負片與正片

負片（又名膠卷或底片，又譯作菲林，film的音譯）。現今廣泛應用的底片是將鹵化銀塗抹在乙酸片基上，當有光線照射到鹵化銀上時，鹵化銀轉變為黑色的銀，經顯影工藝後固定於片基，成為我們常見到黑白負片。彩色負片則塗抹了三層鹵化銀以表現三原色。
以感光類型來分的則有黑白底片、彩色底片、紅外線底片等。
負片的產生，與當時的科技有很大關係。用鹵化銀作感光物料，最容易是獲得相反的影像，因光線會令鹵化銀轉變為黑色的銀。
正片（又名幻燈片) 相對於負片而言，有正片，其記錄顏色和運作原理都與負片相反，正片有一個特色是它在菲林上記錄的影像會與原來的影像顏色一致，所以我們才把幻燈片稱為正片。
認識正片，不得不提Kodachrome，Kodachrome是第一個用减色法而製造的彩色正片，一般底片嘅色彩藥膜係附在片基(乙酸片基)上，沖洗後顏色就顯現，Kodachrome係色彩藥膜並非附在片基上,用K-14沖洗時才將色彩逐層逐層黐上(Layers),所以色彩還原度一流。但一定要專業lab做，所以買Kodachrome是包冲費的。
現是流行的Ektachrome用E-6 process，原理就有點像彩色負片了。
正片有個比負片差的缺點，就是寬容度低，即要求曝光非常準確，而負片一旦under或over 少少都可以用後期黑房技術修正。
正片一般用於投影機投放，而我們看到的電影菲林其實是正片的一種，而直至上世紀末，商業印刷仍是用正片做source，而學生投考各藝術學校，亦被要求以正片形式交出其作品。

In addition for same film speed, negative would give a grainy images, (which maybe a good thing, depending on what u r shooting & what u r lookin' 4.) Also, generally speaking, negative is a high contrast media, compare to slide.

Historically speaking, the first photo is a positive image, which we called slide. It generated by fixing the image in projected on the wall of a church tower in Paris, throughout a pinhole made on the window, 1831. Negative was found around 5 years after that. The most popular negatives during that time, are paper, glass and metal negative. They had various adv & disadv.
Paper is cheap but not likely light sensitive.
Glass the finest image, richest tonal rages, but, heavy to carry and fragile.
Copper, the shortest exposure time but, highest contrast and flammable

They are similar to the film that we are using now. A layer of emulsion was sprayed onto the medium. Many alternative processing/ real "Pin-hole" (not snap shot) photographers still make their own film.