Talk:Photographic lens

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[edit] Lens Classification

Why is the classification of lenses meaningless ? This is based on objective datas. I'm not a great fan of zooms and I agree with the sentence that the best zoom (not lens) are your feet. But well this has senses have you ever tried to shoot handheld at 1/15s with a 300mm ? Or to make portraits with a 21mm ? Ericd 22:44, 7 May 2005 (UTC)

True, but I could create a new classification based on another type of objective data (e.g. focus range at a particular zoom); also, the current classification isn't what one would call scientific ("considerably smaller", "about the same", and "considerably larger" than the than the diagonal of the frame.) And finally, my whole point was that there is no classification when shooting: one just chooses a lens which matches the scene—and your examples illustrate that—, as opposed to some rigid lens "classes". And just to make my point clear: I wanted to write that paragraph for SLR newbies who must (IMHO) understand that the lens choice is theirs, and theirs alone, and it's not dictated in any way by some immutable laws. --Gutza 23:06, 7 May 2005 (UTC)
The classification and the explanation of it is awkward. There needs to be an explanation that lenses are intended for certain formats because of the circle of coverage within which an image does not have significant vignetting. For example, a 90mm lens would be telephoto on a 35mm camera, "normal" on a 6x7cm camera, and wide angle on 4x5 in. camera. However only the lens designed for the 4x5 camera could produce an unvignetted image on all three. A 90mm lens designed for a 35mm camera will produce a small image circle, too small for either of the other formats. Also noteworthy is the fact that because the 90mm wide angle lens is designed for the 4x5 format, it would not resolve as well in the center as a 90mm lens optimized for 35mm cameras.

[edit] "Nitpick"

My "Encyclopedia of Photography" defines a "telephoto" lens one that is shorter physically than its focal length would indicate, and the correct term for a lens that is significantly longer than the diagonal of the film area is "long-focus."

As an aside, "significant" in the above sentence refers to the fact that most normal lenses sold on SLR's for the 135 market ("35mm film") are 50-52mm in length- rather longer than the film diagonal of 42-43mm (film gate measurement).

JD

This is adressed in Telephoto lens. Ericd 08:59, 19 October 2005 (UTC)

[edit] Is "objective lens" inappropriately redirected here?

It doesn't seem like an objective lens is the same as a photographic lens. Some objective lenses are not involved in photography, like those in microscopes and telescopes. -- Kjkolb 01:58, 10 November 2005 (UTC)

I changed it into a sub. -- Kjkolb 02:42, 10 November 2005 (UTC)

[edit] f-number and aperture diameter

I have edited the article slightly to remove the common misconception that the f-number is equal to the ratio of the focal length to the aperture diameter. It is not. The f-number is equal to the ratio of the focal length to the diameter of the entrance pupil of the lens. The latter is proportional to the diameter of the aperture. It is true that doubling the f-number halves the diameter of the aperture but it is not true that the aperture diameter is equal to the focal length divided by the f-number.--Srleffler 04:30, 14 February 2006 (UTC)

[edit] Camerapedia copying OK?

It's not plagiarism to copy the "free content" from camerapedia.org; however, the GNU license they use makes their material unsuitable for the wikipedia:

         Copyright (C) 2000,2001,2002  Free Software Foundation, Inc.
         51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
         Everyone is permitted to copy and distribute verbatim copies
         of this license document, but changing it is not allowed.

If changing the content is restricted, we can not use it here. Dicklyon 04:01, 6 July 2006 (UTC)

Erm, isn't it talking about the license, not content, when it says "changing it is not allowed"? --Imroy 04:12, 6 July 2006 (UTC)
Perhaps you're right. I had read it as "this licensed document", but I see now that's not what it says. OK, so why did you revert the content? Dicklyon 04:32, 6 July 2006 (UTC)
I didn't like a huge pile of content from a different site (and context) being copied verbatim into the article. It also had a lot of redundant information that was already covered in the article and other articles. Camerapedia looks like a useful resource, but I would prefer the information to be integrated into the article - carefully and with consideration of what is already in the article and other articles. Not just regurgitated with the simple click of a button. Wikipedia is not a mere dumping ground for random bits of information found elsewhere. The articles need to be well-written and well-laid-out, both within an article and the way in which information is broken into separate articles. --Imroy 05:06, 6 July 2006 (UTC)
The content of Camerapedia is under the GFDL, exactly like Wikipedia. --Rebollo fr 21:25, 26 July 2006 (UTC)

