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發表於 2007-10-17 21:16
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http://www.fredmiranda.com/forum/topic/572769/2
看起來滿像一回事的post，
看起來50 1.2L \"focus shift\"的issue是球面像差造成的?是設計上的妥協與特性?
文中提到Zeiss ZM 50mm 1.5 Sonnar在設計為了完美的散景，設計時刻意不修正球面像差，代價就是\"focus shift\"，
Zeiss可以依使用者的習慣 - 常開放光圈or縮光圈拍攝，幫忙校正。但是沒有辦法校正到兩邊都好。[s:11]
The newish Zeiss ZM 50mm 1.5 Sonnar is purposely designed with loads of spherical aberration, for a 1930s glamour portrait look. The price is loads of focus shift (Zeiss will calibrate the lens for use wide open or stopped down, your choice - but you can\'t have both).
canonfans應該臥虎藏龍，
有沒有懂光學的大大能幫忙解釋? [s:44]
原文如下:
focus shift has been around forever. it was pretty common in older lenses with 100% spherical elements. The problem is spherical aberration - near the center of the lens elements, the curvature is relatively low and the aberration between ideal and actual is pretty small. When you stop down, the lens only sees through the well-corrected center so you have \"correct\" focus. Near the edges (included in the wide open image), spherical aberration piles up and can cause a shift in the plane of best focus between the ideal (similar to where it would be at say f8) and actual. Since spherical aberration varies by color, there\'s usually also some smearing and loss of sharpness with non-monochrome subjects.
All of this can be fixed with aspherical and APO elements. Problem is, most often those corrections bring along nasty bokeh. For world class bokeh, it\'s a balancing act between leaving aberrations in the wide-open lens (for nice portrait bokeh, since a 50 1.2 is going to be used as a portrait lens on 1.6) and being fully-corrected stopped down. Apparently the possibility of focus shift either eluded Canon, or was deemed to be acceptable.
For more examples of focus shift, read up on the Leica 35mm Summilux-M ASPH and the 50mm 1.0 Noctilux. The newish Zeiss ZM 50mm 1.5 Sonnar is purposely designed with loads of spherical aberration, for a 1930s glamour portrait look. The price is loads of focus shift (Zeiss will calibrate the lens for use wide open or stopped down, your choice - but you can\'t have both). A google seach will reveal a lot of large format guys talking about it (LF guys commonly use 80-year-old lenses, which are very simple spherical symmetric lenses prone to focus shift). The Leica M8 has been exposing focus shift in classic lenses with a vengeance. One theory is that the thickness and waviness of film emulsions has been covering up this kind of focus shift since the dawn of time, and only the ruthlessly flat digital sensors reveal it.
[ 本帖最後由 vario_sonnar 於 2007-10-17 21:28 編輯 ] |
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發表於 2007-10-17 21:00
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