陷阱對焦（trap focus，focus trap），顧名思義，就是先設計好對焦平面，等準備拍攝的對象進入對焦平面，拍攝。這是拍攝運動的人和動物常用的手法。也就是說這是守株待兔，等到目標進入取景範圍後進行拍攝。
陷阱對焦的道理簡單，做起來沒那麼容易。成功的關鍵在於：在被攝對象進入事先設計的焦平面的瞬間，按下快門拍攝。可是怎樣確保在被攝對象正好在焦點上的時候拍攝呢？如果使用手動對焦的鏡頭，只能靠眼睛在取景器中判斷是否合焦，合焦了再按下快門，有個時間延遲，拍攝不易成功。
使用自動對焦鏡頭，在有些相機上可以進行自動陷阱對焦，大大提高成功率。對於沒有專門設計的陷阱對焦功能的相機（比如尼康D700），也可以實現自動陷阱對焦，但需要以下條件：
（1）鏡頭可以自動對焦；
（2）相機的自動對焦功能可以指派給獨立的按鈕，解除半按快門自動對焦的功能；
（3）相機有“對焦優先”的快門釋放模式；在這個模式下如果不合焦相機不會拍攝。
如果以上幾個條件具備，自動陷阱對焦的步驟如下：
把相機的自動對焦功能指派給獨立的按鈕，解除半按快門自動對焦的功能，以保證按快門的時候不影響對焦；
相機啟動對焦優先快門釋放模式（一般在單點對焦模式下）；
設好對焦屏面。這可以通過自動對焦完成，也可以通過手動對焦完成；如果用手動對焦完成，之後要把鏡頭和相機換回自動對焦模式，單點自動對焦模式；
全按快門（別鬆開），等待被攝對象進入焦平面。被攝對象進入焦平面之前，相機不會釋放快門，因為預設了對焦優先。當被攝對象進入焦平面並經過對焦點的時候，相機自動釋放快門，完成拍攝。
原理上是這樣，舉個網友的例子：
這類方法很多朋友都有聽說過，陷阱對焦顧名思義，是預先設置好對焦範圍，然後守株待兔，等到目標進入取景範圍後進行拍攝。這樣就有兩個問題：如何預設對焦範圍和如何快速的進行拍攝。筆者這裡要介紹一下實際操作時的具體方法：
問題一：如何預設對焦範圍
預設對焦大家都會，可是鳥類拍攝時我們經常沒有參照物來確定鳥類位置，這時如何對焦呢？有得朋友認為鳥離我們很遠了，使用無限遠就可以了，其實這是錯誤的。（之前我就是這樣以為）
預對焦方法
正確的方法是，預判鳥的距離範圍，可以通過樹木、建築等進行判斷，在確定鳥類飛行軌跡後，在這一區域保持預備狀態，等鳥類飛過時快速開啟對焦。
問題二：如何快速拍攝
反應速度是需要長期練習的。一般情況下，不要等鳥類到達構圖的位置再按快門，要等鳥類剛進入視野就按下快門，這樣鳥類飛行一段距離後，會形成較好的構圖。
可以設定相機，讓對焦完成後，才能釋放快門（佳能用戶比較尷尬，需要高端機型才有）
如果反應速度不夠，可以把對焦交給相機來進行，我們可以把設置改為對焦優先，這樣當相機沒檢測到對焦時就不會釋放快門。我們設定好對焦，將對焦點設置為中心點對焦，這樣鳥類達到對焦位置時，快門會立刻釋放。
網上還有一篇不錯的文章討論了陷阱對焦，英文的，拷貝在後邊。文中特別指出：陷阱對焦也可以在拍微的時候使用，設好對焦陷阱，按下快門，前後微動相機，在被攝物進入對焦點的時候，相機自動釋放快門，拍出清晰的圖像。該文還講了陷阱對焦和連續對焦模式的不同適用場合。
Trap Focus for Nikon shooters
Trap focus is where you prefocus on a fixed physical location, and the camera refuses to fire (even with the shutter pressed all the way down), until the camera recognizes something that is in a particular bracket as being in focus, before it fires.
Different cameras may have different setups to accomplish this, but for a Nikon D700, you can mimic "Trap Focus" with the following settings:
1. Set custom setting #A5 to AF-ON ONLY
2. Set the focus mode to SINGLE SERVO
3. Set Auto focus area mode to SINGLE POINT (9 point, 21 point, 51 point or 51 point 3-D will all work)
4. If your lens has an A/M switch, make sure it is set to A for autofocus mode.
5. Pre-focus the lens for a particular distance. Don't forget that a more convenient way to use this trick is to pre-focus using not the shutter button, but the FOCUS button on the back of the camera can be used. I find this easier and more effective.
6. Press and hold the shutter release ALL the way down, the camera shoots only when the selected focus bracket is in focus.
"Trap Focus" is only effective if you know exactly where your moving object will stop and you yourself do not move or change position from (for example, a baseball player running towards a specific baseball plate or a branch where you expect a bird to land are good examples (but don't forget the hint that having a deeper DOF is also a strong consideration when very close to the subject, this gives you some extra leeway for getting a better shot!).
Another place where "Trap Focus" comes in handy, and not many people know or use it with, is with hand held macro shots. If you are using the trap focus method, you focus on a particular area, are ready to take a shot, breathe perhaps a little, the scene goes ever so slightly out of focus, the camera will refuse to take the shot until you adjust position, camera focuses and the shot is sharp. In this case, "Trap Focus" saved you from taking a blurry picture.
However, it is not very good for use where we want to capture a moving object and we do not know the exact location where it will stop, or in cases where an object moves from sensor bracket to sensor bracket.
In these cases, continual focus or continuous servo, as Nikon calls it (in conjunction with a Dynamic AF setting and a SINGLE FOCUS AREA ), gives me far more keepers than using "Trap Focus" (ie: bride and groom are moving down the isle and you are moving to keep up with them), as I choose the STARTING focus point and as they move (I may want to recompose the shot on the fly), the focus remains on the subject that I initially set the starting focus point on. It does this because the lens is continually trying to focus as long as I have the shutter pressed instead of waiting on that one point/distance where focus is achieved and the shutter trips to take the shot, and if they move to another sensor, focus tracking is in place and focus is continually modified and locked.
Trap Focus has caused me to lose or miss the moment many times in the past before I learned to understand the D700 focus and exposure system and how it works in detail and when to use each of the proper settings to best effect.
Try them out and see what works best for you and under what circumstances!
