[Law2718]

各位前輩, 請各位可指教, 如何計算當光圈改變時, 快門如何改變而令相片的曝光不變?

[h0123954]

You can try to simply use the "Tv" mode for the camera to recalculate the new shutter speed.

[Law2718]

多謝, 但因為我的G3在慢快門或夜景時要用全手動模式, 所以如果有計算方法, 我想可能有幫助

[Velsper]

先知道快門光圈"一級半級"的關係
大----------------------------------------細 (光圈)
1.0 1.4 2.0 2.8 4 5.6 8 11 16 22 32
慢-----------------------------------------------------------------------------快 (快門)
2' 1' 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000
以上是一級的分別
如果光圈大一級, 快門就相應快一級
反之亦然

[dede]

公式：EV = log2(aperture^2 x (1/shutter speed) x (ISO sensitivity/100))

EV=Exposure Value(暴光指數)
aperture^2=光圈的平方
shutter speed=正確的快門速度全寫(如400s=1/400)

eg.:
f=5.6 -> 5.6*5.6=31.36
S=1/250s = 0.004
ISO=100

EV=log2(31.36 x (1/0.004) x (100/100)
　=log2(31.36 x 250 x 1)
　=log2(7840)
　=12.9
EV=13

以上便是計算的公式，袛要你知道當時的光源有光（即 EV 值），再自訂一光圈或快門，及使用的 ISO 值，便可計算出在這個"光度"下的快門光圈的組合。

但不知你如何得知當時有幾光呢？所以唔實際，一般人是不會在現場用計算法計出來的，專業人仕（如拍電影的）也要用測光錶來測光才計算啊！

也有 EV 表可查出光圈和快門的相對應，但也要先知到當時的 EV 值才可用。也可根據上面的公式，預先計算出 EV 值的查表來。請自己運用吧

[Law2718]

多謝晒 Velsper, dede, h0123954

[NaHO]

呢個 post 好有用

[tedted]

請問有冇網頁有詳細解說呢? 可以介紹嗎?

[mathaeus]

To dede:

According to the formula you provided, could you explain why EV is directly proportional to aperture number and shutter speed? (which I think should be inversely proportional?)

I am also a starter, this is by no means challenging on you.. please kindly give me advice, many thanks!

[nyc0401]

So EV 1 = bright
EV 13 = dark

what's the best EV value for a picture ?

[micros]

ev 值表示o既係光o既強弱。
而我地就係因應光o既強弱﹐用光圈同快門o既配合去曝光。
所以唔係有「最佳EV」﹐而係因為光暗去調節光圈同快門。
暗﹐就開大光圈或者用慢o的快門﹐相反就收光圈或調高快門。

[Stanley2468]

請問 F2.8 減1.5級,是否等如F8

Thanks

[micros]

f/2.8 --> f/8 係三級。
f/2.8 減 1.5 級係 f/4.7。

睇下呢度:
http://content.edu.tw/vocation/art/ks_hc/htm/content/ch17/New_Pages/operation_4_1.html

http://www.nikon.co.jp/main/eng/feelnikon/kumon/06e.htm

有興趣再想知多o的不妨去圖書館或書店搵書睇﹐會學到好多基本知識。

[Stanley2468]

Thanks Micros

[peterchan04]

(1) The larger the EV number, the brighter the light source
(2) The larger the apperture "f/stop values" and the shutter speed (denominator's) values, the darker the pic
(3) That's why they are inversely related.

Shutter Speed "values" are expressed in 1/15... 1/125... 1/2000... 1/8000 seconds. Thats why the shutter speed is faster when the value of the denominator increases (and the resulting image is darker).

For apperture, it is more confusing. Don't mix up the "f/stop values" with the description " smaller or larger appertures". A "smaller" apperture means larger "f/stop vlaue". You will get darker pics and a "larger or wide open" apperture will give you a brighter image.