版权声明：转载时请以超链接形式标明文章原始出处和作者信息及本声明
http://mahiraga.blogbus.com/logs/46496550.html

ok，之前和猫咪对定焦镜头进行了一些基于主观印象的讨论，经查，我犯了一些错误。现在我们来补课。

-------------------------------------------------------------

首先，我们看看中文百度百科对定焦镜头的说法，来源在于http://baike.baidu.com/view/973579.htm

定焦镜头

　　焦距固定的镜头，即定焦镜头（一般指单反机型使用的专业镜头），由于焦距固定，所以可以简化镜头组的结构并有助于提高成像质量。
　　定焦镜头的好处，主要体现在短焦段的使用上：
　　1、定焦的广角或标准镜头一般都比涵盖相应焦距段的变焦镜头口径大。一般的定焦广角和中焦镜头的光圈都在F2. 8以上，通光量大，便于在低照度情况下拍摄。
　　2、定焦的广角镜头一般都比涵盖相应焦距段的变焦镜头最近对焦距离近。比如EF17-35的最 近对焦距离是0 .42m，而EF20/2.8、EF24/1.4、EF28/1.8、EF35/2 的最近对焦距离都是0.25m。最近对焦距离小意味着能够非常近的接近拍摄主体，得到大的结像。而且和它们的大光圈配合能够实现变焦广角镜头不容易实现的 广角背景虚化效果。
　　3、定焦广角镜头一般都比涵盖相应焦距段的变焦镜头体积小，重量轻。
　　4、广角定焦镜头一般都比变焦镜头的广角段成像好，这是镜头的设计所决定的，变焦镜头由于要考虑所有焦距段都有相对好的成像，就要牺牲局部的利益让整体有一个相对好的表现。但定焦镜头不用考虑这些。
　　定焦镜头是在还没有发明变焦镜头前所能制作的镜头，它没有变焦镜头适应性广。定焦镜头是单反相 机使用的镜头，不可以调焦距，说得通俗一点就是不能够调节远近，因为不能调远近，所以调节照片的构图那只能靠走。定焦镜头一般只有专业的摄影师才选择用， 适合拍同一种题材使用。定焦的镜头比变焦的镜头（可以调远近)的成像好，所以专业摄影师偏好于定焦镜头。
　　一般喜欢拍人像的朋友都会选择一只定焦镜头，有较好的成像质量。可以随时抓拍，除了不能调焦以外，对于新手来说省去了许多繁琐的对焦过程。

-------------------------------------------------------------

接着我们来看看维基百科对这个问题的说法。

---------------------------

中文:http://zh.wikipedia.org/wiki/%E5%AE%9A%E7%84%A6%E9%95%9C%E5%A4%B4

定焦镜头

---------------------------

英文:http://en.wikipedia.org/wiki/Prime_lens

Prime lens

From Wikipedia, the free encyclopedia

In film and photography, a prime lens is either a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system.

Confusion can sometimes result due to the two meanings of the term if the context does not make the interpretation clear. Alternative terms primary and fixed focal length or FFL are sometimes used to avoid ambiguity.

Contents [hide] 1 As alternative to zoom lens 2 Traditional meaning as primary lens 3 References 4 External links

[edit] As alternative to zoom lens

The term prime has come to be used as the opposite of zoom; that is, a prime lens is a fixed-focal-length lens, while a zoom lens has a variable focal length.^[1][2][3]

A prime lens of a given focal length is less versatile than a zoom whose range includes that focal length, but is often of superior optical quality, lighter weight, smaller bulk and lower cost. This is because prime lens designs are simpler (fewer moving parts), and can be optimized for one particular focal length, whereas zoom lenses must function at a variety of focal lengths. Also since they sport a less complicated lens formula, unlike zoom lens, they are less likely to suffer from problems related to chromatic aberration.

Prime lenses usually have a larger maximum aperture (smaller f-number) than zoom lenses. This allows photography in lower light (with the same shutter speed) as well as achieving a shallower depth of field.

In 35 mm photography, a "normal" prime commonly refers to a prime lens with 50 mm focal length, though that is somewhat longer than the film format diagonal of 43.3 mm, which is typically taken to define a normal focal length. Small sensor digital cameras will require shorter focal lengths (in the ratio of their crop factor) to have the same angle of view.

[edit] Traditional meaning as primary lens

An alternate and apparently somewhat older meaning of the term prime lens is the main lens in a combination lens system.^[4] When the camera lens is used with some other optical device, such as a close-up lens or teleconverter, the camera lens itself is properly called the prime lens. Prime is here used in the sense of primary, chief, original, first in order, etc.

Lens manufacturers such as ARRI Media^[5], ISCO Precision Optics^[6], Schneider^[7], Carl Zeiss AG^[8], Canon^[9] and others still make variable focal length cine and video lenses regularly catalogued as variable primes. A variable prime is sometimes distinguished from a "true zoom" in that the latter maintains focus as the focal length is varied.

This use of the term "prime lens" is an example of a retronym. Early in photography only primary camera lenses were available, and were merely called "lenses" or "objectives". Later, "auxiliary" lenses were available, which usually fit in front of the front element of the primary, or "prime" lens.

---------------------------

德文:http://de.wikipedia.org/wiki/Festbrennweite

Festbrennweite

aus Wikipedia, der freien Enzyklopädie

Festbrennweite ist im Fachjargon der Name für ein Objektiv dessen Brennweite fest eingestellt ist und sich nicht wie bei einem Zoomobjektiv verändern lässt. Im Vergleich zum Zoomobjektiv ist eine Festbrennweite einfacher und günstiger zu entwickeln und zu produzieren. Deshalb haben Festbrennweiten in der Regel ein besseres Auflösungsvermögen und bessere Abbildungsqualität sowie eine höhere Lichtstärke.

Neben der Fotografie kommen Festbrennweiten auch in der Videotechnik zum Einsatz.