Defishing and Refishing with the Samyang 7.5/3.5 and the Panasonic 7-14/4

Started Jan 23, 2013 | Discussions thread
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Anders W
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Defishing and Refishing with the Samyang 7.5/3.5 and the Panasonic 7-14/4
Jan 23, 2013

Just having received the Samyang (aka Rokinon) 7.5/3.5 fisheye, I couldn't resist the temptation to give it a spin yesterday just to see how it would compare for various purposes with the Panasonic 7-14/4 that I am used to shooting.

Sometimes the fisheye distortion actually adds to rather than detracts from the composition. At other times, however, all you want is the extremely wide angle. The image below, from the main building of Uppsala University, is, to my mind, an example of an image where the fisheye effect is more undesirable than desirable. On the other hand, defishing to the rectilinear projection (the one used by any ordinary lens) wouldn't work well because of the extreme stretching of the corners and edges it implies. The lack of resolution as well as the extreme distortion ("fattening") of objects close to the left and right edge looks horrible to my eyes.

Samyang 7.5/3.5 at f/5.6

But here, and in many other cases, the Panini projection comes to rescue. This is a projection that cannot be natively accomplished by any lens since it is not "azimuthal", i.e., not symmetric in all directions from the center. It keeps vertical lines straight along with all radial lines (lines that pass through the center). However, horizontal lines that aren't radial are still more or less bent. By making that concession, it avoids the extreme stretching of the left and right edge that the rectilinear projection implies when the FoV becomes sufficiently wide.

Samyang 7.5/3.5 after (partial) defishing to Panini

The image below shows the same scene as captured by the Panasonic 7-14 at 7 mm. Although the horizontal FoV is less than that of the fisheye (102 versus 132 degrees), the "fattening" of things at the edges is already obvious, e.g., the girl and the door at the left edge.

Panasonic 7-14/4 at f/4 and 7 mm

But just as you can (partly) defish a fisheye image to Panini, you can of course "refish" a rectilinear image to Panini as well. Here's what the "refished" image produced by the 7-14 looks like. As you can see, the girl and the door to the right have now regained far more natural proportions.

Panasonic 7-14/4 at f/4 and 7 mm after (partial) "refishing" to Panini

As you can see, the "refishing" of the rectilinear image to Panini implies a loss of vertical FoV in the center of the image. This can, if we want, be avoided by modifying the Panini projection by stretching it vertically at the edges. The same operation simultaneously makes horizontal lines less strongly bent at the upper and lower edge. The concession we make (there must always be one) is that radial lines are no longer straight, as illustrated by the image below.

Panasonic 7-14/4 at f/4 and 7 mm after "refishing" to modified Panini projection

Comments and questions of all kinds about the utility of UWAs and various projections (more than about the images as such, which are just meant to serve as modest examples) are of course welcome.



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