[edit] 4 Canon lenses

NPOV require us to show other brands ! Ericd 23:19, 3 August 2006 (UTC)

[edit] Lens size in inch

We could use some input. An anonymous editor has submitted a request to create a new article Lens size in inch. The content is sourced, but doesn't provide much context. The relation between lens "size" and image size is clearly not applicable to lenses in general. I'm not clear whether this relation is related to photography or video cameras or some other application. Could someone who knows more about photography please review it. If it is appropriate to photographic lenses, perhaps this content should be added as a section in this article. If not, perhaps someone from here could provide input on where best to put it.--Srleffler 21:05, 21 March 2007 (UTC)

[edit] Diagram request

 It is requested that a diagram or diagrams be included in this article to improve its quality.
For more information, refer to discussion on this page and/or the listing at Wikipedia:Requested images.

This article would greatly benefit from a diagram showing things like focal length, aperature, entrance pupil, and film format size. -- Beland 18:57, 5 May 2007 (UTC)

[edit] Clean up versus delete silently...

If you don't like my edit Dicklyon, then the place to discuss it is the talk page. Please do not delete legitimate additions to the article.

If somebody wants to translate the PDF I cited they can, I don't really know how to do it conveniently.Sukiari 02:17, 4 November 2007 (UTC)

It was discussed in the edit comments; there's no obligation to bring it here until someone feels the need to or needs the extra space or a dialogue. So, now we have one. As far as the edit conflict goes, I believe that WP:RSUE covers this - unless there's something special that makes this information exclusively available from reliable Italian sources, better citation needs to be found first. Girolamo Savonarola 06:45, 4 November 2007 (UTC)
I did a quick google search and found this company's page that talks about "optical components in a broad range of materials including Calcium Fluoride, Magnesium Fluoride, Barium Fluoride, Zinc Selenide, Zinc Sulfide, Germanium and many more exotic materials too." You could list those materials with that as a source if you want; but it would be better to find one that talks more specifically about photographic lenses. Dicklyon 06:57, 4 November 2007 (UTC)

[edit] Focal length clarification

The focal length of the system is related to the angle of view by

\tan ( \alpha / 2 ) = \frac {d/2} {f}

where f is the focal length and d is the image diameter. But that doesn't make sense to me. That seems to imply that every chief ray entering the entrance pupil exits the exit pupil in a parallel direction. Couldn't one, in theory, have a lens in which all of the exiting chief rays were "bundled tighter together", giving you a lens with a wider field of view than the focal length and the sensor size would imply? 155.212.242.34 (talk) 16:23, 11 December 2007 (UTC)

