Re: fisheye or roundshot?

[Glenn Barry <glenn@xxxxxxxxxxx>]

Hi Jook,

I have the Sigma 15mm f2.8D EX fisheye that I use on 120 for spherical panos, the
nodal point is very close to the front of the lens, mine is the AF version which
may differ from yours, but the nodal point is locatd around the middle of the
focus ring which is approximately 32mm from the lens mounting flange. This value
is approximate, but may give you a good starting value.

Glenn

Jook Leung wrote:

> To provide this discussion list with a visual example, I posted an image
> taken with a Roundshot 220vr camera and a Sigma 15mm full frame fisheye
> lens.
>
> You'll notice that the base of the camera (the battery) is in focus while
> everything further out is smeared.
>
> I had no published info on where to set the rear nodal point "h value" for
> this lens, so I used "a take a guess value" for the 14mm Sigma 2.8EX lens
> but that's the rectilinear version.
>
> http://360vr.com/pages/rs_fff_test.html
>
> regards,
> Jook
>
> >On 15 Apr 00 at 13:49, Glenn Barry wrote:
> >
> >> The easy way to see this is to put the lens on look through the viewfinder
> >> or ground glass and watch the degree of image distortion as you rotate the
> >> camera. Fisheye mapping and compression requires that this is quite severe,
> >> more so at the edges agreed, but it is present throughout the frame
> >> including the centre. Just look at a straight line in the middle of the
> >> frame. It must begin to curve the moment you rotate the lens due to the
> >> fisheye projection. There will always be some blurring.
> >
> >It is claimed that only a single-pixel wide digital 'slit' can overcome
> >this blurring problem....in that case, no pixel is exposed at more
> >than one rotational position, unlike any photo-chemical registration
> >with a meaningful slit-width (a slit would have to be smaller than a
> >hair to mimick the digital one-pixel concept, which would kill the
> >image due to diffraction (remember that the digital slit is not a
> >slit, but only a recording single row of pixels, so diffraction is
> >not relevant there).
> >
> >Not quite sure whether it is valid to explain the blur itself with
> >the varying magnification of a fisheye....a rectangular lens has the
> >same magnification from corner to center, a fisheye does not I
> >believe (this even goes as far as the claim that a 14mm fisheye has
> >less variety from corner to center than a 17mm fisheye with the same
> >overall angle of view, hence the fisheye 'effekt' of the 17mm is
> >larger).
> >And since film speed of a rotating camera is depending on
> >focal length, this is exactly the problem....no speed can suit both
> >center and edge at the same time.
> >
> >--
> >Bye,
> >
> >Willem-Jan Markerink
> >
> >
> >      The desire to understand
> >is sometimes far less intelligent than
> >     the inability to understand
> >
> >
> ><w.j.markerink@xxxxx>
> >[note: 'a-one' & 'en-el'!]
>
> =============================================================
> JOOK LEUNG
> 360VR Photography
> 44A Honeck Street
> Englewood, NJ  07631
> http://360vr.com   voice: 201.894.5881    fax: 201.894.5882
> http://bigapplepenthouse.com

--
Glenn Barry Photography
2/13 Gerroa Avenue
Bayview
N.S.W. 2104
Australia
Ph (612) 9211 3080
Mobile 0415 279 366
E-Mail: mailto:glenn@xxxxxxxxxxx
Web: http://www.acay.com.au/~glenn