Talk:Focal length

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
B	This article has been rated as B-Class on the assessment scale.
High	This article is on a subject of High importance within physics.
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

Film Portal

This article is within the scope of WikiProject Films. This project is a central gathering of editors working to build comprehensive and detailed articles for film topics on Wikipedia. If you would like to participate, you can edit the article attached to this page, or visit the project page, where you can join the project and/or contribute to the discussion.

This article has been rated as B-Class on the quality scale.

???

This article has not yet received a rating on the priority scale.

Editing Guidelines

Please remember these guidelines when editing a film article:

If a non-film article already exists with the name of the film that you are trying to create an article for, disambiguate and use (film) in the title: Film Title (film)
- For more information see: Wikipedia:Naming conventions (films)
When writing an article about a particular film, the general format should be a concise lead section, followed by a plot summary of no more than 900 words, production details, a cast list, a reception section, and references.
Create an Infobox that tells all pertinent information about the film.

Additional Information:

	This article has been checked against the following criteria for B-Class status: Referencing and citation: not checked Coverage and accuracy: not checked Structure: not checked Grammar: not checked Supporting materials: not checked To fill out this checklist, please add the following to the template call: `<!-- 1. It is suitably referenced, and all major points are appropriately cited. -->` `\|B-Class-1=yes/no` `<!-- 2. It reasonably covers the topic, and does not contain major omissions or inaccuracies. -->` `\|B-Class-2=yes/no` `<!-- 3. It has a defined structure, including a lead section and one or more sections of content. -->` `\|B-Class-3=yes/no` `<!-- 4. It is free from major grammatical errors. -->` `\|B-Class-4=yes/no` `<!-- 5. It contains appropriate supporting materials, such as an infobox, images, or diagrams. -->` `\|B-Class-5=yes/no`
Related task forces (general):
	Filmmaking task force

1 Diagram
2 herschel telescope of the 1700's
3 EFL
4 Convex & concave
5 Too Technical
6 millimeters?
7 Disambiguation
8 Definition
9 Unclear sentence in the Photography section
10 Wrong sign convention for formulas
11 Normal Lens Myth

[edit] Diagram

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)

[edit] herschel telescope of the 1700's

Has anybody any idea where I can find a picture of the early telescope by Herschel ?

send to malcolm.bennett@ntlworld.com thanks

[edit] EFL

In my own experience, the term "EFL" is used to express the equivalent size of a digital lens in 35mm terms. For example, on some Olympus cameras, since the size of the sensor is smaller, a 150mm lens will have the same angle of view as a 300mm lens would on a 35mm camera. That lens would be described as having an EFL of 300mm.

I'm an amateur photographer and have been working in laboratory optics for the last four years and have never heard of this. EFL has a very well-defined meaning in terms of both traditional optics as well as consumer camera optics—it's the distance from the focal point to the first principle point. I strongly doubt an optically-saavy company like Olympus would deviate from the accepted standard on this...although I admit that I have been wrong in the past. --Milkmandan July 6, 2005 18:46 (UTC)

[edit] Convex & concave

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)

[edit] Too Technical

I find this article very heavy on the technical side, and it doesn't give an explaination in simple terms. Other photography terms have good basic descriptions, followed by more in-depth information.

This is not a photography article. It is an optics article that happens to be of interest to photographers (at least those who want to know more about how their cameras work.)--Srleffler 00:40, 8 November 2006 (UTC)

Agreed. It's great that we have the knitty-gritty of focal length here, but how about the simple description that it is, for practical purposes, quantified zoom? --User:RobertM525 00:26, 8 November 2006 (UTC)

Part of the problem is that we have several communities intersecting in this article. Focal length is an extremely important concept in optics, and deserves a full explanation. If all photographers care about is that it is "quantified zoom", that's fine, but then there is little need for them to read this article.--Srleffler 00:40, 8 November 2006 (UTC)

[edit] millimeters?

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)

I'm not sure what information you are looking for. Did you try Photographic lens?--Srleffler 23:51, 26 February 2007 (UTC)

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)

[edit] Disambiguation

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)

Fixed using a

This article is about {{{1}}}

template. --KYN 15:10, 17 August 2007 (UTC)

[edit] Definition

In photography, the focal length of a lens is defined as as the distance from the optical center of a lens (or, the secondary principal point for a complex lens like a camera lens) to the focal point (sensor) when the lens is focused on an object at infinity.

