點解影相會有紅眼?

首先要知道，為什麼會有紅眼的問題出現：

1. 紅眼是閃光燈打在眼睛的視網膜後的微血管組織上，所反射回來的自然現象，只要是「鏡頭」與「閃光燈」之間的夾角設計得太小，便很容易產生「紅眼」的現象。

2. 在比較暗的環境中，人眼的瞳孔會為了看清環境而自動放大，此時如果閃光燈與相機鏡頭的光軸（optical axis）比較近，強烈的閃光燈光線會通過人的眼底反射入鏡頭，眼底有豊富的微小血管，這些血管是紅色的，所以就形成了紅色的光斑。

3. 數位相機在設計上，較著重於輕薄短小、攜帶方便的考量上，因此「鏡頭」與「閃光燈」常常靠得很近，也就更容易產生「紅眼」的現象。


如果要使用閃光燈，倒也有幾種方法去應付這個情況：

1. 使用相機內置的閃光燈防紅眼功能，閃光燈會在正式閃光之前預閃一次，使人眼的瞳孔感受強光而縮小，減少光線從眼睛那邊的反射，從而減輕紅眼現象。

2. 另一防紅眼方法，便是使用漫射的光線，例如，不要讓閃光燈直接照人眼，讓閃光燈做透過某些反射（如照向天花板，再反射於人像上）。

3. 此外，利用外部的閃光燈，加大鏡頭與閃光燈之間的距離，都可以有效地避免紅眼。

The red color comes from light that reflects off of the retinas in our eyes. In many animals, including dogs, cats and deer, the retina has a special reflective layer called the tapetum lucidum that acts almost like a mirror at the backs of their eyes. If you shine a flashlight or headlights into their eyes at night, their eyes shine back with bright, white light. Here is what Encyclopedia Britannica has to say about the tapetum lucidum:

Among many nocturnal vertebrates the white compound guanine is found in the epithelium or retina of the eye. This provides a mirrorlike surface, the tapetum lucidum, which reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities. Tapeta lucida produce the familiar eyeshine of nocturnal animals.

Humans don't have this tapetum lucidum layer in their retinas. If you shine a flashlight in a person's eyes at night, you don't see any sort of reflection. The flash on a camera is bright enough, however, to cause a reflection off of the retina -- what you see is the red color from the blood vessels nourishing the eye.

Many cameras have a "red eye reduction" feature. In these cameras, the flash goes off twice -- once right before the picture is taken, and then again to actually take the picture. The first flash causes people's pupils to contract, reducing "red eye" significantly. Another trick is to turn on all the lights in the room, which also contracts the pupil.

Another way to reduce or eliminate "red eye" in pictures is to move the flash away from the lens. On most small cameras, the flash is only an inch or two away from the lens, so the reflection comes right back into the lens and shows up on the film. If you can detach the flash and hold it several feet away from the lens, that helps a lot. You can also try bouncing the flash off the ceiling if that is an option.

當攝影環境較暗時，被攝者的瞳孔會張得很大，而在閃光燈閃亮的瞬間，瞳孔來不及收縮，視網膜後面的紅色血管受到強烈閃光照射，令反射到相機鏡頭的閃光也呈現紅色，因此所拍到的照片就會出現紅眼現象。如下圖所示，紅眼現象使拍攝到的影像大大失色。


紅眼消減技術 (Red-eye reduction technology)

我們可以利用預閃閃燈技術減低紅眼現象：當利用閃光燈拍攝時，閃光燈會在快門開啟之前，先發出一次或數次較小光量的閃光，使被攝者的瞳孔受到強光刺激而縮小；而當快門開啟時（即正式曝光的時候），閃燈才正式發出閃光。由於這時被攝者的瞳孔已經縮小，所以紅眼現象便得以消減。

不說不知，這種消減紅眼技術是 Kodak 的專利發明，其他相機廠每售出一部設有消減紅眼功能的相機，都要付 Kodak 版權費呢﹗