It's not clear what you mean by that. The formula is correct only for rectilinear lenses, that is, those that image like a pinhole would. Focal length is defined such that the magnification is the same as a pinhole at the distance of the rear nodal point from the focal plane. Look it up and add it to the article to improve it. Dicklyon (talk) 19:08, 28 December 2007 (UTC)
Would that be to say that rectilinear lenses are exactly those lenses for which the nodal points and the pupil centers are aligned? —Ben FrantzDale (talk) 16:56, 2 January 2008 (UTC)
No, I don't think there's any such relationship between a lens's distortion and its nodal points and/or pupils. Dicklyon (talk) 05:47, 3 January 2008 (UTC)
You seem more knowledgeable than I on the topic, but I'm skeptical, at least if we restrict ourselves to optical systems for which the entrance and exit pupils stay put as a function of off-axis angle, I think I'm right. If the pupils don't move around, then it seems like you can mentally align the center of the exit pupil with the center of the entrance pupil and you have the equivalent of a pinhole camera if and only if the nodal points are aligned with the pupils. —Ben FrantzDale (talk) 01:20, 4 January 2008 (UTC)
It does seem plausible that that would be a sufficient condition for rectilinear imaging. But it's not a necessary condition. Now that I think about it, it seems that if the nodal points don't move with angle, then that's sufficient for no distortion, and might even be necessary; but I don't see any connection to the pupils. Dicklyon (talk) 02:44, 4 January 2008 (UTC)
I think I can explain better now. As I understand it, you can have an image-space telecentric lens with what is effectively a finite focal length. If that is correct, then the back focal length of the lens in an optical sense would be infinite even if a photographer would tell you it has a finite focal length. That is, it sounds like an image-space telecentric lens with an arbitrarily small aperture would rays entering the entrance pupil with angle θ with respect to the optical axis and turn them into outgoing rays parallel to the optical axis offset by sin(θ). Does that sound right, or am I missing something?
Whether the offset is sin(θ) or not depends on the distortion of the lens, but yes, that's about how it works. But "back focal length" is a different concept, not the same as distance to exit pupil. Actually, I'm not sure what "back focal length" is, except that sometimes diagrams show a back and a front focal length and then say they have to be the same, equal to the focal length. Other times it may refer to a distance from the back element or something like that. Dicklyon (talk) 15:45, 11 May 2008 (UTC)

[edit] Total Focus Camera Lens?

I wonder if anyone here can help. I remember watching a documentary on a lens a few years ago (that I *think* was designed by an Australian) that allowed for objects to be in focus from macro distances to infinity. I watched the shots of a caterpillar eating a plant up very, very close and then a plane sprayer flies in from the distance and over head - all the while with both parts in focus. I seem to recall also that it was a lens used on Cameron's Titanic and that they ended up modifying and mounting it with a swivel/pivoting head. Has anyone else heard of this or got any info for the article? —Preceding unsigned comment added by 58.179.175.155 (talk) 03:55, 6 February 2008 (UTC)

[edit] References

Ref. 4 was in Italian. Please could someone get an intelligent native English speaker to translate it? The current text shown in the article is unintelligible. EEye (talk) 18:34, 13 February 2008 (UTC)

[edit] Focal distance

Is there a word for the distance at which a lens focuses? When I was just learning this material, I assumed that was what focal length meant, which was confusing. So, is there a word to fill out "Take landscape pictures with a _____ of infinity; take macro pictures with a _____ of an inch or so"? Is it just "focus distance"? —Ben FrantzDale (talk) 18:15, 22 April 2008 (UTC)

Depth of field says this is called subject distance. —Ben FrantzDale (talk) 23:44, 28 April 2008 (UTC)
This really appears to be the case; it shows up in the EXIF standard (although nobody supports it, for some reason).—Ben FrantzDale (talk) 23:57, 28 April 2008 (UTC)
What you're looking for is the "front focal distance" (sometimes just "focus distance" when it's clear that you mean outside the camera, not inside). When it's equal to the subject distance, your subject will be in focus. Sometimes in DOF analysis we take a shortcut and assume that equality. Dicklyon (talk) 03:42, 29 April 2008 (UTC)
Is it really front focal distance? I've seen that page before but didn't think it was what I was looking for. Are you saying that the "subject distance" is the distance from the camera to the subject even if the subject is out of focus? Other hits on the web seemed to suggest that "subject distance" was right. If it's not, then I'm all for redirecting subject distance to front focal distance and making that distinction there. Thanks. —Ben FrantzDale (talk) 11:43, 29 April 2008 (UTC)
Good question; not as defined on that page, I'd say. Better study some books... Dicklyon (talk) 14:38, 29 April 2008 (UTC)
Optics by Hecht describes this distance as object distance. (Being an optics book not a photography book, it talks of objects, not subjects.) It also points out that the "object and image must be on opposite sides of their respective focal points" for finite distances (p. 163). So this does seem to be distinct from front focal distance. —Ben FrantzDale (talk) 14:09, 30 April 2008 (UTC)