你拍的照片清晰嗎？

影相第一件重要的事是拍出來的照片要清晰，不過，有時事與願違，是什麼原因呢？ 原來相機有一個對焦的過程，不是遠近的東西都同時可以清晰記錄的，像下面的一張花的照片，當對焦位置在花時，其他的部份就未必/不會清晰了。

We always hope for a sharp picture but this is not always the result.  So, what makes a picture sharp? When taking a picture, there is a process called focusing.  This means that not all the things in the picture can be sharp.  Only the points near the place we focus is sharp.  Like the picture below where the focusing is made on the flower the other parts look blur.

﹝1﹞

但有些時候我們見到一些照片，是無論遠近都同樣清晰的，好像下面的一張，那又是什麼原因呢？

However, we can also have pictures like the one below with the whole picture in focus.  So, what makes the difference?

﹝2﹞

原來這兩張相片拍攝的參數大不相同﹝可以觀看原相片的EXIF資料，若果你有exif viewer之類可以真接觀看，若沒有，可以到以下網址便能看到原圖的基本拍攝資料 圖1 圖2﹞

The main difference lies in the way we take these pictures.  You can learn this from the exif information of the pictures.  Use your exif viewer to learn the parameter of them or you can click fig 1 fig 2 for the details

景深Depth of Field (DOF)

圖1是所謂淺景深的作品，清晰範圍有限，圖2是長景深的作品，由最近的物件到最遠處都在清晰的範圍內。 下面幾張相片，你該可以說出誰是淺景深，誰是長景深的作品：

Fig 1 is of typical shallow dof picture where only a very small part of the picture is in focus.  On the other hand, fig 2 is what we said a picture of long dof.  For a picture with long dof, nearly all the things in the picture should be in focus. For the pictures below, you should be able to tell which one is of shallow dof and which one is long dof.

﹝3﹞

﹝4﹞

﹝5﹞

﹝6﹞

何時用長景深、何時用短景深沒有準則的，視乎你的設計和表達的方式，一般而言，風光照片當然最好盡量的長景深，以便大家把當地的風光看得一清二楚，但也沒有人說不可以用淺景深令大家集中視線在一個位置上。同理，拍攝花卉時我們會想用淺景深把主體以外的東西糊起來不要分散注意力，但這也是一個普遍的準則，沒有非守可的規定。

It is only a personal choice in when to use shallow dof and when not to.  But for a general rule, people like to use long dof in landscape photos as the main purpose of this kind of photos is to show the beauty of the land.  For taking a photo of flowers, you may consider using shallow dof as to concentrate on the flowers themselves.

景深的控制 Control of DOF

用圖1及圖2的拍攝參數來說明如何控制景深： 圖1鏡頭的焦距是23mm，光圈是f5.6；而圖2的鏡頭是18mm，光圈則是f8。兩圖都是用非常短的焦距來拍攝，圖1是歸入廣角鏡之類，圖2則是超廣角範圍了，一般來說，廣角鏡的景深較長。 光圈是相機透光的孔徑的大小，f5.6叫大光圈，當然不是很大，再大的有f4, f2.8, f1.8之類，你會發現數字越小，光圈越大。f8是細光圈了，但也不是很細，還有f11, f16, f22之類。一般來說，光圈越大，景深越淺。 所以圖1的兩個參數都趨向淺景深，得出來的結果就是遠景較糊了；圖2的兩個參數都趨向長景深，因此可以整個畫面都清晰。 為了讓大家更清楚鏡頭和光圈的效果，我特別用了一支135mm焦距的中長鏡用f2.8及f22各對著門前的籬笆拍了一張，你猜得出那是2.8光圈，那是22光圈嗎？

Let's start from looking at the exif data of fig 1 and fig 2. From the exif information, fig 1 was taken by a lens of focal length 23mm with aperture f5.6 while fig 2 was from 18mm lens at f8.  Both lenses very short in focal length.  23mm lens is what we called wide angle lens while 18mm lens falls in the super wide angle lens category.  In general, the dof of a wide angle lens is long. Aperture measures the size of the hole in the camera for the light to enter it.  f5.6 is a large aperture but not quite large as there are f4, f2.8 and even f1.8 which corresponding to larger hole and thus larger aperture.  You may find this is quite interesting, the larger the value used in aperture is, the smaller is the aperture.  Thus, f8 is a small aperture but not quite small as there are smaller apertures like f11, f16 and f22.  In general, the larger the aperture is, the shallower is the dof. Thus, fig 1 with 23mm lens and f5.6 favors shallow dof which fig 2 with 18mm lens and f8 gives longer dof. To make the effect of focal length and apeture on dof more clear, I use a 135mm lens to take a picture of the fence at the front of my home with two extreme apertures, f2.8 and f22.  Below are the result (fig 7 and fig 8).  Can you tell which one is using f2.8 and which one is using f22 aperture?

﹝7﹞

﹝8﹞

其實，除了焦距及光圈對景深有影響外，對焦的距離也是一個重要參數。下面兩幅相片，也是135mm焦距及f2.8和f22的結果，但我走遠了才拍，結果當然還是有景深的差異，但好像看起來就沒有圖7及圖8的顯著了。

Besides lens focal length and aperture, there is one more important factor that affects dof.  That is the distance the camera from the object (or the focusing distance).  The two photos below are both using 135mm lens at the two apertures as before but at a greater distance from the fence.  You can still see the difference in dof but the effect on the change of dof is not so obvious when the distance increases.

