為什麼膠片機長時曝光會導致倒易律失效？
好問題
這問題要回答兩方面：
1、極低光照（既然需要長時間曝光，當然是低照度的情況）
2、極短曝光時間
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通俗的講，要形成一個潛影中心（之後能顯影還原），你需要一定數量的光子「打中」鹵化銀。而這些光子到來的時機和方式，決定了這個潛影中心是否能夠形成。
1、極低光照
我們舉個形象的例子好了。
打麻將要四個人湊一桌，客人隨機各坐各的桌子。不巧的是最近客流量非常稀少，張三和李四坐在5號桌，等了半天沒人來，李四等不下去，走了。過了一會王五、趙六坐到了5號桌，但人還是不夠四個。張三等不下去了，走了…
5號桌始終沒有湊齊四個人，開不了胡。
當光子數量不足無法在「一定時間」內連續「擊中」鹵化銀顆粒，吸收的能量會逐漸衰退，無法激活足以讓潛影中心形成的所需的至少四個銀原子（金相原子參與下為三個），形成不了潛影中心，鹵化銀也就不會顯影。
因此低光照下部分鹵化銀運氣不好，始終無法被激活，倒易率會失效。
2、極短曝光時間
還是舉打麻將的例子。
這個麻將館比較奇怪，第一個人坐下來之後，第二個人得等一分鐘才能坐下來。
今天客流量爆炸，門一開一下湧進來幾百號人，各自奔著桌子去了。張三坐到了5號桌，李四和王五走了過來，發現要等一分鐘才能落座，便直接從後門出去玩了。但人流就這麼一會，還沒到一分鐘，後面沒客人了，張三孤獨守著桌子一會，等不及也走了。
5號桌還是沒有湊齊四個人，開不了胡。
鹵化銀不能連續的吸收光子，吸收一個要「歇一會」，這種短暫間隔在極短曝光時間情況下就變得舉足輕重。假設曝光時間短到只有這種間隔的10倍時長，就會導致「丟失」50%的光子，如果曝光時間短到只有這種間隔的5倍時長，會丟失60%（以上數據只是示意，並不精確）。
鹵化銀對光的「吸收率」是一定的，因此在極短時間內，很多光子無法被吸收，縱然光線很強。倒易率故而失效
就是這樣。
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你們要學術化的解釋？
覺得我舉的例子太蹩腳？
既然這麼好學，英文原文簡單的一逼，我就不翻譯了：
...(a) A photographic emulsion consists of tiny crystals of silver halide, called
grains, suspended in a gelatin medium. Distributed through each grain are mobile
interstitial silver ions, freed from the lattice structure of the grain, and stationary
traps, thought to be impurities or crystal dislocations (or both), having a positive
electric charge. Deep traps have a larger charge than shallow traps.
(b) When light strikes a silver halide grain, electron-hole pairs are created which
are free to independently drift through the grain. Such electrons are called photoelectrons.
(c) A photoelectron may become caught in one of the traps, more likely a deep
trap where it is held more tightly. A potential field is set up around the deep trap
which repels other photoelectrons, but attracts interstitial silver ions. Such an ion
in the vicinity of the trap will be attracted to the trap, and a silver atom will result
at the trap. The time interval between the trapping of the photoelectron and the
arrival of the interstitial silver ion is called the dead time.
(d) This silver atom is, however, unstable and will decay after a time interval of
length, called the decay time, into a silver ion and an electron. However, if before
this silver atom can decay, it is joined by another silver atom formed in the same
manner at the trap, a more stable two-atom silver speck results.
(e) Successful repetition of the process in (c) and (d) above leads eventually to
the formation of a stable silver speck. The aggegrate of all the stable silver specks
in the grains forms the latent image. Only those grains which contain a stable silver
speck will be reduced to pure silver during subsequent chemical development of
the emulsion.
It is believed that step (c) is a source of high intensity reciprocity failure, since
during the dead time, those photoelectrons prevented from entering the trap can
recombine with holes and are lost to the photographic process. Step (d) is thought
to account for low intensity reciprocity failure, since at low intensities, it is unlikely
that a second silver atom can be formed at the trap before the first decays. Based
on these assumptions, the first author in [l] introduced a model combining both
low and high intensity reciprocity failure at a single trap, and in a more detailed
model of high intensity failure which allowed for recombination with holes.
Furthermore, while steps (c) and (d) contribute to reciprocity failure, they need not
be the only factors. For example, whereas whole grains almost certainly function
independently of one another during latent image formation, individual traps in
a grain certainly do not, and the competitive action of deep traps in the same grain
as a factor in high intensity reciprocity failure has recently been investigated...
(source:A QUEUEING MODEL FOR RECIPROCITY FAILURE IN THE PHOTOGRAPHIC GRAIN
W. J. ANDERSON and A. M. MATHAI)
另外有興趣的話去精讀那本Tadaaki Tani寫的《Photographic Science》，純學術。
問題問的是長時間曝光啊。。。。各位好好審題。。。
長時間曝光互易律失效主要是因為鹵化銀的敏感性降低了。
不知道進一步的解釋，不過比較新的膠片都支持較長的曝光時間互易律不失效，特別是常用於拍風景的。。。。具體的互易律失效情況要查表。
低於極限靈敏度的話。。。就相當於在膠捲盒子裡面啊
鹵化銀顆粒在超過「一定」時間的曝光情況下，會產生「一定」的惰性。但這個惰性雖然是必然的，但不是有規律的常數。具體到某種膠片，如果是專業膠片，膠片生產公司的網站上基本能查到。下面是幾種專業膠片網站上查到的補償數據。以富士為例：
fuji provia 100f:
fuji velvia 100f:
依據上表，在倒易率失效的情況下，補償的時候，除了時間上還需要在濾片（色彩）上有補償。專業膠片在倒易率的問題上比民用膠片做的好很多，比如provia在128秒以上才建議補償曝光。民用膠片這方面就做的要差不少，舉例superia200彩色負片，官方資料給出的數據如下：
superia200,在曝光4秒的情況下就需要補償三分之一檔。跟同公司的provia和velvia不可同日而語。
光的絕對量不足，銀鹽反應不了
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