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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 07 Apr 2009 (Tuesday) 10:06
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180 degrees diagonal fisheye

 
shutterfiend
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Apr 07, 2009 10:06 |  #1

What does it really mean?

If this means, that if I were to stand at the door and take a photograph of the room with my camera tilted, I'd be able to see edges of the door frame at two corners of the photograph, that isn't happening. At least not with my Canon 15mm f/2.8 fisheye.

Perhaps the 5D sensor is slightly slightly smaller than 35mm film.


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JelleVerherstraeten
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Apr 07, 2009 10:22 |  #2

there are some fisheyelenses that will perform that way. the 4,5mm of sigma will do that and the sunex 5,6 to, maybe others, I don't know that.


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tkbslc
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Apr 07, 2009 11:32 |  #3

It might be 178 degrees. Manufacturers are known to round up on specs.

Also, are you talking the finished image, or viewfinder view? The veiwfinder is not 100% coverage.


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gjl711
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Apr 07, 2009 11:48 |  #4

15mm on a crop is only going to give you a 75 degree FOV across the frame. Going to a FF camera will give more, about 100 degrees, but still far from 180 degrees. A 1mm lens on a full frame approaches 180 but still is not quite there.


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tkbslc
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Apr 07, 2009 12:03 |  #5

gjl711 wrote in post #7685162 (external link)
15mm on a crop is only going to give you a 75 degree FOV across the frame. Going to a FF camera will give more, about 100 degrees, but still far from 180 degrees. A 1mm lens on a full frame approaches 180 but still is not quite there.

focal mm on a fisheye is different than normal lenses. Canon lists the 15mm fisheye as having a 180 degree FOV.


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gjl711
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Apr 07, 2009 12:07 |  #6

tkbslc wrote in post #7685240 (external link)
focal mm on a fisheye is different than normal lenses. Canon lists the 15mm fisheye as having a 180 degree FOV.

Hmm.. interesting.. really?


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gjl711
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Apr 07, 2009 12:09 |  #7

Son of a gun.. it does.. :)
http://www.usa.canon.c​om …id=7321#ModelTe​chSpecsAct (external link)


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tkbslc
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Apr 07, 2009 12:11 |  #8

gjl711 wrote in post #7685259 (external link)
Hmm.. interesting.. really?

Yeah, the Tokina 10-17mm fish is like twice as wide as the 11-16mm as well. The big round fisheye front element throws off normal FL calculations.


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shutterfiend
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Apr 07, 2009 13:13 |  #9

tkbslc wrote in post #7685085 (external link)
It might be 178 degrees. Manufacturers are known to round up on specs.

Also, are you talking the finished image, or viewfinder view? The veiwfinder is not 100% coverage.

That's what I think too, more like 170. Advertising fluff. I didn't get to play around with focusing. It may give a wider FOV when the lens is fully retracted.

I'm talking about the image uploaded on my computer. I try getting my numbers in order when I'm playing around. It's only when I'm working that I throw the book out the window :D.


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rdenney
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Apr 07, 2009 15:22 |  #10

shutterfiend wrote in post #7684538 (external link)
What does it really mean?

If this means, that if I were to stand at the door and take a photograph of the room with my camera tilted, I'd be able to see edges of the door frame at two corners of the photograph, that isn't happening. At least not with my Canon 15mm f/2.8 fisheye.

Perhaps the 5D sensor is slightly slightly smaller than 35mm film.

A fisheye lens makes a circular image that (by definition) provides 180 degrees of coverage edge to edge of the circle. Within a small range of tolerance, a fisheye lens will have a focal length of about one third the diameter of the circle at infinity focus. Thus, an 8mm fisheye makes a circle about 24mm in diameter, and a 15mm fisheye makes an circle about 45mm in diameter.

If we include the circle entirely within the frame, the diameter of the circle can be no larger than the narrow dimension of the frame. That's why an 8mm fisheye makes a full circle on a 24x36 frame.

If we include the frame entirely within the circle, the diagonal of the frame can't be any larger than the diameter of the circle, or we'll cut off the corners. The diagonal dimension of a 24x36 frame is 43mm, which fits entirely within the image circle of a 15mm fisheye. Because the corners of the frame very nearly reach the edges of that circle, the 180-degree coverage extends very nearly from corner to corner diagonally across the frame.

We call a fisheye whose image circle fits within the frame a "circular fisheye", and the fisheye whose circle is large enough for the frame to fit within it a "full-frame fisheye", but really they are the same. The "full-frame fisheye" of 15mm focal length would create a 45mm image circle, which would (very nearly) fit within the narrow dimension of a 6x4.5 format camera. So, the same 15mm fisheye is a full-frame fisheye on 24x36, but a circular fisheye on 6x4.5. The diagonal of 6x6 is about 80mm, and thus it takes about a 30mm fisheye to serve as a full-frame fisheye.

