淺談機背按鈕對焦功能 / Back Button AF

去年和今年兩次北極之旅最大的收穫為攝取了不少的極地生態,包括鳥禽和北極熊等野生動物的影像,是非常難得的經驗,拍攝野生動物不同於風景、靜物或人像,使用的器材和相機的設定均有不少的差異,今天來談談有關利用相機背部自動對焦按鈕來拍攝的優點和方法。

I did 2 trips from last year to explore Svalbard and what I got most are a lot of images of the wild lives in the polar area, which includes birds and animals etc. specially the polar bears was a valuable experience, shooting wild lives, landscape, still objects or portrait are all different in the camera lens and setting, Now I would like to talk about the “back button auto focus (AF)” which is a very versatile way to do the wild lives shooting.

 

何謂相機的背部按鈕,一般高階單眼相機的背面有兩個按鈕一個是”AF-ON”另一個是星號”*”分別是自動對焦及測光鎖定鈕,此兩個按鈕的功能可在相機自定功能選單裡編輯成個人喜歡的功能,一般使用相機的自動對焦方式為操作相機前方的快門鈕,此鈕為複合功能鈕,即按下一半時相機進行測光/對焦,續按到底打開快門攝取影像,我們都熟悉這樣的操作但它有以下的缺點:

  1. 一個按鈕同時操作兩個動作,手指如操作不當容易出現錯誤如對焦不準或構圖錯誤等。
  2. 在拍攝移動主體時半按快門時由於主體的移動相機須不段對焦容易拍出模糊的影像。
  3. 當對準主體對焦後要移動相機重新構圖時,由於像機繼續對焦會造成主體以外的部份對焦而非主體。

What is back button AF? there are 2 back buttons in the most professional DSLR which are “AF-ON” and “*” with the default setting, these 2 buttons can be customized in the manual setting, normally we take photos by pressing the shutter button in the front side of the camera, when we press the shutter button half way firstly the camera measuring the exposure and do the focusing, then further pressing to take the photo, we are all familiar with this way of taking photos but it have some drawbacks:

  1. the shutter button equips 2 functions sometimes finger can cause the trouble during the focusing and re-composition.
  2. when you shoot the moving subjects using the shutter button, camera will keep on and stop focusing, you might shoot some blur photos.
  3. When you press the shutter half way and focusing the subject then move the camera for compositing, your finger might be loose and camera is refocusing and get a sharp photo which is not on the subject.

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因此將快門鈕的對焦功能移到機背按鈕由機背按鈕如”AF-ON”或星號”*”鈕來鎖定目標主體,再由快門鈕來攝取影像可快速準確的攝取須要的影像,操作機背自動對焦按鈕可歸納以下的優點:

  1. 將”自動對焦”和”快門”功能分開成兩個按鈕,可由機背自動對焦鈕迅速對焦主體再按下快門鈕攝取影像。
  2. 當拍攝移動主體時可採取”連續自動對焦模式”按住機背自動對焦鈕持續跟焦再按下快門鈕攝取影像。
  3. 在光線不足的環境下,利用機背自動對焦鈕一旦鎖定目標後即可按下快門攝取影像,相機不會發生因光線不足而產生無法按下快門的情形。
  4. 利用機背自動對焦鈕可鎖定目標再重新構圖不會使主體脫焦。
  5. 使用機背自動對焦鈕,對焦模式可不須選擇”單張對焦”或”連續跟焦”,將相機設為固定的”連續跟焦”模式,拍”單張”時在對焦後放開機背自動對焦鈕再按快門,如須”連續跟焦”多張拍攝則按住機背自動對焦鈕不放,再按快門鈕連續拍攝。

So we separate the function of auto focusing from the shutter button and have the back buttons like “AF-ON or “*” to focus the subject and then the shutter can take the photo fast and precisely, to operate the back button AF have the following advantages:

  1. Separate the AF function from the shutter button using the back button AF can quickly focus the subject and press the shutter button to take the photo.
  2. When shooting moving subjects, put camera in “AL servo mode” and keep holding the back AF button the camera will stay focusing and won’t get blur images as you press the shutter button.
  3. In the low light situation sometimes camera is difficult to get the focus and a hunting noise from it, but using the back AF button once getting the focus it will stay focusing until you press the button again.
  4. With “one shot” mode, operating the back button AF after getting the focus you can release the button the camera will stay focused when you re-compositing the frame.
  5. You can choose and stay in the “Al servo” shooting mode for “one shot” or “AL servo” modes, for one shot mode, you just release the back button AF and press shutter to take the photo, for “AL servo” modes you can hold on the back button AF and press the shutter to continue taking the photos.

不同相機各有”機背自動對焦鈕”不同的設定方法,以下以Canon的5D MK III作例子來說明:

  1. 首先按Menu鍵轉動選單到”自訂控制”選項後按”set”鍵。
  2. 在畫面上邊第一個選項的功能項目中由”測光/對焦”改為”測光”。
  3. 然後在畫面上邊第二個選項的功能項目中設定為”自動對焦”。

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設定好後即可將”快門”和”自動對焦”的操作分開,使影像拍攝更快速有效率尤其是拍攝移動的主體,而拍靜止的主体只要對焦後即可放開按鈕,此時如須要也可重新構圖後再按快門鈕攝取影像。

There are different ways to operate the back button AF by the camera brands, the following explains the setting for back button AF of the Canon 5D MK III:

  1. First press the Menu button, and choose the “custom controls” and then press “set”.
  2. From the jumping out diagnosis screen, change the function of shutter button to “measuring” only.
  3. then change the function Of the ” AF-ON” button to ” measuring & auto focus”.

Now the function of “shutter” and “Auto Focus” are separated, you can focus and take photos very quickly and efficiently, especially when taking the photo of the moving subject, for still subject you just release the back button AF after focusing and re-composting if needed, then press the shutter to take the photo.