攝影與真實的看見, 其實有所不同. 因為人眼對光線的寬容度是比菲林和感光原件高的. 假設人眼可同時看見光度1至10的畫面, 相機拍下的, 可能只有3至8的光度, 過光的會全白, 過暗的會全黑. 因此, 我們不可能見到什麼就拍什麼, 還要先問問自己想拍下的什麼. 最基本的, 就是以想拍攝的主體去選擇曝光的基數.
The photography is different form the reality. It is because the human's eyes have a higher dynamic range on light than film and CMOS. Let say, if human can see the light level form 1 to 10 at the same time, camera can only record maybe 3 to 8 light level. It will be totally black or white if above or below the range. Therefore, we can not snap everything we saw. We must ask our self, what I am want to show in my picture. You can choose the object you want to show on the picture as the basic exposure measure. It is the foundation of exposure.