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2008年11月16日星期日

十二月份月曆

又快到2008年最後一個月了，用PhotoCap做左幾款12月份的月曆，可以印出來用又或者放在桌面做wall-paper。

至於點樣用PhotoCap，就唔再講了，有興趣去睇返D舊blog就OK....

如果你有興趣直接用我做的月曆卡，隨便下載便可以，想要更高解像度的原圖，留言比我吧。

2008年11月13日星期四

To Make it Sharp

曾經介紹過景深的觀念，拍攝的物體，若在景深範圍之外，就不會清晰了。其實，除了景深之外，還有一些別的因素，影響到我們照片的清晰與否。
不如，由一張我昨晚在屋外拍的飛蛾說起。

We have talked about how dof is used in obtaining a sharp photo. Actually there are other factors affecting whether a photo is sharp enough or not. Let us start by considering a photo of a moth that I took last night outside my home.

這是一張微距拍攝的照片，大概是1:2的大小，亦即是說在感光材料上的大小，是原飛蛾的一半。在物理學上說，這麼大的放大率，相機要十分接近飛蛾，所以景深十分淺，要用極細的光圈拍攝。據exif顯示，拍攝光圈是f24。

This is considered as a macro photo. It is 1:2 meaning that the magnification is 50%. From optics we can easily conclude that the picture is taken at a very close distance. From the previous discussion, this gives a shallow dof. The exif information shows that f24 is used to get around this shallow dof so that the whole moth is clear and sharp.

f24是極細的光圈，以晚上的光線計，用f24光圈，曝光時間就上等上十多秒了。試想想，在這十多秒期間會發生什麼事呢？

一是飛蛾會移了位
二是手持的相機也會移位

無論是誰移動了，得出來的相片就無法再如此清晰了。

就像下面一張長時間曝光的相片一樣，當然，這可能是你預期要有的效果。

f24 is a really small aperture. To have correct exposure, the shutter speed may over 10 sec. What would happen in this period of time?

may be the moth will move
may be the camera will move

No matter who moves, once there is relative motion, the picture will no longer be sharp.

The following picture is a typical long exposure time picture. This may, or may not, be the effect you desired.

拍照時，相機必然有震動，怎樣的快門速度才夠快讓我們不用害怕相機移了位呢？

原來經過了長時間的實踐，我們已找出一個所謂安全快門的公式，那就是看一看你所用的鏡頭焦距，若是50mm，你的安全快門就是1/50秒，比這慢的快門如1/30秒等，都有可能因相機的移動導致相片不清晰。

如果改用500mm的長鏡，安全快門就只有1/500秒，所以鏡頭的焦距越長，越易因相機的震動而得不到清晰的照片。

The vibration of the camera cannot be avoided. So, how fast a shutter speed is fast enough to have a sharp picture?

There is a safe speed for every lens of a certain focal length. For a 50mm lens, the safe speed is 1/50 sec. This means that using a shutter speed lower than this, say 1/30 sec, will probably result in a blurred picture.

When a tele lens like 500mm lens is used, the safe speed reduced to 1/500 sec. Thus, the longer the focal length, the harder to get a sharp picture due to camera vibration.

要擺脫安全快門的限制，可以考慮以下各點：

利用腳架，無論是三腳的或是獨腳的，都可以有限減少相機的震動。三腳架是首選，若遇上仍不安穩時，可以在腳架上掛上重物，增加腳架的穩定性；獨腳架要配合拍攝者的身體使用，使形成三腳架的效果，不過，這樣做的穩定性不高亦與使用者的技巧有關。
找承托/支持，如倚在柱邊或俯伏在地上等，上面那飛蛾的相片，正是倚在門邊拍的。
考慮用有防震的設計相機及鏡頭。防震系統在Nikon裡叫VR，在Canon裡叫IS，都可以有效減低約2到3級快門，亦即是說若安全快門是1/500秒，可以改為1/125秒，這些防震功能對手持拍攝有極大的幫助。

To go around the safe speed, we may do any of the following steps:

Use a tripod. This would give a stable camera. The ability of the tripod can be further improved by hanging a heavy bag under it. When tripod is not suitable for the situation, we may consider using a monopole. The monopole together with the photographer forms a tripod. However, the effectiveness of it depends on the technique of the photographer.
Find external supports. We can lean against a wall or lay down on the floor to have good support. The first picture in this blog is taken when I am leaning again the door of my home.
Use machines/lens with vibration reduction system may also help. In Nikon this is called VR while IS in Canon. They are said to be able to increase the speed for 2 to 3 readings. In this way, a safe speed of 1/500 sec without such a system is changed to 1/125 sec. This surely help the hand-held type photographer very much.

