 am 
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Joined: Sat Oct 04, 2008 6:19 pm
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Do you have any recommendations in a fixed focal length (non-zoom)? Important things for me would be very sharp, can stop down to f/22 or smaller, and exact infinity focus at the exact end of the focusing turn (my current kit lens goes just a bit beyond infinity focus). I also do not need autofocus, though having it can be a plus for some casual uses.


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PostPosted: Fri Oct 17, 2008 3:01 am 
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I have now found that there are at least 3 different types of fisheye lenses as discriminated by their "mapping functions". 2 of them are of particular interest (equidistant and equisolid). You can see the math for these at http://en.wikipedia.org/wiki/Fisheye_lens#Mapping_function


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PostPosted: Fri Oct 17, 2008 11:35 am 
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Location: Grenoble
Phil Howard wrote:
I'm starting to question what a given focal length even means for a fisheye lens. For a rectilinear lens, it should behave geometrically like a pin-hole lens at a distance from the image plane equal to the focal length in consideration. But a fisheye has no such comparison.

As far as I understand:

For any lens using any projection mode, the focal length corresponds to the distance betwen said lens and the image of a very distant object located on the lens axis (infinity.) For a 16mm fisheye this distance is twofold the 8mm fisheye distance. (BTW, this distance correspond to a segment, one of the ends of this segment is located on the sensor but I don't know how the other one is defined !)

For rectilinear lens, image size depends only of the focal length. Doubling the focal length always results in doubling the image size (because all rectilinear lenses share the same "pinhole like" projection mode.)

No such rule for fisheyes. Different fisheye use different projections. Different fisheyes having the same focal length but different projection mode have different FOV and the corresponding image circle diameter varies.

Before using a rectilinear lens, photographers should know its FOV but, because the FOV only depends of the focal length, they usually just ask for its focal length. For normal and tele lens this works very well but the wider the FOV the more they rather should ask for the FOV...

Before using a fisheye, photographers must ask for both the radius of the image circle and for the corresponding FOV (see important note.) When the image circle diameter is larger than the sensor diagonal, horizontal FOV, vertical FOV and diagonal FOV are usefull characteristics ...

Important note: APP knows fisheyes focal length and camera sensor size but often fails (or don't attempt?) to place the image circle. Here is an example:





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Last edited by GURL on Fri Oct 17, 2008 11:44 am, edited 1 time in total.

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PostPosted: Fri Oct 17, 2008 7:33 pm 
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GURL wrote:
Before using a rectilinear lens, photographers should know its FOV but, because the FOV only depends of the focal length, they usually just ask for its focal length. For normal and tele lens this works very well but the wider the FOV the more they rather should ask for the FOV...

Before using a fisheye, photographers must ask for both the radius of the image circle and for the corresponding FOV (see important note.) When the image circle diameter is larger than the sensor diagonal, horizontal FOV, vertical FOV and diagonal FOV are usefull characteristics ...

Photographers must also ask for the mapping function of the fisheye lens, too. There is more than one type of fisheye. A rectilinear lens is just a wide angle lens with a different mapping function (known as "gnomonical"). Choices in fisheye lenses include "equidistant" (what I described in an earlier post before I found all these variations, which keeps distances the same), "orthographic" (gives the appearance of looking at an orb), "equisolid" (keeps areas the same, instead of distances), and "stereographic" (I'm not exactly sure I understand this one).

GURL wrote:
Important note: APP knows fisheyes focal length and camera sensor size but often fails (or don't attempt?) to place the image circle. Here is an example:

It will also need to know the mapping function of the lens in order to know how to simply correct it's geometry, if that differs from the intended geometry produced by stitching (which itself could be any of these mapping functions, including gnomonical for FOV well less than 180 degrees). My goal is to select a lens that has a mapping function equal to the mapping function of the panorama that is intended to be produced. I believe that would be the equidistant one. Apparently, a number of fisheye lenses are of the equisolid type, instead.

These mapping functions are similar in issue to what map makers have to deal with. For a map of a town, that is like a telephoto lens and the differences are too small to notice. For a map of a continent or larger, the differences are substantial.


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