放大攝影倍數運算一問

[littleming]

想請教一下大家, 我有冇理解錯
C8 2000mm + 2X barlow + 25mm eyepiece =>
2000X2/25 = 160 倍?

剛買了個CA1 extension tube放埋個25mm eyepiece 再駁DSLR 影movie,
粒木星都係細細粒., 還未試到加埋2x barlow的效果(因為天氣不太好, 加埋barlow 後暗到直情見唔到粒木星,

唔用eyepiece, 就咁用extension tube 加barlow 會唔會都做到放大?

謝謝.

[Wah!!]

1. 攝影不同目視, 攝影的圖像沒有標準的倍率可言
2. 若要放大, 延長管應放在感光器和目鏡(/巴羅)之間
3. DSLR不是優良的行星攝影工具, 應該轉用webcam

P.S. 若要買webcam, 可以pm我

[鄧登凳]

如果你是用目鏡後拍攝(afocal method), 即用相機連鏡頭對著目鏡去拍, 就有程式可算出等效焦距: (見: http://www.hkastroforum.net/viewtopic.php?t=18596)

有效焦距 = 望遠鏡倍數 x 相機焦距

你的例子: C8 2X barlow + 25mm eyepiece = 2000X2/25 = 160 倍?
假定你的相機是35mm鏡頭, 等效焦距 = 160 x 35mm = 5600mm

焦比是 5600/200 = f/28 (所以很暗)

有人認為由於相機由35mm鏡頭變了5600mm, 等同加了一個160X的增焦鏡, 所以說是有160倍放大, 即放大倍數 = 目視的放大倍數。

如果你是用目鏡投射法(eye-piece projection), 即相機拆除鏡頭, 讓目鏡的影像投射在相機的感光片; 在乎你投射有多遠, 即目鏡和相機感光片的距離(設定為PD), 目鏡焦距為EF, 那放大倍數M

　　　M = (PD - EF)/EF

你用25mm目鏡加2x增焦, 目鏡有效焦距為12.5

例如你的目鏡進相機感光片100mm, M = (100-12.5)/12.5 = 7
那等效焦距 = 望遠鏡焦距 x M = 2000mm X 7 = 14000mm, 焦比是14000/200= f/70 (更加暗)

也有人說由於相機"原來"的鏡頭是35mm, 現變了14000mm, 所以是放大了14000/35=400倍(或相比35mm鏡頭拍, 視場闊度變為四百分之一)。但這個倍數和目鏡投射的倍數是不同的。

不用目鏡, 折下相機的鏡頭去拍, 叫直焦法(prime focus), C8+2Xbarlow = 2000mm X 2 = 4000mm焦距, 焦比f/20。

也有人說由於相機"原來"的鏡頭是35mm, 現變了4000mm, 所以是放大了4000/35=114倍。

ps. 拍行星, 還是用專門的攝影機或用webcam好。

[littleming]

謝謝各位解答.

[vr2xsc]

這張相我是用 152/1900mm 折反 + 3X barlow + Canon 600D 影的

[扒飯撈麵]

為數不多，不過都有人用dslr影行星

http://www.astropix.com/GDPI/INTRO.HTM

[littleming]

剛剛影多左張
C8 + 2X barlow + extension tube + Canon 60Da 1000 frame stack

[Wah!!]

光軸有偏差, 所以見到左邊有彗尾.

[littleming]

光軸有偏差, 所以見到左邊有彗尾.

請問會否是barlow 放置不妥造成?
抑或是focus 較得唔好?

氣流導致影像浮動也有關嗎?

謝謝.

[luka]

DSLR 唔合適影行星係因為粒CMOS 太大?
因為而家d機，都可以拍到高清片，如果一條高清，羅中間的質素來疊加，又會唔會解決到CMOS 太大令到行星太細粒?

如果用現在的無反 , eg : EPL1 、 GF1 果類，又如何?

用webcam 點都要連電腦，其實太多野

[Wah!!]

光軸與對焦/氣流關係較小, 一般是主鏡和副鏡影響最大.

DSLR不適合的原因不是因為CMOS太大, 而是它的QE較低, 影片也是有損壓縮, 未能把應有的信號保存. 相機體積和重量也會有影響.

[扒飯撈麵]

不用目鏡, 折下相機的鏡頭去拍, 叫直焦法(prime focus), C8+2Xbarlow = 2000mm X 2 = 4000mm焦距, 焦比f/20。

也有人說由於相機"原來"的鏡頭是35mm, 現變了4000mm, 所以是放大了4000/35=114倍。

same concept with this calculator?

[Water]

扒飯撈麵 兄：

請恕本人見識膚淺，未聞於市面上有在硬件設計上專為行星攝影而設的電腦銷售。

[扒飯撈麵]

sorry for the missing link: http://www.astro.shoregalaxy.com/dslr_calc.htm
I was talking about the magnification calculation but not any hardware.

[鄧登凳]

sorry for the missing link: http://www.astro.shoregalaxy.com/dslr_calc.htm
I was talking about the magnification calculation but not any hardware.

The calculator seems to assume that a standard lens for APS-C (1.6 crop factor) is FL=28mm (actually 27.5mm, instead of 35mm I suggested) and that for full-frame is FL=44mm (a bit strange as 35mm film cameras usually have 50mm or 55mm standard lenses)

Therefore, the magnification of APS-C with FL=4000mm is 145 (4000/27.5=145). The magnification for full-frame is 92 (4000/44=91).

I speculate that it is making use of a field of view (FOV) ratio to do the calculations involving an arctangent function. Conceptually, it is similar to mine but mine use direct ratio rather than arctangent. The use of arctangent should be the mathematically correct way to do it but the errors in using simple ratio are not too serious