Talk:Focal length

message: I only started learning about focal length today, so if the diagram is wrong or can be improved let me know. - Redjar 01:36, 14 Oct 2004 (UTC)
thread_title:  Diagram 

message: I've re-generalized things a bit. It's important to note that convex and concave lenses are examples of converging and diverging lenses, not definitions of them. It's fine saying that in the picture caption, because the picture actually shows convex and concave lenses. But generally things are more complex. For example:
The third lens in this picture is a converging lens, but is both convex and concave. Therefore it's incorrect to define converging lenses as purely convex. --Bob Mellish 17:00, 25 October 2005 (UTC)
thread_title: Convex & concave

message: Where would I find information about camera focal length measured in millimeters? I followed some links from this page but didn't find the right page. I'd think this page should link to wherever that answer is. —Ben FrantzDale 22:30, 26 February 2007 (UTC)
replies: text: I'm not sure what information you are looking for. Did you try Photographic lens?--Srleffler 23:51, 26 February 2007 (UTC)
replies: text:  That's enough to get me started. Thanks. I'll ad a DAB at the top of this page. —Ben FrantzDale 21:19, 27 February 2007 (UTC)
thread_title:  millimeters? 

message: The concept of "focal length" is also used in relation to pinhole cameras to describe the distance between the camera pinhole and the image plane.  To me this is a not really the same this as the focal length described in this article which is more related to convergence of lenses and optical systems in general.  Even though a pinhole camera is a type of optical system, I still don't see these two focal length concepts as the same thing.  In any case I want to be able to make a short description of "focal length" in relation to the pinhole camera somewhere so that it is possible to use a simple wikilink to find it.  Possible alternatives:
* It can be done in a separate section in this article + adding the alternative defintion in the lead section.
* Turn this article into a disambiguation page which links to "Focal length (optical systems)" which will host this article and "Focal length (pinhole camera)" which takes care of the pinhole case.  Maybe "optical system" is not a good qualifier since a pinhole camera is an optical system as well?  Perhaps "lens systems" is better?
Any opinions?
--KYN 08:43, 7 August 2007 (UTC)
thread_title:  Disambiguation 

message: There is either an error in the formulas here or the sign convention is described wrong. The formula for the effective focal length is clearly wrong given the sign convention described for the radii of curvature. It's correct for the sign convention described at Radius of curvature (optics), and that used at Lens (optics), however. Assuming I'm not making a mistake, I think we should fix the description of the convention used rather than changing the formulas.--Srleffler (talk) 05:44, 24 February 2008 (UTC)
thread_title:  Wrong sign convention for formulas 

message: Stewart's Optical Principles and Technology for Engineers highlights a crucial (and logical) difference between front focal length (FFL) and front focal distance (FFD):
* p.34 : " The focal length of a lens is measured from the focal point to the corresponding principal point, not to the surface of the lens. The focal distances measured from the lens surfaces are the back focal distance and the front focal distance."
* p.63 : "The distance from the first focal point to the first principal plane of the combination is the front focal length, fFF." 
which is supported in Grievenkamp's Field Guide to Geometrical Optics p.6 
*"The front and rear focal lengths (...) are defined as the directed distances from the front and rear principal planes to the respective focal points" "
Using this terminology, a lens' EFL = BFL = FFL only when the object and image space media have the same refractive index - "in air" - which is by far the most common case. However, when the refractive index of the media in front (object-side) and behind (image-side) of the lens are different: the effective FFL ≠ BFL, so the distinction is an important one.
Whilst no-one would ever dream of saying "focal length = focal distance", some authors (, ) are still quite content to use "FFL = FFD" and (are forced to) use "first focal length" and "second focal length" when front and back media have different refractive indices. Not only is this logically messy, but I can see this numbering system causing problems with a multiple-lens system: it would seem helpful to be able to compare the "first back focal length" with the "second front focal length", etc ...
In "common" usage, then, it seems that "front focal distance" and "first focal length" are safely unambiguous, but "front focal length" could be used to refer to either of them. 
* Menzel, Fundamental Formulas of Physics, vol.2 p.384 : "The quantity f is called the equivalent focal length of the system in object space, or the first focal length, and often the front focal length. Similarly the quantity f' is called the equivalent focal length of the system in image space, or the second focal length, and often the back focal length. (It should be noted that the terms front and back focal length are often used at the present time to describe the distance from the first focal point to the front vertex of the lens, and from the rear lens vertex to the second focal point. This confusion is not desirable, and the terms front and back focal distances are recommended instead. The terms front focus and back focus are also used.) Where initial and final media have identical indices, it is clear from the above formula that the two focal lengths are equal." 
--Redbobblehat (talk) 14:30, 23 August 2009 (UTC)
replies: text: Yes. It's hard to treat this well though because usage varies between different authors. I believe at one time this article used Greivenkamp's definitions, but the text was adjusted because some authors do use FFL for the vertex-to-focus distance. The text does correctly describe how some authors distinguish between FFL and FFD. While textbooks lean toward the "FFL=principal plane to focus" usage, vendor catalogs and the like tend to use FFL/BFL for vertex to focus distance. --Srleffler (talk) 00:47, 24 August 2009 (UTC)
replies: text:  Perhaps, following Menzel's treatment, we should relegate front focal length and back focal length to an explanatory (disambiguation) section, and the rest of the article should encourage - for wikipedia purposes - the unambiguous use of front focal distance, back focal distance, second focal length and first focal length? ... IMO this wrankles :( but for the sake of wikipedians with different 'background reading' it's the best practical compromise I can think of. --Redbobblehat (talk) 11:36, 24 August 2009 (UTC)
replies: text: It rankles me too, mainly because "first focal length" etc. is (I think) a much less commonly used term. I'm not sure I have ever seen it before. --Srleffler (talk) 17:00, 24 August 2009 (UTC)

