You are correct there, Shel. In addition actual focal length changes with focus. External focus lenses get longer as you focus closer. Internal focus lenses get shorter. I suppose one could do a cam coupled interal/external focus lens that did not change focal length with focus, but I have never seen one.
graywolf http://www.graywolfphoto.com "Idiot Proof" <==> "Expert Proof" -----------------------------------
Shel Belinkoff wrote:
Plus, there's something of a flaw in just using math to determine the AOV ... while a lens may be nominally rated at a particular focal length, it may not in actuality be that focal length. Depending on the actual focal length of the lens the difference may be significant. As a Leica user I'm sure you've noticed that Leica has the actual focal length coded on the barrel of some of their lenses. It seems that, apart from the nominal FL often being an approximation, variations in the build of each sample can be different as well. And, if you recall, not long ago Cotty put up some pics made with lenses of similar focal lengths that seemed to show a greater difference than would have been expected had the nominal FL of the lenses been precisely as indicated by the manufacturer.
While it's nice to know how to figure out all these technical things, I
feel that nothing beats making a photograph to get to the truth when it
comes to lenses and all their characteristics.
Shel
[Original Message]
From: Rob Studdert
On 23 Apr 2005 at 10:10, Shel Belinkoff wrote:
Too much math, not enough photography .... I don't understand the need
for
equations, nor do I understand the math, the equations, or your
conclusion.
Like I said, the field of view, regardless of your math, is greater with the Zenitar than it is with a 20mm lens on a 35mm camera. Have you
looked
through both focal lengths on a 35mm camera? Have you actually compared photos side-by-side?
From a practical perspective you are of course correct Shel, the fisheye
lens
will provide a wider AOV than the a rectilinear lens of equal focal
length.
However due to the physics you will find that the fisheye effect
diminishes as
the image is cropped (and this can be easily described in mathematical
terms,
as Cory showed). So a 16mm fisheye on an APS sized DSLR frame looks far
less
fishy than you might expect.
Cory said: "So, what I *meant* to say is that FOV of the fisheye is about
equivalent to a 13-14mm rectilinear lens on the -DS, or a 20mm
rectilinear
lens on a full-frame 35mm body."
How I interpreted that paragraph was that a 16mm lens on a D or DS body produces an angle of view roughly equivalent to the AOV a 20mm lens will provide on a full frame 35mm body, and I agree.
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