Talk:Lens

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 [...]

Links

• focal length (educational fun)
• focal length (don't worrý)
• focal length (the world still worries)

Astigmatism

User:Grzesio

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)