拍產品的討論 A discussion on product photography

在大陸的某攝影網站上有個討論問公司要拍筆筒般大小產品的細節，只有5千元預算，還要兼顧拍會議等場面，可以用什麼鏡頭？ 有人說該用微距鏡，有人說該用移軸鏡，有人說什麼鏡都不是問題，一定要有閃燈，真是意見紛云。	There is a discussion in a Mainland China photographic forum on the choice of lens for taking the details of product of the size of pen holder but the budget is limited to 5k.  What makes it even harder to make the decision is that the lens should be used in event photography too. Some suggest macro lens, some said PC lens is a must and some insist lens is not a problem but an external flash is a must.
對於選用微距鏡的人來說，質素是他們的首要考慮，因為要拍出好片子，鏡頭要有足夠的銳利程度，片子中心跟周邊的效果不可以相差太大，這些正正是微距鏡的強項。可惜，微距鏡也不是太平的，舊款的60mm micro 2.6k，新款的60mm micro G鏡就6k，105 VR也超預算，所以，微距的選擇似乎不可能。	For the choice of macro lens, quality is the keyword.  If we need a picture with sharpness at all part of the picture, there is no other choice than using a macro lens.  However, it is expensive to buy a macro lens.  The old model 60mm micro is selling at 2.6k while the new model is over 6k.  The 105 VR is also over budget.  Thus, it seems not possible to use macro lens in this case.
提議用移軸鏡的，當然是考慮到景深控制的問題，在有限景深下，用移軸鏡可以選擇最有效的焦平面，可惜，移軸鏡價值不菲，5k只可以買半支舊款的移軸鏡。	PC lens would give a very good control in the using of the limited DOF.  However, PC lenses are very expensive.  Even for the old model, 5k can only purchase half a lens, if they willing to cut it for you.
我卻偏向相信有兩個解決方案： 若果要用DSLR，可以用套頭作會議之用，這樣大概用去2k，在二手市場找一只55mm micro作產品拍攝用，這會用1k左右，餘下的2k買一支外閃。這組合可以作平面出版之用。 若果可以棄用DSLR，則DSLR用套頭作一般用途，另購一DC作產品拍攝，餘下約1k可以購置一三腳架。這個組合對一般網上發表經已足夠。	To me, I believe there are two solutions. If DSLR is used, use the kit lens for event shooting (about 2k), use a second-hand 55mm micro lens for product (about 1k) and add an external flash to the system (about 2k).  This would be good enough for book and magazine publishing uses. If it is not necessary to use DSLR, buy a DC for product use (2k) and use kit lens for event shooting.  The remaining 1k would be good enough for a tripod. This system would be good enough for distributing pictures in the internet.
我的選擇其實是基於景深的考量，由於拍攝的距離短，景深十分有限，所以要盡量令組合可以給出最大的景深。 為了了解當中的情況，我們要看一下光學中的一些事實。 在物理裡，有兩公式綜合了我們的討論。 (1) 透鏡公式 : 1/f = 1/u + 1/v (2) 放大率 : m = v/u 利用這兩條公式可以知道，若要利用55mm鏡頭拍攝1:1的圖片，物距是110mm，將這個資料放入計算dof的工具，發覺在f22光圈時景深有0.27cm，對大致沿焦平面放置的產品大概可以了；但若果只有f5.6光圈，則景深變為0.07cm，這對一般的拍攝變為不可能了。當然，不需要1:1的放大率時景深會寛容許多，不過重點是要用細光圈，所以閃燈不可少，這正是第一個方案的原因。﹝注意，上面雖然用了55mm鏡頭舉例，但在相同機器下，相同放大率時，焦距不影響結果。﹞	My choice of solution is based on the fact that when shooting close to the product, the depth of field (dof) is very limited.  In order to make the mission done, even not to the best quality, we need to obtain the greatest dof. Let us look at some facts in optics. The basic physics in this problem is summarized by two equations: (1) Lens formula : 1/f = 1/u + 1/v (2) Magnification : m = v/u Let us use an 55mm lens as an example (but the result is the same when other focal lengths are used as long as the camera does not change).  To have a magnification 1:1, the above two equations give the object distance v to be 110mm.  Put these data into a dof calculator, we found that  at f22 the dof is 0.27cm while it reduces dramatically to 0.07 at f5.6.  Thus you can see f22 seems necessary for any practical shooting at this magnification.  Of course, when the magnification reduced, the dof will be greater for you to work with.  But the truth of using small aperture still applies.  Thus, this justifies to have an external flash in the system is a must.
除了可以利用細光圈獲得大景深外，亦可以用細感光元件來達至。以G9為例，若要拍出像本文第二幅照片的效果，在F8光圈時﹝G9的最細光圈﹞景深為5cm左右，但若用DSLR在f22時拍，景深也只有1cm左右，因此，我們可以見到DC在產品拍攝的優勢，當然，細感光元件也有缺點，就是雜訊高質量低。所以，在只需在網上發表作品的情況下，DC是一個好的選擇。	Besides using small aperture, we can use small sensor to obtain great dof.  When a DC like G9 is used, to create a picture like the second picture in this blog, the dof is about 5cm at F8 (the smallest aperture can be used) while it is only about 1 cm when using DSLR when f22 is applied.  You can easily see the advantage of using a DC in product shooting.  Of course, the quality of the picture would not be as high as the camera with large sensor.  Thus, this solution is good only when we need to put our pictures in the internet only.
I know lots of people studying physics also like photography.  May be it is because we have another understanding of it from the very fundamental way. 我認識許多讀物理的人也是攝影的愛好者，或許這是因為我們讀物理的人對攝影根本就存在著一些十分另類的基礎認知。