rice7361 wrote:
我相很多人被"片幅"這個名詞給誤導了
都認為片幅指的是"相機"的感光片幅
事實上在計算景深時所指的最小模糊圓(以片幅來標示)
指的確是"鏡頭"的片幅
..............恕刪.................
"片幅"指的是"鏡頭"的片幅
不是DSLR的感光片幅
說了這麼多如果還是聽不懂就真的沒辦法了
模糊圓是一個很簡單的概念：它是光束（或一個焦散面）聚焦在影像平面上時，所能形成的最小直徑………中型片幅的模糊圓為60微米；35mm相機的模糊圓是33微米；而像Minox間諜相機片幅（8×11）的模糊圓則為15微米
......So a 200mm lens on a digital SLR with FLM of 1.5X will have the field of view of a 300mm lens on a 35mm film camera which would be heavier and more expensive. Also, because the 35mm equivalent field of views are achieved with shorter focal lengths, depth of field is larger (1). This advantage on the tele end becomes a disadvantage on the wide ange end. For instance, a 19mm lens fitted onto a digital SLR with FLM of 1.5X will only generate the field of view of a 28mm lens fitted on a 35mm film camera......這段是說1.5X的DSLR加上200mm鏡頭，視野(field of view)和35mm底片相機加上300mm是相同的
........(1) Assuming the aperture and subject distance remain constant, the increase in depth of field (DOF) due to the reduction in focal length is partially offset by the reduction in the maximum permissible Circle of Confusion (CoC). For a smaller format (e.g. a sensor with FLM of 1.5X is a smaller format than 35mm film), the maximum permissible CoC is smaller (2), so DOF will be smaller. However, this reduction in DOF is smaller than the increase in DOF caused by the reduction in focal length, so overall DOF will increase, and more so with larger FLMs. You can verify this by using the depth of field calculator on this site........註1說明了固定光圈、物距時，因實體焦長較短所以景深會增加，但一部份會因所需的模糊圓(CoC)變小所抵消
.........(2) In Case 2 it is easy to understand that if you print an 8" x 10" of both images, the optical information collected by the smaller sensor has to be enlarged more than the optical information collected by the 35mm film, so the maximum permissible CoC for the sensor is smaller.註2說明了如果同樣放大相片到8"x10"，小片幅的放大倍率較大，因此所需的模糊圓CoC會比較小
踢西斯 wrote:
上圖pentax s5z 14.6mm, 約135 88mm焦距, f4.4
下圖canon 300D, 50mm, 約135 80mm焦距, f4.5
(恕刪)
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