But the lenses are designed with a little slop, because the very edges are often degraded in performance. A true 180-degree full-frame fisheye on the 24x36 format would be about 14.5mm focal length. Given how much of that coverage is crammed into the very edges of the circle, that half millimeter becomes significant. Most lenses nominally provide 180-degrees of coverage, but in practice provide a hair less in order to provide good sharpness in the corners.

As I recall, the 5D sensor is only about a tenth of a millimeter smaller in both dimensions than 35mm film's nominal size.

Rick "working with approximations" Denney


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ben_r_
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Apr 07, 2009 15:53 |  #11

Yea I was thinking it did too, thats why I remembered buying it! Pretty cool!


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BscPhoto
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Apr 07, 2009 16:11 as a reply to  @ ben_r_'s post |  #12

I've always had a fisheye lens in my line up until recently. The 14mm is actually slightly longer than the 15mm and Canon has made it to control the barrel distortion really well but it still gives you this crazy wide look to your shot.
FOV isn't everything.


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Ook
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Apr 07, 2009 16:15 |  #13

rdenney wrote in post #7686411 (external link)
A fisheye lens makes a circular image that (by definition) provides 180 degrees of coverage edge to edge of the circle. Within a small range of tolerance, a fisheye lens will have a focal length of about one third the diameter of the circle at infinity focus. Thus, an 8mm fisheye makes a circle about 24mm in diameter, and a 15mm fisheye makes an circle about 45mm in diameter.

If we include the circle entirely within the frame, the diameter of the circle can be no larger than the narrow dimension of the frame. That's why an 8mm fisheye makes a full circle on a 24x36 frame.

If we include the frame entirely within the circle, the diagonal of the frame can't be any larger than the diameter of the circle, or we'll cut off the corners. The diagonal dimension of a 24x36 frame is 43mm, which fits entirely within the image circle of a 15mm fisheye. Because the corners of the frame very nearly reach the edges of that circle, the 180-degree coverage extends very nearly from corner to corner diagonally across the frame.

We call a fisheye whose image circle fits within the frame a "circular fisheye", and the fisheye whose circle is large enough for the frame to fit within it a "full-frame fisheye", but really they are the same. The "full-frame fisheye" of 15mm focal length would create a 45mm image circle, which would (very nearly) fit within the narrow dimension of a 6x4.5 format camera. So, the same 15mm fisheye is a full-frame fisheye on 24x36, but a circular fisheye on 6x4.5. The diagonal of 6x6 is about 80mm, and thus it takes about a 30mm fisheye to serve as a full-frame fisheye.

But the lenses are designed with a little slop, because the very edges are often degraded in performance. A true 180-degree full-frame fisheye on the 24x36 format would be about 14.5mm focal length. Given how much of that coverage is crammed into the very edges of the circle, that half millimeter becomes significant. Most lenses nominally provide 180-degrees of coverage, but in practice provide a hair less in order to provide good sharpness in the corners.

As I recall, the 5D sensor is only about a tenth of a millimeter smaller in both dimensions than 35mm film's nominal size.

Rick "working with approximations" Denney

Thus concludes the thread!


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shutterfiend
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Apr 07, 2009 16:16 |  #14

BscPhoto wrote in post #7686708 (external link)
FOV isn't everything.

It is when your single assignment in 3 months was a sweet sixteen party!!!

In other words, I have no place to go except sit at home and measurebate.

... and be glad that I have a day job... still.


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rdenney
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Apr 07, 2009 16:49 |  #15

BscPhoto wrote in post #7686708 (external link)
I've always had a fisheye lens in my line up until recently. The 14mm is actually slightly longer than the 15mm and Canon has made it to control the barrel distortion really well but it still gives you this crazy wide look to your shot.
FOV isn't everything.

The 14 is a rectilinear lens, not a fisheye. It does not provide a 180-degree coverage at the edges of the image circle, nor is it possible for a rectilinear lens to do so.

You just can't really compare the two, because their geometric projection is utterly different. It's (rather literally) like comparing a square to a circle. When is a square bigger than a circle? When the square breaks out of the circle, when the square fully contains the circle, or somewhere in between?

I found that my 16mm fisheye seemed to sweep up about the same amount of scene as my 14mm rectilinear lens, but the central subject is larger and the corners reach further to the sides. That was on the small sensor, which cut off the most compressed part of the fisheye projection. On the full-frame camera, they are just too different to meaningfully compare.

Rick "a long-time user of fisheye lenses for much more than the fisheye effect" Denney


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