p.1 #1 · Differences between circular and diagonal fisheyes?
Is there a difference between the projection of circular and diagonal fisheyes?
I know circular fisheyes inscribe their circular image inside the rectangular frame of the sensor, leaving a portion of black frame in the output image.
I know diagonal fisheyes project their image circle so the entire sensor is "covered" by the fisheye image.
But if I were to crop down a circular fisheye so there was no black empty frame in the image, would it show the same diagonal 180 degree angle of view as the diagonal fisheye? (With fewer pixels of course.) Or would there be a difference in the look of the image?
In other words, is a diagonal fisheye simply a circular fisheye with a larger image circle?
p.1 #2 · Differences between circular and diagonal fisheyes?
It depends on the lens. Most modern fisheyes have a 180 degree coverage, across the 44mm diagonal, which results in the full coverage. Many older fisheyes, such as SMC Takumar and FE-Takumar 17/4, and 18/11 have 160 degree coverage on the 44mm diagonal, so they're also "full image", but don't have quite as wide coverage. There's even a few fisheyes (I can't remember which) that have 36mm diameter coverage, so that most of the top and bottom edges of the image are flat, but you can see the curvature on the left and right edges (I don't recall if they're 180 deg).
p.1 #3 · Differences between circular and diagonal fisheyes?
Yes, no, and maybe. Yes, because if you think a bit, that can be done, but not easily. No, because of the differing focal lengths. Maybe, maybe not because I'm not a lens designer/engineer.
Make a print of the circular fisheye image and superimpose a frame of suitable size, and you will get something approximating the full-frame fisheye. However, the hurdles involved to get the much larger image circle at the sensor plane are probably vastly different from getting a small image circle. Remember, I'm not an engineer, nor did I sleep at a Holiday Inn Express last night. I do have a wonderfully sharp logical mind clouded by age and years of heavy drinking and drugs. So there.
p.1 #4 · Differences between circular and diagonal fisheyes?
CKrueger wrote:
In other words, is a diagonal fisheye simply a circular fisheye with a larger image circle?
Yes. As Jim pointed out, there have been various designs with different focal lengths, and not necessarily covering a full 180 degrees (at least not within 22 mm of axis, which covers a 24x36 mm frame). There was even a Nikon fisheye that "saw" beyond 180 (200 degrees?), and projected all of it in a circle within the frame (only film when that was made). But your guess is basically right on the money. As far as I know, they all have similar patterns of distortion, and would all look pretty similar if you chose image sensors to reveal the full circle for each.
BTW, I have a Peleng 8 mm fisheye which does the 180 circle on full frame. On a 1.3 crop, it is almost frame filling, and on 1.6 it doesn't quite see 180 diagonally. Unfortunately, it doesn't approach the edge quality of the fancier fisheyes until you get to the 1.6 crop, at which point it makes a very usable fish with pronounced barrel and maybe 160 degrees or so of diagonal view.
p.1 #5 · Differences between circular and diagonal fisheyes?
Cool, thanks guys. That's exactly what I wanted to know.
I'm mulling a circular fisheye, a diagonal fisheye, or even Tokina's fisheye zoom (now that I've learned it can still cover 180 degrees on a FF camera simply by zooming in slightly). I'd rarely shoot a circular frame, so knowing there's no real difference in projection (assuming both lenses cover 180 degrees) simplifies my decision.
p.1 #6 · Differences between circular and diagonal fisheyes?
I have a Sigma 15MM diagonal fisheye for full-frame. Now with digital, I got a Tokina 10-17MM fisheye. Granted, they aren't everyday lenses, but they can be a lot of fun.
