主题：谈谈偏振镜的原理及使用 -- 北田

不至于产生所谓的"Ghost Image"。

参见http://www.bhphotovideo.com/bnh/controller/home?O=filters.jsp&A=getpage&Q=Product_Resources/filterIntro.jsp

Multi-coating

Multi-coating is an excellent way to protect your images from unwanted flare. However, not all filters come with multi-coating. Some manufacturers produce the same filter both with and without multi-coating and when you want to buy that particular filter, you must specify whether you want multi-coating or not. Unless you specifically request multi-coating you will, most likely, be sold a non-multi-coated filter. In addition, many filters are not even produced with multi-coating as an option. That depends on the manufacturer. And remember, a multi-coated filter will cost you more than a non-multi-coated filter. However, many pros feel the added benefits of multi-coating outweigh the added expense.

What is multi-coating? Frequently, “ghost images” appear in backlit photographs or when light enters at extreme angles from the side. This is caused by a reflection created on the surface that occurs when light enters the filter glass. Normal untreated glass causes light loss of approximately 2-4% due to reflection. Along with light loss, flare can occur which causes the picture to appear flat and washed out. The reason for this is due to the different refractive characteristics of glass and air.

Filter coating is necessary to compensate for this difference and to allow reflection-free light transmission. To achieve this, a scratch-resistant metallic oxide coating is applied to the surface in a high-vacuum process. A single layer coating can reduce reflection by more than half. Further reduction of internal reflections throughout the entire spectral range requires several layers.