In the pinhole camera model the focal length is the distance between the pinhole (optical centre) and the image plane.

I think these definitios should be somehow inlcuded in this article. —Preceding unsigned comment added by 217.154.24.242 (talk) 16:56, 4 September 2007 (UTC)

The definition in photography is the same as the one used in physics. This article is more general in scope, and has to be more careful about how things are defined.

From the article:

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens ... the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot.

and

For an optical system in air, the effective focal length gives the distance from the front and rear principal planes to the corresponding focal points.

I'll try to add a more contextual description to the section on photography.--Srleffler 03:03, 5 September 2007 (UTC)

By the way, photographers should probably drop the phrasing that the focal length is the distance from the "optical center" of the lens to the focal plane. It is misleading, particularly for camera lenses. The focal length is always the distance from a principal plane to the corresponding focal plane. The principal planes can generally only be taken to be at the center of the lens when the lens is much thinner than the focal length, which is almost never the case for camera lenses. In (almost) every other case there are two principal planes and it is misleading to refer to either of them as the lens' "optical center".--Srleffler 03:14, 5 September 2007 (UTC)

[edit] Unclear sentence in the Photography section

“

When the lens is adjusted to photograph objects closer to the camera, the actual focal length of the lens changes.

”

This sentence seems incorrect to me. When the lens is adjusted to photograph objects at a less-than-infinite distance, the focal length of the lens doesn't change... it's just that the lens moves further from the image plane (film or sensor), so that the rays from the object converge in points on the appropriate surface.

As in this diagram from the lens article:

If $S_1 < \infty$ , then $S 2 > f$ , so the lens must be moved further from the image plane.

Maybe some photographic lens designs employ a complex construction that doesn't require moving the lens further from the image plane, but all of the ones I own seem to obey this principle.

Should this sentence be changed? I'm trying to think of a clear way to explain how the camera lens moves in order to focus objects at different distances. Any suggestions? ǝɹʎℲxoɯ (contrib) 03:50, 17 January 2008 (UTC)

I think you're right. I wrote that sentence, and I'm not sure what I was thinking.--Srleffler (talk) 04:17, 17 January 2008 (UTC)

I changed it. It's likely that for some lenses it was true, but I don't think that's what he was trying to say. I'm not sure about principal planes, but I'm pretty sure that what I said about the rear nodal point is true, and it agrees with the diagram on that page. Dicklyon (talk) 04:13, 17 January 2008 (UTC)

The cardinal points of a thick lens in air.
F, F' front and rear focal points,
P, P' front and rear principal points,
V, V' front and rear surface vertices.

I'm certain principal plane is correct for a lens in air. I can't recall if the nodal points coincide with the principal planes in that case. Which diagram are you referring to?--Srleffler (talk) 04:25, 17 January 2008 (UTC)

Yeah, I'm not sure which is more correct and standard here... principal point or nodal point? In air, they're at the same place. ǝɹʎℲxoɯ (contrib) 04:36, 17 January 2008 (UTC)

Thanks! I expanded on your changes by adding an example to show how a 50 mm lens must move about 2.6 mm to go from infinity to close focus, hopefully this will be a useful real-world example. ǝɹʎℲxoɯ (contrib) 04:36, 17 January 2008 (UTC)

Note that when the lens is focused on objects not at infinity, the rear focal plane no longer coincides with the image plane. The focal planes are explicitly the planes containing the focal points—they are separated from the principal planes by f. Also note that the equation

$\frac{1}{S_1} + \frac{1}{S_2} = \frac{1}{f}$

in your example is not actually the thin lens equation. It has the same form, but you are interpreting the variables differently. In the thin lens equation S₁ and S₂ are measured from the center of the lens. Camera lenses aren't "thin", so the locations of the principal planes have to be taken into account.--Srleffler (talk) 04:58, 17 January 2008 (UTC)

Yeah, the optical term focal plane doesn't quite agree with what photographers call the focal plane, which is their film is, which is really the image plane. Should we mention the different interpretation of the term? Dicklyon (talk) 05:59, 17 January 2008 (UTC)