再看那雙蛾，若你看我的exif，你會發覺我的快門速度並沒有十多秒那樣長，其實，我們還有一個法寶，就是用外部光源照亮物體，那麼，我們就可以返回安全快門。這相片因為用了閃光燈，快門速度只是1/250秒。

有時，我們會以為閃光燈只是一個補光的工具，但實際上它可以做更多的工作，在這裡，我就是用它來加快曝光的完成，除了防手震外，還有兩個作用：

我因為有了大門的倚靠，其實一兩秒的曝光是可以保持相機不移動的，但有誰保証飛蛾會合作呢？
更重要的一點是我向著屋內拍攝，透過玻璃門我是會拍到家內的景物，用來做飛蛾的背景會十分討厭。原則上我用的相機在高iso中有良好表現，手持一兩秒再加高iso的話，任務還是可以完成的，不過，屋內的光度比屋外高，這樣一來，大家只會看到家中的東西而看不清主角，用了閃燈可以選擇性地對飛蛾加光，看起來，背景就會是一片漆黑了。

其實，拍照用閃燈也是一個大學問，用得洽當，可以增加相片的可讀性。

Back to the moth again. If you peek at my exif information, you will find that the shutter speed is 1/250 sec instead of 10 sec. The reason that I can do this is by an external light source. I use flash to reduce the shutter speed well below the safe speed.

Flash itself is not merely a tool for adding light to darker part of a picture. I used it to reduce the time I need to take the picture. Besides that this would help me in reduce the chance of camera motion in giving a blur picture, it also helps in other two ways:

Although I have a support from the door which would guarantee as low shutter speed as 2+ sec, who can guarantee the moth won't move in this time period?
A more important point for using flash is to reduce the effect of the light inside the house on forming a visible background for the moth. I selectively light up the moth by flash so that the rest of the picture looks dark. What would happen if I use high iso and 2 sec shutter speed to take my picture. I would say the moth would be as sharp as you see here but the background is bright too. We would lost our concentration on the moth when we look at the picture then.

I can say that there is a great knowledge in using flash. When it is used correctly, it would increase the attraction of our photos.

===>

2008年11月11日星期二

乘夜前來的雪人及其朋友

自從雪人出來了之後，我家就引來了一大班雪人及其家族，他們不單止有意思留待聖誕來臨，還不斷通知他們在遠方的友好，我們這裡的客房都給他們住滿了，在他們未有動向前，實在招呼不了更多的雪人及他們的朋友，不過，就在我們趕得及跟他們溝通前，他們已通知了另一批朋友漏夜趕來，惟有收留他們並介紹一下他們。

1. 一雙一對雪人伴侶 ─ 他們從不分開，見佢地咁恩愛，昨晚邀請左佢地同我地一齊食晚飯。真係好sweet sweet。

2. 雪帽禮物雪人同佢地靴 ─ 人地由北方趕來香港一定帶手信，點知見到佢個袋空空如也，問佢點解，佢話聽講香港是購物天堂，當然來了香港先買，何況依家中國是世界工廠，無謂在北極買件中國製造的禮物又帶返香港，聽落真係好有哲理。

3. 掌上明珠雪人BB1號、2號、3號及4號 ─ 佢地雖然出世無耐，都趕到尾班車來，佢地話北極聖誕冇氣氛，香港始終係東方之珠先配得上佢地掌上明珠，聽到我一頭霧水。

4. 雪人老爺奶奶 ─ 唔好以為後生仔先對香港有興趣，其實老人家都好想來香港的呀，你睇雪人老爺鬚都白晒，都要來湊下熱鬧，佢太太當然跟住來照顧佢，真係恩愛。

5. 工程師雪人先生 ─ 咁多雪人同佢地的朋友來了香港，有乜意外，香港真係冇邊個識得救佢地，所以工程師先生就請膺自己來幫佢地，佢話如果香港雪地唔夠，佢會剷雪，不過同北極唔同，香港要剷D雪比佢地行wor。

6. 瘦身雪人家族 ─ 估唔到雪人都興瘦身，見佢地高佻的身形又真係好吸引，唔知會唔會乘機做下瘦身廣告代言人呢？

7. 肥仔雪人 ─ 唔好以為雪人都係瘦身的，原來也有肥肥白白的雪人，你睇佢件衫拆左線補返就知佢今年的身形又加左幾個碼，真係家肥屋潤，又幾好意頭。

8. 快樂寶盒雪人 ─ 佢地三個真係攪笑，每人一個盒，聽講話係快樂寶盒，對住個盒，就算空的，都會好快樂。

9. 雪橇雪人家族2號 ─ 原來佢地自己有私家車，唔怪之得趕到來。

10. 快樂三人組 ─ 佢地又唱歌又跳舞，都仲趕到來，真係叻叻叻

11. 掌上明珠聖誕老人版1號及2號 ─ 原來唔單止冇掌上明珠雪人，仲有聖誕老人，我地香港有咁多小朋友，佢地真係要生多D掌上明珠聖誕老人版先可以應付得來呢。

12. 快高長大雪人 ─ 依個小朋友認真勁，我點影都影唔晒佢，惟有委屈下佢，佢話要同一個快高長大的小朋友一齊先會開心，但係我家都冇小朋友，真係煩。

2008年11月10日星期一

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。