Re: Nikon wide angle lens question - confused on one aspect


[This is a rewriting of my response after rethinking parts of it.]

"dj_nme" <dj_nme@xxxxxxxxxxx> wrote in message news:467a91bb$0$22433$5a62ac22@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Don Stauffer in Minnesota wrote:
On Jun 21, 8:24 am, DeanB <deanbrow...@xxxxxxxxx> wrote:

I don't understand this:
http://www.nikonusa.com/template.php?cat=1&grp=5

Why is it the 14mm lense has a 114 degree FOV, while the 16mm has a
full 180? Is there something special about fisheye lenses (the 16mm in
this case) vs. non-fisheye?



There are various types of perspectives (rectangular, spherical,

orthographic, reverse-spherical, cylindrical, isometric (one best

approximated with very long telephotos, as in old Japanese

paintings and drawings of buildings made without perspective

effects), etc. In all, made by all lenses, the focal lengths are rated

similarly (by the distance of the lens optical center from the

sensor at infinity focus) - but the perspective types (and distortions

within the perspective types caused by lens design shortcomings)

may cause straight lines of subjects running off axis of the image

center (of an unshifted lens relative to the sensor) to be curved,

which causes a shift in image magnification away from the center

of the image, sometimes permitting a greater angle of view to be

photographed using some lens perspective types. Or, a truly

"distortion-free" rectangular-perspective type of lens will show

the same magnification in the center as a truly "distortion-free"

spherical-perspective type of lens (fisheye) of the same FL, but

as you look away from the image center, the fisheye will show

increasingly less subject magnification, and therefore more

included subject area (a greater angle of view).



Indeed, when we get to extreme fisheyes, the idea of focal length
controlling field of view breaks down.



Not really - see above...



In order to truly get 180
degree FOV with conventional optics, it would take zero focal length,
which is of course impossible. So extreme fisheye lenses MUST have
distortion.



Not "distortion", but a different set of perspective rules - see above...



Effectively, the focal length is not constant with field
angle. That "distortion" gives the field of view at the more extreme
field angles.

One other possibility is that the 14mm f/2.8D ED AF isn't a full frame lens, as this isn't specifically mentioned in the
description for this lens.



It is full-frame...


The 16mm f/2.8D AF Fisheye-Nikkor is specifically described as having "Full frame 180 degree fisheye coverage...", so I can only
guess from this that it will definitely cover the whole 24x36mm 135 film frame.



It will...


Another possible reason for the 14mm f/2.8D ED AF not being as "wide" as the 16mm lens is that it might be a rectilinear
(distortion corrected) lens.
This tends to "stretch out" the edges of the image to make objects appear the same size at the same distance regardless of how
off-axis it is and make the angular coverage less than a non-rectilinear lens.



Ah, close - but it doesn't really "stretch-out" the image edges. Try

a pinhole camera - it renders perfect rectangular perspective, but has

no optics to "stretch" things...;-)


I don't have either lens and can't do anything other than speculate.

Perhaps looking through pbase.com for images tagged as being made with these lenses will answer this aspect of the question?



See above...;-) I also have an article, "On Seeing and Perspective" on

my web page, at www.donferrario.com/ruether/articles.html#perspective

in which I try to explain how we see, and in which perspective type (it

may surprise you, but it is easy to prove that we see in spherical

perspective...;-).
--
David Ruether
d_ruether@xxxxxxxxxxx
http://www.donferrario.com/ruether


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