I want to try out NIkon's new 2.8mm lens!

480sparky

Senior Member
Yep. 2.8mm!

It's on their website:

Newnikonlens.jpg



:p
 

FastGlass

Senior Member
Yes, very interested in seeing what it can do. Sounds like you're tempted enough to try one. Can't wait to see some wild shots.
 

Bob Blaylock

Senior Member
Obviously, someone made a silly mistake entering data into the web site, entering the lens' widest ƒ stop in a field that ought to have been the focal length.

Untitled-1.jpg

It does make me wonder about something.

In an SLR, there is only so close that the lens can actually be to the focal plane without running afoul of the mirror. Just off the top of my head, it seems that a good rough estimate of this minimum distance would be the height of the frame multiplied by the square root of two—so with a DX-format sensor, about 22 millimeters, and for an FX-format sensor or a stone-aged 35mm film camera, about 34 millimeters. Taking a look at the mirror chambers in my D3200 and my F2, it looks like the minimum distances are significantly greater than these.

Aside from the mislabeled “2.8mm” lens (which is really a 300mm lens), Nikon does have, listed on its web site, DX-format lenses as short as 10.5mm, and FX-format lenses as short as 14mm. Clearly, it is not possible for any part of these lenses to physically be that close to the film plane.

It occurs to me that I ought to know a term that refers to the point within a lens which, with the lens focused at infinity, is the distance from the focal plane that is that lens' focal length, but I can't recall such a term, and a quick perusal of some articles on the Wikipedia that ought to tell me that term, if it exists, do not.

Whatever the term is for this point, is it possible for it to be outside of the actual physical structure of the lens? Is it possible for a lens to have a focal length of, say, 10.5 millimeters, without any part of that lens needing to be any closer to the focal plane than 22 millimeters?

Now that I think of it, I suppose even the stock 18-55mm lens that came with my D3200 must be an issue, here. There's no way that any part of it is ever as close as 18 millimeters to the focal plane. I'm leery of needlessly sticking anything far enough into my camera to make a usable measurement, but eyeballing it, it certainly doesn't look like anything less than about 30 millimeters at least, from the focal plane to the very rearmost reaches of how far into the camera any part of the lens goes.
 

aroy

Senior Member
.....

In an SLR, there is only so close that the lens can actually be to the focal plane without running afoul of the mirror. Just off the top of my head, it seems that a good rough estimate of this minimum distance would be the height of the frame multiplied by the square root of two—so with a DX-format sensor, about 22 millimeters, and for an FX-format sensor or a stone-aged 35mm film camera, about 34 millimeters. Taking a look at the mirror chambers in my D3200 and my F2, it looks like the minimum distances are significantly greater than these.

.......

The wide angle lenses for SLR cameras are usually "retrofocus" design, which means that there is enough space behind the last lens element for the reflex mirror.
History of photographic lens design - Wikipedia, the free encyclopedia
retrofocus lens (optics) -- Encyclopedia Britannica
 

Bob Blaylock

Senior Member
The wide angle lenses for SLR cameras are usually "retrofocus" design, which means that there is enough space behind the last lens element for the reflex mirror.
History of photographic lens design - Wikipedia, the free encyclopedia
retrofocus lens (optics) -- Encyclopedia Britannica

It doesn't appear that I can read the Encyclopedia Britannica article without paying for a subscription. As I was pondering the question at the time of my previous posting, the word “retrofocus” popped into my head, leading me to look that up on Wikipedia.

As far as I can tell, no article I've yet read seems to suggest that what, lacking any other known term, I am going to call the “optical center” of a lens can be outside of the physical structure of that lens. The point of “retrofocus” was to move that as far back as possible, but the articles left me with the impression that it was still no further back that the rearmost element.

On the other hand, it does seem that I have one empirical, observable sample, in the form of my 18-55mm stick lens that came with my D3200. It claims to have a minimum focal length of 18mm, but it is fairly obvious to me that when mounted on the camera, there is not any part of it that is anywhere near that close to the film plane. I suppose, at some point, I'll have to try some experiments and measurements, to verify that when zoomed out to 18mm, it really does produce an angle of view that is consistent with an optical center that is 18mm from a DX-format sensor, but for now, I'll take it as a given that it does.

I guess, if you think about it, any fisheye lens must be similar in principle to what I am wondering about. Geometrically, to get a 180° field of view, you'd have to have an effective focal length of zero. Nikon's famous rare 220° fisheye would have to have an effective focal length that is negative. Obviously, there is some trick in optics to take in a wide field of view, and convert it into something that is then projected to focal plane that is farther away from the lens than the focal length that would geometrically produce that field of view; otherwise, fisheye lenses would be impossible. I guess any lens with an effective focal length that is shorter than the physical distance between that lens' rearmost element and the film plane must rely on such a trick, and is effectively a less-extreme version of a fisheye lens. Perhaps that's what “retrofocus”*really is about, and I've just not yet read any article that did a good job of explaining this to me.
 
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