Probably a good idea.--Srleffler (talk) 06:29, 17 January 2008 (UTC)

Good point. Is there a name for the equation when used with non-thin lenses? ǝɹʎℲxoɯ (contrib) 07:35, 17 January 2008 (UTC)

I was looking for a technical description of focal length as it applies to photography and didn't find it here. The most odd thing for a newbie is that focal length in photography has little to do with focus. I think there should be a page dedicated to that, say focal length (photography), since it is used to describe view angle, among other things. There is some information on photographic lens, but it doesn't go into detail. For example, is the focal length of a lens a real thing you can measure (e.g., the distance from the exit pupil to the image plane) or is it really used as a surrogate measure to describe view angle. For example, I think you can have a wide-angle lens that is image-space telecentric. I'll be bold and start that page. —Ben FrantzDale (talk) 01:33, 18 January 2008 (UTC)

What do you mean by "focal length in photography has little to do with focus"? It has everything to do with focus; also with magnification. It determines where to put the film to get an image in focus, and it determines what the subject magnification will be (mapping of distant object angle to image size, or object size to image size if you know object distance). See pinhole camera model. Dicklyon (talk) 05:24, 18 January 2008 (UTC)

Focal length is a real measureable thing, but doesn't have to do with exit pupil, nor does it determine the field of view (even when combined with format size, you still need to know the distortion to get an accurate angle of view). Yes, you can have a short focal length lens that's telecentric on the image side, since distance to exit pupil is unrelated to focal length; focal length is distance to rear nodal point. Simple definition: it's the distance from image plane to a pinhole that would image a distant small object the same size as the lens in question. Dicklyon (talk) 02:40, 18 January 2008 (UTC)

I just checked out your new focal length (photography), and converted it to a redirect to focal length. Use the Template:Split-apart or Template:Splitsection tag (in double curly braces) on this article if you'd like to propose a content split. I'll probably oppose it. Dicklyon (talk) 02:53, 18 January 2008 (UTC)

Ben, are you perhaps confusing focal length with focus distance? The former (the subject of this article) is a property of the lens, while the latter is the distance to the object that you want to focus on. Those two are then of course related to the distance to the image plane, by the lens equation. I think it might be common to mix them up... ǝɹʎℲxoɯ (contrib) 08:48, 18 January 2008 (UTC)

I'm not confused now, but I used to be. To someone new to the subject (i.e., someone who just got an SLR), it seemed baffling that people would talk of "focal length" when it seemed more natural to think of angle of view. While it does have something to do with focus, once you have a camera in hand, you can focus at almost any distance without changing the focal length, so for someone coming at this from a photography perspective, calling it "focal length" seems unnatural. That's why I wanted to see a page dedicated to the photographic use of the term.

I think the first paragraph of the photographic description of focal length should say something like "The focal length of a camera lens describes the distance from the film plane to the pinhole of a pinhole camera that would produce images with the same projection." To many, that (phrased better) would be sufficient. But personally, I am still having trouble seeing how the "focal length" of a lens system necessarily corresponds to the projection you see on the sensor. (Thanks for all your patience in helping me understand this. Once I understand it, I'll see to it that the article is clear to the next person who comes upon it from my perspective.) —Ben FrantzDale (talk) 15:26, 19 January 2008 (UTC)

Ben, I agree that it *is* more intuitive for photographers to think of focal length in terms of angle of view. However it is also quite intimately related to the distance between the lens and the image plane. I have recently improved (hopefully) the angle of view article to include a diagram that explains the relationship between distance to the image plane, focal length, and angle of view:

What do you think of the accompanying text? Does it clarify the situation somewhat?

Perhaps there is a good way to integrate the information there into this focal length article, or link to it appropriately... ǝɹʎℲxoɯ (contrib) 23:07, 19 January 2008 (UTC)

I'm thinking that the symbols and names used in that article may lead to confusion. In optics, the focal length is conventionally represented by a lower case f. The upper-case F is the symbol for the focal point itself. Additionally, "focal length" and "effective focal length" are the same thing. (There are other focal lengths in an optical system, such as the front focal length and the back focal length, which differ from the e.f.l.) The angle of view article defines effective focal length f, but then distinguishes from the stated focal length of the lens F. The text implies that the focal length changes, in particular that f and F differ for macro photography, where

$f = F \cdot ( 1 + m )$ .

