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This is the discussion page for Lens. Click here to start a new topic.

Discussion pages are for discussing improvements to the article itself, not for discussions about the subject of the article.

Should the list of lens mounts go to the main page? --rebollo_fr

I would leave it where it is.

Ref: All very fast lenses are interchangeable lenses, so if this will be important to you, you will need an interchangeable lens camera.

I understand what you were trying to say here but this is not entirely true. Many digicams have fast lenses especially for zooms. My Sony f717 has the 35mm equivellent of a 38-190mm zoom at f2-2.4 In the 35mm world that would be a very fast zoom.

Lens coating has nothing to do with color correction

I've corrected this myth both here and in the glossary. For a very good explanation of how antireflection coatings work, check Rick Oleson's discussion of the matter here. (He's a camera and optical guru.)

From his page:

One last, unrelated question: Does coating make a lens "color-corrected"?
No. Color correction is a function of the optical design of the lens, and is not affected in any way by coating. Color correction is the property of a lens that causes all colors to focus at the same [...]




A caption about astigmatism: "Astigmatism is when a point sending light through a lens cannot be projected as one point behind the lens. It appears as a line on the focal plane. That's the result of faulty lens elements. The best correction of astigmatism is to use faultless lens elements."
It's a complete nonsense! Astigmatism appears - no matter of the lens quality - when beam of light passes through the lens at an angle to the optical axis. In this case vertical and horizontal curves of the lens element, in the place where the beam is transmitted, are different and the beam is focused as two perpendicular lines in TWO different places behind the lens - one by vertical and one by horizontal curve. These lines DO NOT appear at the focal plane but at two different curved surfaces (often close to spherical), as the astigmatism is closely connected with field curvative.

"Long-focus" vs. Telephoto

While some have attempted to enforce this distinction, it is my sad duty to report that your cause is lost. At least in American English, the term telephoto is universal, and it has been for decades. I have beside me a 1958 photo magazine which blithely uses "telephoto" in describing 8mm movie camera lenses, which are almost certainly not of telephoto optical construction. The distinction has also broken down because increasingly photographers use long-focus zooms whose optical design is unknown.

I have edited the Long-focus section to acknowledge the distinction, but foreground the more familiar word. This is a subject that I've blogged about, and I give a fuller defense of accepting "telephoto" there. --Vox 07:21, 1 March 2011 (PST)

Depth of field

As someone familiar with optics I will have to correct the common misconception that depth of field (dof) is related to focal length.

This is wrong, dof is related to magnification and aperture only, the apparent increase in dof with wide angle lenses is due to reduction in magnification and vice versa with telephoto lenses. Small formats have greater dof because of reduced magnification (images on a 35mm film are smaller than those on roll film 6x6).

  • If any lens is used close up dof decreases, why? Because magnification has increased
  • Take a 50mm lens and photograph an object exactly filling the frame. Now take a 500mm lens, move back 10 times and with the same aperture dof will be exactly the same

Of course we are all aware of the apparent dof changes using lenses of different focal lengths, but the proper explanation should be given. Subject to discussion, I intend to do this. Do please note that this is not a matter of opinion, it is optical fact. - DesmondW 09:07, 29 January 2012 (PST)

The DOF explanation given here is rather simplified, I agree. However to treat the subject in full detail can become quite complex. There's an excellent 2010 article by Dr. Hubert Nasse of Carl Zeiss which clarifies many points, including the importance of the entrance pupil diameter. I would welcome some expansion of the DOF section, but Lens is already a long article. Any changes should be succinct, and helpful in terms of practical photography. --Vox 09:51, 29 January 2012 (PST)
How's that? DesmondW 11:20, 29 January 2012 (PST)
The article Lens has had a number of problems in the past, and I thank you for making several good improvements. The relationship between focal length and DOF may still need one clarification. But I will want to think a bit before wading in with advice. I once wrote a blog post (rather long, I apologize—skip 1/3rd of the way down the page) which may give an idea what is on my mind.--Vox 16:17, 29 January 2012 (PST)
Take a 50mm lens and photograph an object exactly filling the frame. Now take a 500mm lens, move back 10 times and with the same aperture dof will be exactly the same That looks like a neat explanation of how DoF is related to focal length.
Suppose that A is related to X, Y, and Z, and that one of X, Y and Z is not necessary for the calculation of A. One can then truthfully say that X is not necessary and that Y and Z are sufficient. One can just as truthfully say that Y is not necessary and that X and Z are sufficient.
How is the Wikipedia article "Depth of field" defective? -- Hoarier 15:37, 29 January 2012 (PST)
At longer distances the approximation of dof to focal length is roughly true. At closer distances magnification is the important factor, Leica's highest magnification near focus lens is 12.5mm focal length with up to 15:1 magnification at 7mm object distance and tiny dof - DesmondW 11:15, 30 January 2012 (PST)
In response to your question about the Wikipedia article: it isn't defective and I have added some clarification. In summary, changing focal length without moving viewpoint changes magnification and dof. Changing distance from the subject without changing lens also changes magnification and dof. Saying that dof is related to focal length explains one of these cases, saying that it is related to magnification explains both.
Incidentally, focal length is a very poor lens measure as the final image depends on the format. Quoting angle of view would be much more meaningful as this is inclusive of the format. - DesmondW 11:07, 31 January 2012 (PST)

Purple Fringing

If anyone wants to add purple fringing to the list of lens distortions covered, I have a nice set of photos demonstrating the effect and showing how a UV filter (or coating) will improve it purple fringing - no UV filter, purple fringing - with UV filter Steevithak 07:24, 30 January 2012 (PST)

I would be a little cautious about this topic. Chromatic aberration is a purely optical phenomenon (where the fringing might also appear as green or red), and we might expand that article section. But my understanding is that purple fringing involves some interaction with a digital sensor. Before adding anything, I would want to see well-researched sources explaining what is going on.--Vox 15:55, 31 January 2012 (PST)

Groups and Elements

The standard block of specs we request on every lens page includes the number of groups and elements in a lens. Should that be covered in this article perhaps with some information on how one can determine the correct values from looking at the lens itself? Or is that even possible without disassembling the lens? I see element is in the article already but nothing explaining the official definition of a group. Steevithak 07:24, 30 January 2012 (PST)

I'm not aware of any reliable way to count the elements of a lens from the outside. The number of reflections which appear (e.g. of a bright light source over your shoulder) do give some qualitative sense of whether a lens has many groups or few. In any case, I'd agree that some intro text about elements/groups would be appropriate. Early lens designs (before lens coating) tended to favor more cemented groups, because it reduced the number of air-glass surfaces--Vox 15:49, 31 January 2012 (PST)