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.

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