原來RF對焦轉向的不同會影響對焦!!!!

大家好，我是我主人最愛的C Sonnar。他這個孤陋寡聞的人剛剛看到一個疑似野人獻曝的資訊，要我上來代PO。

http://www.rangefinderforum.com/forums/showthread.php?t=51742&page=2

請看第30篇Dante Stella的發言：
"Try this focusing technique with the Sonnar. At f/1.5, focus from near to far and stop at the farthest distance where the RF arguably lines up. At f/2 and smaller, focus the other way and stop at the nearest distance that the RF lines up. Your break point may be f/2.4 or f/2.8, but you can figure this out pretty quickly.

On M cameras, there is also some microscopic play in the RF system that can sometimes make a difference based on the direction in which you focus. Unidirectional focusing is explicitly discussed in the instruction leaflet for the Mamiya 6 150mm lens, so it's not anecdotal or speculation."

以及第50篇Gertf的進一步解釋：
"Huck, what Dante is saying is that it makes a difference as to whether you rotate the focus ring from nearest focus or from infinity due to play in the rangefinder mechanism on Leica Ms.

I confirmed this by marking the position of the lens focus ring when focusing on an object 3 metres away. When focussing from infinity to 3 metres the rangefinder lines up a bit further away than 3 metres on the focus scale of the lens. When focussing from nearest focus to 3 metres the rangefinder lines up a bit nearer than three metres markings on the lens.

You can use this way of focussing to make up for a bit of front or backfocus in the lens if you know its behaviour."

也就是固定一個三公尺外的物體，當你針對它由無限遠至近對焦，判斷疊影合焦了，一看鏡頭上測距尺會發現自己對焦其實是在稍微後面一點得地方，比如說3.1公尺。也就是你對焦在3.1公尺的地方，實際焦點會落那個三公尺外的物體上。簡言之，RF的對焦系統在由無限遠轉向近對焦的時候，會造成焦點前移。反之，當你由最近轉向無限遠對焦，判斷合焦了，實際上測距尺會出現你對焦在前面一點，比如說2.9公尺。也就是你對在2.9公尺的地方，實際焦點會落在那個三公尺外物體上。簡言之，RF的對焦系統在由最近轉向無限遠對焦的時候，會造成焦點後移。

我們Sonnar的問題出在大光圈時會焦點前移，那麼在大光圈的情況下時，由最近至無限遠對焦，RF對焦系統會造成焦點後移，剛好可以補償一下我們焦點前移的問題。這是除了自己憑感覺經驗補償外另外一種較省事的補償方法。如果你的其他鏡頭有移焦問題然後你已經知道他移焦的特性，也可以利用這種方式補償。

不管你手上有沒有Sonnar或者會移焦的鏡頭，這都是有趣的知識。
RFF真是個好地方，而RF真的是要很下工夫研究的一個器材。
所有真心要玩RF的人其他地方可以不逛，RFF強烈建議一定要上。
就當順便練英文吧，而且RFF三不五時都會有很靠盃的老外幽默。