✔ 最佳答案
There is no problem in defining exposure value (EV) = log(base 2)[N^2/t] where t is the exposure time in second.
The reason is because it is a definition. The time t is specified in unit of "second". As such, the value of t is unique. It could thus be regarded as a number.
The reason in applying a log function to [N62/t] is to avoid a large range of numbers that would ocuur in [N^2/t], as t is usually a small number (in order of ms) and may span over a wide range (from ms to s). This practice is not uncommon in science, just as when plotting a graph with wide range of y (or x) values, we usually plot log(y) again x (or y against log(x), whichever the case will be) to facilitate grapg plotting. Here, y or x are generally physical quantities with dimension.
2013-11-25 10:41:07 補充：
I think you still not grasp the cencept behind.
The time t in the definition is unique, because it is specified in unit of second. Even if you use other time unit, you need to convert it into second before calculating the EV....
2013-11-25 10:43:43 補充：
(cont'd)...But in your previous post, the unit in not fixed. As such, would get different values if different units are used.
After fixing a unit in the definition, the value in the parameter thus becomes unique and be regarded as a number.
