Talk:Parfocal lens

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Cinematography[edit]

"It is almost impossible to stay in correct focus (as done manually by the focus puller)" — sorry, but this is false. Entire features have been shot on varifocal lenses since the introduction of Canon 5D mark II, including my own feature Kvadrat (film) which used a 24-105 mm" (I'm a director / DP). You may want to use the sources like ShotOnWhat, IMDb, or even the sources in Kvadrat (film) to correct this (look for varifocal Canon zoom lenses like the 24-70, 24-105 etc).

In other words: using a parfocal zoom is highly preferable in cinematography, but not necessary. A varifocal zoom lens can be used, including zooming while shooting, though requiring much more skill from the focus puller (you zoom and pull focus simultaneously). It's a question of comfort, not technical feasibility.

Anatoly IVANOV (talk) 08:29, 19 October 2014 (UTC)[reply]

Deleted external link 'Focusing trick can lead to blurry images'[edit]

I deleted external links section, containing only 1 link to a now dead domain. The content was relevant but didn't add anything to the article IMO. Archive.org snapshot available here Katana (talk) 00:50, 5 November 2012 (UTC)[reply]

Opening sentence of main article may need a re-write/improvement[edit]

Although I understand what the article is trying to say, as currently written (A parfocal lens is a lens that stays in focus when magnification/focal length is changed.), the opening sentence may be incomplete at the very least, and possibly incorrect.

To make my point, see the Ehow.Com entry entitled 'Why Is It Desirable That Microscope Objectives Be Parfocal?'. Paying particular attention to the paragraph 'Determining Parfocality,' note that practicality dictates that images viewed through a parfocal optical system must first be brought into focus by viewing the subject using the system's maximum magnification. Thereafter, if the optical system is indeed parfocal, then changing the focal length to lesser magnification levels will continue to yield a focused image.

So ... While the current opening sentence may technically be correct, without mentioning how optimal focus is achieved, this is the reason I consider the article's opening sentence incomplete. TonyRony (talk) 13:28, 7 March 2014 (UTC)[reply]

How do they work?[edit]

Lots of text on what they are and who uses them, no information on how the optics works under the hood!

Can anybody help fill that out? I want to know more. Miserlou (talk) 21:31, 26 August 2020 (UTC)[reply]

The variable focal length lens uses the principle that the combined focal length of a converging lens plus a diverging lens depends on their separation, the air gap between them being an effective third component. Using this principle, the "variator" group can be designed. That's the easy bit! The problem is that changing the focal length, by definition, changes the position of the plane of focus and to compensate for this other moving elements comprising the "compensator" group are incorporated. It is well known that a telephoto lens has a physically shorter length than its effective focal length, for example, and that a retrofocus lens has a physically longer length than its effective focal length. The compensator therefore needs to be designed with variable telephoto, retrofocal properties or both, depending on the zoom range of the design. So more positive and negative elements with variable air gaps between them, The compensator action is therefore rather more complex than that of the variator, hence true parfocal zoom lenses are less common than partially compensated varifocals. 87.75.117.183 (talk) 17:31, 1 January 2021 (UTC)[reply]