ecloud 发表于 2013-11-25 12:30
看图就能明白，除了倒数第二个是能够把色散控制在弥散圈以内的真APO，其余的都或多或少有些色散溢出，包 ...
APO的定义并不统一，所以没法说什么叫做“真APO”。19世纪阿贝提出一个定义，即对三条距离较远的谱线校正色差，对两个相距较大的波长校正色球差并控制彗差，这个要求很严格，现代几乎没有哪个产品能真正达到，所以各个厂商都按照自己的理解使用缩水的APO定义。
色差校正是折射镜的一个重要指标，但也不能过分追求。有些最优秀的折射镜产品，其色差校正看起来并不是很完美。而有些色差校正很完美（至少是纸面上）的镜子，因为别的方面损失较多，也未必是最优秀的设计。
TMB生前曾经对此有过一段很好的论述：
“With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma.
现代一个流行的Apochrocmat定义是，一个物镜对于C谱线656.3nm到F谱线486.1nm的波长范围，散射范围不大于四分之一光程差，对于g谱线435.8nm，不大于二分之一光程差，对于可见光内三条间隔较大的谱线校正色差和彗差。（对这一段的用词持保留意见，当然这段不是重点）。
Here is a more detailed analysis for those that are interested. The term "Apochromat" is loosely used by many manufacturers and amateurs astronomers. Let's look at the history of the definition, and maybe a more modern one. Ernst Abbe, in 1875, met and worked for Carl Zeiss, a small microscope, magnifier and optical accessory company. They realized that they needed to find improved glass types, if they were going to make progress with the optical microscope. In 1879, Abbe met Otto Schott. Together they introduce the first abnormal dispersion glasses under the name of Schott and Sons. Abbe discovered that by using optically clear, polished natural fluorite, in a microscope objective, that apochromatism could be achieved. These first true apochromatic microscope objectives were so superior to the competition, that Zeiss gained nearly the entire high end market. So secret was the use of fluorite, that Abbe marked an "X" on the data sheet for the fluorite element, so as to keep it secret from the other optical companies.
Apochromat这个词被生产商和爱好者广泛、或者难听一点说滥用了。光学大师阿贝，1875年遇到蔡司并为之工作，1879年遇到了肖特并发明第一种异常色散玻璃。阿贝发现了使用天然萤石制作显微镜镜头，可以达到他所期望的apochromatism效果，第一台apochromatic显微镜物镜远远超越了同时代的其他竞争对手，以至于蔡司获得了几乎全部的高端市场。Abbe使用X来代表萤石，以保守这个秘密。
Abbe's definition of apochromatism was the following. Apochromat: an objective corrected parfocally for three widely spaced wavelengths and corrected for spherical aberration and coma for two widely separated wavelengths. This definition is not as simple as it sounds. I have designed thousands of lenses: simple achromats, complex achromats, semi-apos, apochromats, super-achromats, hyper-achromats, and Baker super-apochromats. Abbe's definition, to put it in clearer terms (I hope) is that a true apochromat is an objective that has three color crossings that are spaced far apart in the visual spectrum (~4000A, deep violet to ~7000A, deep red). However, just because a lens has three color crossings, doesn't mean that it is well corrected. Let's say that a 4" lens has three color crossings at the F, e and C wavelengths (4861A, 5461A and 6563A). Fine, this objective is now considered an apochromat by most amateurs and even optical designers because it has three color crossings in the blue, green and red -- the common definition of an apochromat. But what about the levels of spherical aberration at each of these wavelengths? If the lens is 2 waves overcorrected at 4861A, and 1.5 waves undercorrected at 6563A, is it still an apochromat? No. It is no better than an achromat, as the OPD wavefront error is worse than a 4" f/15 achromat.
阿贝对apochromatism的定义如下：Apochromat是物镜对可见光三种大间隔的波长（谱线）校正色差（焦点合一），并且对两种大间隔波长校正色球差和彗差。要满足这个定义远没有听上去那么简单。我设计过成千种镜头，简单普消，复杂普消，semi-apo，APO，超消色差（四条谱线），超超消色差（五条谱线）以及Baker super-apochromats. 阿贝定义要求校正三条谱线，但校正了三条谱线不等于APO。一个四寸物镜，对F，E，C三条谱线校正色差，大部分爱好者以及一些光学设计者现在都认为它是APO无疑，因为红绿蓝三条谱线都校正了嘛。但是色球差又如何呢？如果物镜对486.1nm两个波长过校正，对656.3nm 1.5波长欠校正，它还是一只（真正的）APO镜吗？，我的回答是否定的，它比一只4寸F15的普消更差。
Abbe, in his definition of apochromat, states that spherical aberration must be corrected for two widely spaced wavelengths. Now I will tell you what happens when you correct spherical for two widely spaced wavelengths: you correct for all the wavelengths between them too. This is called correcting for spherochromatism (the variation of spherical aberration with a change in wavelength). Only with extremely long focal lengths, advanced Petzval designs, aspherics, large air spaces, or a combination of these designs/factors, can you correct for this aberration. It is the designer that must come up with a good compromise of color correction, lack of spherical aberration (3rd order and zonal) and controlling spherochromatism, so as not to degrade the image contrast.”
阿贝，在他对Apochromat的定义中，强调了色球差对两条宽间隔谱线必须得到校正。我（根据自身经验）得到：当你做到这一点时，你对这两条谱线之间所有的波长都有了很好的色球差校正。只有对于长焦、先进的匹兹万设计、非球面、大空气间隔或者这几个综合，才能达成这种校正。所以在现实中，设计者必须对色差校正、球差校正、色球差校正权衡和互相妥协，并保证影像反差的不降低。
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