[hellenkit]

http://www.dcfever.com/photosharing/view.php?id=998684

點解用了 2.8光圈都可有咁深既景深 ?

[h0001244]

當focus 於無限遠就，F2.8都變得好深景深
當focus 於0.3m就 F8都係淺景深

[hellenkit]

但見到佢個 focus係 : 焦距 : 15.0mm (相對於 35mm: 49mm) , 呢個係咪就係無限遠 ?


EXIF 作品拍攝資料
檔案大少 : 118878 bytes
日期時間 : 2011:01:04 00:11:01
相機品牌 : Canon
相機型號 : Canon EOS 7D
拍攝日期 : 2010:12:27 18:00:11
解像度 : 630 x 420
閃光燈 : No
焦距 : 15.0mm (相對於 35mm: 49mm)
CCD width : 11.11mm
曝光時間: 0.050 s (1/20)
光圈 : f/2.8
ISO 值 : 400
白平衡 : Auto
測光模式: pattern
曝光模式 : 光圈先決 (semi-auto)

[tam_dominic]

以下節錄自 "攝影基礎訓練" notes, 歡迎索閱

====================================================
B. 景深 (清晰範圍的大細) = 焦距(反比) + 物距(正比) + 光圈(反比)

背景移遠移近也不影響景深,不過背景離開焦平面愈遠 影像愈模糊
感光原件(sensor)的大小只影響視角(Field of View) 與景深無關

要最細景深 來 簡化背境時, 只要
1. 用最大焦距 (ie zoom 到盡)
2. 用最大光圈
3. 用最細物距 (ie 最近對焦距離拍攝)
就是該鏡頭 細景深的極限

焦距 - 寫在鏡頭上的 mm 數值.
DC景深很大 是因為 焦距很短. (eg.Canon G11 是 f=6.1 – 30.5mm),

物距 - 微距拍攝時, 物距很短. 清晰範圍都會很細.
大景深不利拍攝背景模糊人像. 但有利拍攝微距

景深範圍計算 (Circle of confusion 0.019mm=模糊的一般定義):
http://www.tawbaware.com/maxlyons/calc.htm

======================================================

該相的焦距很短, 只有15mm
該相的物距很大, 接近無限遠
假設物距 100呎, 就算光圈是 f2.8, 其景深約是 12呎 到 無限遠都清晰

Happy shooting !

[怪俠]

但見到佢個 focus係 : 焦距 : 15.0mm (相對於 35mm: 49mm) , 呢個係咪就係無限遠 ?

=========================================================
15.0mm 呢個係焦距唔係焦點，

焦點放在無限遠以後都係清晰嘅，衹有在無限遠之前先至有景深效果，

越接近最近拍攝距離景深越短，越接近無限遠距離景深越長。

[hellenkit]

好詳細既解釋．．thanks!

但那個 Circle of confusion (mm)，　是什麼來?

[lokism3]

Try to read this, the COC is closely related to DOF and film/sensor size too; it is about camera & lens design, and nothing about taking photos (unless, you're using 4" x 5" technical camera with film). I clipped it from http://www.photozone.de/dof-confusion-

DOF (Depth of Field): The area in front of and behind a focused subject in which the photographed image appears sharp. In other words, the depth of sharpness to the front of sharpness to the front and rear of the subject where image blur in the film plane falls within the limits of the permissible circle of confusion

Mathematically,
Front DOF = d • F • a2 / (f2 + d • F • a)
Rear DOF = d • F • a2 / (f2 – d • F • a)

f: focal length
F: F number
d: circle of confusion diameter
a: subject distance (distance from 1st principal point to subject)

COC (Circle of Confusion): Images are formed from a composite of dots (not points, though I’ll call them that for simplicity) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called “circles of confusion.” Thus, one way of indicating the quality of a lens is by the smallest dot it can form, or its “minimum circle of confusion.”

The maximum allowable dot size in an image is called the “permissible circle of confusion.” In other words, how much can a point be magnified and still look like a point and not a disk.

Please note COC, thus DOF, depends on the use of the images and is somewhat subjective. This is no different that resolution.