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Old May 16, 2010, 11:02 AM   #1
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Default Circular vs Rectilinear Fish Eye

What's the differnece? THe DA 10-17 is listed as a Circular Fish Eye.
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Old May 16, 2010, 1:03 PM   #3
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I'll try to explain. Original and true fisheye lenses produce a 180 degree circle on the image plane (film, sensor). With a square frame (like a 24x24mm or 6x6cm film camera) the corners are black. With a rectangular frame (like a FF/full.frame, or HF/half.frame APS-C camera) much of the frame is black. For FF, a ~10mm fisheye lens produces such a circle; for HF/APS-C it takes a ~6mm fisheye lens.

Camera- and lens-makers know that most customers don't like to 'waste' so much of the frame, nor to deal with the exposure problems caused by so much blackness, so they s-t-r-e-t-c-h the len's image circle to fill the frame, without correcting the fisheye distortion. The Zenitar 16/2.8 prime and Pentax DA10-17 zoom are such lenses. The Zen16 fills both FF and HF frames. The 10-17 only fills a HF/APS-C frame. [NOTE: the 10-17 could fill a FF frame if its little lens hood could be removed. As is, I can avoid FF vignetting only in the 14-17mm range.]

Fisheye distortion remains in those lenses. And camera- and lens-makers know that many customers don't like such distortion, even though it can be removed by de-fishing software. So the makers go to great expense to s-t-r-e-t-c-h- and b-e-n-d the image circle to straighten out the edges. Such a straightened untrawide lens is called rectilinear. And you the customer will go to great expense to buy such a lens. Even I might, one of these days. Isn't the new Sigma 8-16 rectilinear? And I only have to rob a couple mini-marts to pay for it?

The reason I bought the Pentax K20D as my first dSLR was because I really wanted to shoot fisheye / ultrawide, and the DA10-17 was the most affordable lens for that. But it's not a circular fisheye. So I got a huge Kenko 180 degree adapter, mounted on zooms in the 40mm neighborhood -- and it pretty much sucks. And I've tried other FE adapters, and they all pretty much suck. Maybe after knocking over a couple more mini-marts, I'll be able to afford a REAL fisheye lens. Sigh...
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Old May 16, 2010, 1:18 PM   #4
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Thanks IE and RioRico: I recall seeing someplace the DA 10-17 referred to as a "circular" FE, but in looking for the reference, I can't seem to locate it. Perhaps it is a "full frame" FE since in my experience, it never produced an image that was inscribed within a circle.

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Old May 17, 2010, 11:30 AM   #5
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Quote:
Originally Posted by jelpee View Post
Thanks Scott and RioRico: I recall seeing someplace the DA 10-17 referred to as a "circular" FE, but in looking for the reference, I can't seem to locate it. Perhaps it is a "full frame" FE since in my experience, it never produced an image that was inscribed within a circle.
Hi jelpee,

The DA 10-17 is a 180 diagonal fisheye, which gives you the 180 on the diagonal as opposed to a circular fisheye, which would capture 180, in both the vertical and horizontal planes, thus giving you a round image within the rectangular frame. I believe the Zentitar 16 FE is a circular, but only on 135 format, so it covers the entire APS-C frame due to the crop. I don't know what the actual diagonal coverage is, but it's still pretty wide.

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Old May 18, 2010, 6:13 AM   #6
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Quote:
Originally Posted by snostorm View Post
I believe the Zentitar 16 FE is a circular, but only on 135 format, so it covers the entire APS-C frame due to the crop. I don't know what the actual diagonal coverage is, but it's still pretty wide.
As I mentioned above, the Zen16 fills both HF and FF frames; on FF it's slightly less than 180 degrees diagonally, very much like the DA10-17 set to 16mm. Put the Zen16 on a HF/APS-C camera like the K20D and it has the same coverage as a 24mm lens would on a FF camera, about 60 degrees, but with the characteristic fishy distortion.

I use and love the Zen16 but it's problematic -- wide open at f/2.8, it's not quite as sharp as the DA10-17 (which is at f/4.5 there). The Zen16 *is* quite sharp from f/4 onwards, but by sacrificing its main advantage, speed. The DA10-17 is much more flexible for various situations, and has fine IQ. The Zen16 forces a bit more disciplined approach to shooting.

What is any fisheye / ultrawide FOR? To gather in a wide view; to capture a small space; to close in on a subject, pulling it from its background; to push distance away; to exploit near-infinite DOF. And with a fisheye, to distort. And they're tricky to use. Every mm, every slight angle, makes a great difference in the captured image. They're not for the shy nor shaky.
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Last edited by RioRico; May 18, 2010 at 6:17 AM.
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