text: A side point: note that in Greivenkamp's formalism in the most general case, the focal length (or EFL) is not the distance from a principal plane to a focus. I have often thought that it would be more sensible to abandon the effective focal length altogether, and use optical power instead. For real, general lens systems, it is not really a measure of physical length, but rather a measure of how strongly the lens converges light. It's only in the simple case of a thin lens in air that it becomes so intuitive to think of "focal length".--Srleffler (talk) 00:47, 24 August 2009 (UTC)
thread_title:  difference between 'focal length' and 'focal distance' 

message: To my understanding, a pinhole camera cannot have a "focal length" because it has no refractive or reflective optics. I can see how the pinhole kind of converges and diverges light - in that the pinhole itself is a kind of focal point, but I doubt this fits the accepted definition of focal length. Can anyone clarify ? --Redbobblehat (talk) 15:21, 7 February 2011 (UTC)
replies: text: Using a semantic argument based on the gaussian definition, you could say that the distance from nodal point to focus is zero, as is focus to toepler point - so focal length of a pinhole = 0mm ? It doesn't seem very useful though ... --Redbobblehat (talk) 15:34, 7 February 2011 (UTC)

text: When someone says a pinhole camera has a focal length, they almost certainly are referring to the distance from the pinhole to the image plane. That is the only number that makes sense, and that lets you work out what the projection will look like with the pinhole camera model. In terms of its ability to focus light, of course, a pinhole has zero power (infinite focal length) in Gaussian optics and has effectively negative focal length if you consider diffraction. —Ben FrantzDale (talk) 14:32, 28 October 2011 (UTC)
replies: text: This is one of the reasons why I dislike the pinhole camera model—a pinhole camera is really very different from a conventional camera, and the model leads people to incorrect assumptions about how cameras work.--Srleffler (talk) 17:22, 28 October 2011 (UTC)
thread_title:  focal length of a pinhole camera ? 

message: Many of our discussions above have touched on the common confusion of terminology that "focal length" means the distance from the subject to the entrance pupil when really that's referred to variously as "subject", "focus", and "object" distance. I can't find a subject distance page on Wikipedia, and it may not warrant its own article, but I think it needs something (perhaps on this page?). Related is that some camera lenses have a fixed focal length (such as the Cannon EF 50mm f/1.8 II), and it is focused my moving all elements together (presumably changing the angle of view as it focuses because the pinhole camera model focal length would be changing (unless it were image-space telecentric). On the other hand, there may be lenses that change their focal length (?) via internal focusing, and then there are liquid lenses that can change their focal length and so could be used to focus or (in principal) to zoom. Thoughts? —Ben FrantzDale (talk) 14:32, 28 October 2011 (UTC)
replies: text: Is this a common confusion of terminology? Focal length is not the distance from the subject to the entrance pupil. I'm not sure that the fact that it is not is worth mentioning,unless there is evidence that many people think it is.--Srleffler (talk) 17:29, 28 October 2011 (UTC)
thread_title:  Subject distance aka focal distance aka focus distance aka object distance 

message: There should be a section with a sample of how the focal length affects the background and subject of the photograph if the photographer moves forward and backward.  See this article http://photo.tutsplus.com/tutorials/photography-fundamentals/exploring-how-focal-length-affects-images/  — Preceding unsigned comment added by 74.64.108.61 (talk) 23:20, 16 January 2012 (UTC)
replies: text: See also: Dolly zoom. —Ben FrantzDale (talk) 18:29, 17 January 2012 (UTC)
thread_title:  In Photography 

message: The lede says,  For an optical system in air, it is the distance over which initially collimated rays are brought to a focus.   Do we need to specify, in air?  Isn't this the same definition in a vacuum (or any other medium)?  -- RoySmith (talk) 15:36, 31 May 2015 (UTC)
replies: text: If the medium has an index of refraction other than one, the point where those rays are brought to focus is not one focal length away from the lens. This is unintuitive. See Greivenkamp's book for details.--Srleffler (talk) 05:05, 1 June 2015 (UTC)
thread_title:  Definition in lede?