I haven't tried to analyze it, but this looks wrong. At the least, unless the focal length of the camera lens actually changes, the article is using "effective focal length" in a way that disagrees with standard optics usage. Perhaps this is again just confusion over the fact that the film is not always at the focal plane of the lens?--Srleffler (talk) 00:40, 20 January 2008 (UTC)

Depends on what you mean by "wrong". The term "effective focal length" is sometimes used to mean the focal length adjusted for lens extension for close focusing, which narrows the angle of view. I think the formula is right, at least in the thin-lens or pinhole approximation. The terminology may be "wrong", however; do you have a better term for this distance? Dicklyon (talk) 00:56, 20 January 2008 (UTC)

Yes, that's the case here, "effective focal length" means "distance to image plane adjusted for close focusing." That terminology was in the article before I edited it. It should just be called "image plane distance", but since photographers are accustomed to that being roughly equal to focal length, that might lead to confusion. ǝɹʎℲxoɯ (contrib) 01:30, 20 January 2008 (UTC)

The terminology has been in angle of view since 2003, this diff. I would have written it the same way; there's also the "effective f-number" concept for f-number adjusted by lens extension (same 1+m factor, but in the denominator). Dicklyon (talk) 01:49, 20 January 2008 (UTC)

[edit] Wrong sign convention for formulas

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)

[edit] Normal Lens Myth

“

A lens with an angle of view similar to that of the human eye is often referred to as a normal lens [...]

”

This whole paragraph is so wrong. Hasn't got anyone a standard/normal lens anymore? Please, mount one on your camera and take a look -- the ANGLE-OF-VIEW is much smaller than that of the human eyes. The apparent PERSPECTIVE of a standard lens is similar to the human perception apparatus, i.e., the things seem to have the same spacial relations to one another in a picture taken with a standard lens as when we view them in real life. For wide angle lenses, these relations seem exaggerated (close things seem closer than they really are, and farther things look farther), whereas teles show a flattened perspective where things look closer together.

Our two eyes have a angle-of-view of about 180°, so a 16mm lens or so is needed to render the same field-of-view. But the human perception apparatus is very different from an optical lens. We have only a narrow "center of attention" which probably equals a 200mm or 300mm lens. For example, when reading, we see the complete page in our field-of-view, but still need to "move" from word to word instead of just perceiving the whole text in our vision. It's all a quite complicated matter, but the myth that a standard lens is anywhere close to equivalent to our angle-of-view needs to be eradicated. —Preceding unsigned comment added by 145.253.2.232 (talk) 01:54, 5 April 2008 (UTC)

I suspect that our sense of "normal" has a lot to do with how we like to look at pictures. My suspicion is that a picture taken with a "normal" lens viewed at a usual distance for that medium will produce the approximately the same image on your retina as if your pupils were at the same location as the camera's was. With 135 film and a normal lens, you have an angle of view in the long direction of 40° (2*atan((36mm/2)/50mm)). That's the same angle as the long direction of a 4"x6" print held at 8.3". If you printed the same picture on paper 10" wide and held it 13.9 inches from your eye, you would have the same image on your retina. If you took the same picture but with a 25mm lens, the middle half of the image would be identical to the image taken with the 50mm lens. If you printed it at 8"x12" and held it 8.3" from your eyes, you would again have the same image on the middle of your retina, but you would have a wider field of view.

I haven't seen citations to support my argument, but in general, perception of "normal" must depend on viewing conditions. I imagine the most "normal" seeming picture would be taking a picture with a 180° angle of view and printing it on the inside of a hemisphere. —Ben FrantzDale (talk) 17:36, 5 April 2008 (UTC)

The myth has been so often repeated that some people accept it in spite of its contradiction. But the subject is discussed more sensibly, sort of like what Ben is talking about, here. Dicklyon (talk) 18:08, 5 April 2008 (UTC)

I fixed the article per that source. Dicklyon (talk) 18:32, 5 April 2008 (UTC)

Dicklyon, thanks for the reference. We should be sure that normal lens includes that information as well. —Ben FrantzDale (talk) 03:13, 6 April 2008 (UTC)