移軸鏡的新奇功效 Special Effect by PC Lens

上期的網誌我跟大家玩了一個沒有獎的問答遊戲，我給大家兩張用相同鏡頭以相同參數拍攝的照片，但照片顯然有所不同，現在，我們再一次看看那兩張相片吧	Last time in my blog I asked what makes the difference of the two pictures using same lens, same setting.  Let us see the two pictures again.
下面是那兩張照片，(A)全部清晰，(B)只有水平中間部份清晰： The following are two pictures I used as example.  (A) being clear and sharp while only the central horizontal part of (B) is sharp:
(A)	(B)
其實(A)是一般的拍攝方法，由於網門與鏡頭及感光元件平行，只要中間部份對焦，其他的部份也可以對焦，因此整個網門都影得一清二楚。 不過(B)圖中的鏡頭與感光元件並不平行，所以縱然調教到中央部份對焦，網門的上半部及下半部都不能合焦，形成這樣一個特別的效果。	Photo (A) is just taken by normal camera setting where the lens and the light sensitive element (CCD/CMOS) are parallel to each other.  As the door is also parallel to them, this will guarantee once a point is in focus, the whole door will be in focus too. In (B), the lens is not parallel to the CCD/CMOS.  Although the central part is in focus, the other part does not.
上面的貓照片，大家會發覺很奇異地只有最近鏡頭的眼睛是合焦的，其他部份，雖然與鏡頭的距離相若，依然不能合焦。原理也是一樣，是相鏡的鏡頭與感光元件並不平行所做成的。 不過，你會問，為何要弄到鏡頭與感光元件不平行呢？又如何才可以弄到他們不平行呢？ 其實答案是移軸鏡。	The same effect can be seen from the above picture of a cat.  To make the eye and only the eye in focus, I use a lens that is not parallel to the CCD/CMOS when I take this picture. Of course, you may wonder why we need to arrange them not in parallel and how we can do this. The answer lies in the production of PC (perspective correction) lens.
移軸鏡其中的一個作用就是讓鏡片的軸可以移離相機的主軸，這樣就可以產生一些特別的效果。 一般來說所謂移軸，可以是鏡片的主軸與相機主軸保持平行的平移，亦可以是把兩條主軸形成一個角度的傾移，前者不在這裡說明了，這裡只想說說傾移的一些效果。 下面是一幅我拿著相機在書枱前拍攝平放在書枱上一份文件的結果，由於相機與文件並不是相互平行而景深有限，所以只有部份文件是清楚的。	PC lens is a special type of lens that allow the lenses in it deviate from the principal axis of the camera so that special effect can be achieved. There are usually two types of movement of the lenses.  One is shift and the other is tilt. The effect of shift will not be covered here.  I will only say something about the effect of tilting the lenses. The picture below shows the effect of taking the photo of a document on a table when holding the camera near the table.  As there is an angle between the camera and the document and the sharpness is limited by the dof, only part of the document is sharp.
利用移軸鏡的傾移作用，在相同的情況下，沒有減少光圈，我可以得出下面的結果。	With the help of the tilt of the lens of a PC lens, I can obtain a much better result even thought I have not reduce the aperture of the setting.
原來光學裡有這麼的一條特別的原理，就是當鏡片的面與感光元件的面有所相交時，就會出現一個與他們一同相交的面，而這個面的所有部份都是合焦的。	The secret lies in a theory saying that there is a plane with which it meets the intersection of the lens' plane and film's/sensor's plane that the whole plane is in focus.  When the document is that plane, all the words in it can be read clearly.
好好的利用這個原理和一枝移軸鏡，我們就可以拍出比一般鏡頭有更廣清晰範圍的照片了。 不過，你亦可以反其道而行知，利用移軸鏡頭的傾移效果，把原本該是一清二楚處在焦平面的東西移離焦平面，使只有一小部份清楚，這正是我在上一篇網誌中所做的。多了一層技巧，也許在你設計你的照片時你可以做得比別人更加不同了。	Thus you can see we can obtain photos with wider "dof" by the tilting of a PC lens. However, no one limits you from doing just the oppose.  You can use a PC lens to deviate a plane that is in focus originally so that only a small part of it is in focus.  My last blog is showing pictures of this type.  Hope this will widen your ability in designing your photos.