﹝9﹞

﹝10﹞

綜合來說，要淺景深的話，就要用長焦距鏡頭、大光圈和近距離拍攝。

So far, we have reached a conclusion that for taking photos of shallow dof, we need to use long focal length lens (tele lens), great aperture and taking the picture at close distance.

85mm焦距鏡頭跟135mm的景深比較
A comparison on the DOF between 85mm and 135mm lens

要計算景深，可以找一個網上的計算器幫手，例如：depth of field calculator 利用這類計算器，我寫了個簡單的Excel worksheet，劃了兩幅圖給大家比較：

There are many online depth of field calculator that we can use to find dof.  The following graphs are from the Excel worksheet I write to show the focusing range for 85mm lens and 135mm lens.

先看85mm鏡頭的效果，這是光圈5.6的。圖中x軸是對焦的距離，藍色曲線與綠色曲線的範圍是景深所在。可以見到在5米距離時，景深只有一米左右；但當離開15米是，景深已經大於10米。

The above graph is for 85mm lens at f5.6.  The x-axis measures the focusing distance while the blue curve is the far focusing distance and green one for near distance.  The range between these two curve is the dof.  For an object at 5m, dof is about 1m but increases to 10m when object is 15m away.

換上135鏡頭，5米時景深當然不足1米，就算15米時，亦只有5米的景深；所以，鏡頭的焦距，確會嚴重影響景深的長或淺。

For 135mm lens at a distance 5m, dof is less than 1m.  However, even when we move to 15m from the object, dof does not increase much beyond 5m.  Compare these two lenses, we can conclude that focal length of a lens would have great effect on dof thus obtained.

為何DC仔比DSLR易拍出清晰的照片？
我在OCOY1234的網誌教你成為最上鏡狗狗上留言說過用DC比用DSLR易拍出出色的狗狗照片，就是基於上面景深的討論。狗狗的照片若是不清晰的話，一定不會說得上是好的作品﹝另有表達除外﹞，而DC機的焦距一般都要比DSLR短好多倍，所以景深特別長，就算對焦不太準確，亦可以把狗狗收納在景深範圍之內，所以能保証照片清晰。你會奇怪我為何說DC的焦距短呢？就如Canon的G9，不是由35mm到210mm的6倍變焦嗎？不錯，宣傳上是這樣說的，因為現在習慣將不同規格的DC統一轉換為135相機的規格來比較，那個焦距是即在135相機﹝DSLR也用這個規格來說明的﹞時相當於35mm到210mm的變焦，其實G9的鏡頭真正焦距是由7.4mm到44.4mm的，在長焦時只有標準頭的焦距，景深已不淺，在廣角端是7.4mm，相信沒有一支DSLR的鏡﹝非魚眼鏡﹞可以這麼超廣角了，因此，G9的景深比一般DSLR都長，易於有清晰的照片；其實，其他一般的DC仔由於感光材料更細，鏡頭的真正焦距更短，景深更長。

所以，想易於拍出清晰的照片，用DC仔會更著數。

順道說明一下影風光相不要用相機向無限遠自動測距的原因
有時會聽到別人說影風景相時不是以無限遠來對焦的，也有些相機已加入景深先決的拍攝模式，令我們拍風光照時可以有最長的景深，原因何在呢？

參見上面兩張說明景深的圖表，大家會發覺當對焦在某點時，例如85mm鏡頭在20米對焦的情況下來說，景深是在17米到24.5米左右，亦即是說大概有一半的景深在對焦點的前，另一半在其後。所以若對無限遠對焦，落在無限遠之後的一半景深是浪費了的，為了要獲取最長的景深，我們應把景深的最遠點放在無限遠上，那麼就可以充份利用景深了。

這個做法叫做超焦距hypofocus，利用上面介紹的計算器，可以計算出超焦距。下面的三幅圖，是分別為50mm及14mm劃出的超焦距圖表：

x-軸是光圈，藍線是對焦的距離，即超焦距，虛線是最近景深距離，亦即是由虛線到無限遠為景深範圍。

例如14mm鏡頭在f8光圈時，若把對焦點設在0.83米處，可以獲得由約0,.4米起到無限遠處清晰的影像，換了是50mm的鏡，只可以由5米開始清晰；所以，14mm f2.8定焦鏡在Nikon的網頁上被稱作記者專用鏡頭，就是若你設了MF手動對焦並把光圈定在f8，對焦位在0.83米處﹝此鏡最近對焦只在幾cm﹞，便可以由40cm開始所有物體都清晰，記者不用對焦，不用看觀景器，只要伸出相機便能把場景拍下來發表。這是超焦距的一個實際運用。

上面是用14mm拍的一張失敗之作，因為沒有用上超距焦的技巧，前景的花花沒有合焦，令人看得不舒服。同樣，遠方的樹也不合焦，由於是利用相機自動測距AF，照片的成效自己就控制不了。

當然，還有很多別的因素影響相片清晰與否，留待以後再說。