Back button focus, also known as thumb focus, is a technique where you separate the functions of focus and shutter release onto two separate buttons. You disable the focus function from the shutter or index button and assign focusing to a dedicated button on the back camera. Most DSLR, mirrorless, and even a few compacts are capable of back button focus either through a dedicated AF button or another button on the camera back that can be programmed. In the simplest terms, back button focus allows us to auto focus by pushing a button on the back of the camera body with our thumb and taking the picture by depressing the shutter release button with our index finger.  So let's take a closer look and see if back button focus is for you.

后点对焦（使用相机背面的对焦功能），也称为拇指对焦，是一种将对焦和快门释放功能分离到两个独立按钮上的技术。您可以通过禁用快门或索引键上的对焦功能，将对焦功能分配到相机背面的一个专用按钮上。大多数单反，无反甚至一些小型相机都可以通过一个专用的对焦按钮或相机上另一个可编程的按钮来实现后退按钮对焦。简单来说，“后点对焦”让我们可以用拇指按相机背面的按钮来自动对焦，用食指按下快门按钮来拍照。现在我们来仔细研究一下后点对焦是否适合您。

With back button focus you can pre-focus were you think the blenny will be and as long as your distance from your subject does not change you can fire off a shot every time the fish shows itself.

使用后点对焦，您可以预先对焦，比如在拍摄blenny时预先对焦，那么只要您和鱼之间的距离没有变化，您就可以抓紧机会在鱼每次出现的时候按下快门键进行拍摄。

So why would you want to separate focusing from the shutter release? If you have ever shot something like a small signal blenny popping in and out of a hole you know how frustrating it can be. You wait and wait only to have the auto focus hunt and fail as the fish finally emerges. No matter how hard you push the shutter button the camera won't take a picture because it can't lock focus. You can pre-focus, and either hold focus with a half press or focus lock button, but this can get tedious after just a few minutes. On the other hand, you can use back button focus, pre-focus on your subject and as long as your distance from your subject does not change you can fire off a shot every time the fish shows itself.

那么，为什么要把对焦和快门分开呢?如果您曾经拍摄过像小信号弹一样在洞口进进出出的blenny，您就能体会到整个过程是多么令人沮丧。您等了又等，直到自动对焦捕捉失败，鱼终于出现。但不管您怎么按快门，相机都不会拍照，因为它无法锁定对焦。当然，您可以预对焦，也可以通过半按或锁定对焦按钮来保持对焦，但这种方法在几分钟后就会变得索然无味。但是，如果您使用后点对焦，预先对焦在拍摄对象上，只要您与拍摄对象的距离没有变化，您就可以在鱼每次出现的时候按下快门。

Similar to the scenario above, when shooting super-macro it can get frustrating if your lens is constantly hunting. One very useful technique is to pre-focus for the basic composition and magnification and then rock a little back and forth to achieve the sharpest focus. We call this “Lock It and Rock It!”. When the important area becomes sharp, you take the picture. Back button focus is the smart choice for this type of shooting. You can concentrate on composition, rock back and fourth to achieve sharp focus and take a shot whenever you like. You will need to be able to see when your subject is in focus, so an enhanced viewfinder can help with this technique.

类似于上面的场景，在拍摄超微距镜头时，如果您的镜头一直在寻找焦点也会让人感到沮丧。一个非常有用的小技巧是确定好基本构图和放大效果后进行预对焦，然后把相机稍微来回晃动一点点，以获得最清晰的焦点。当重要的区域变得清晰时，您就拍照。使用后点对焦是此类拍摄的明智选择。您可以专注于构图，借助后退按钮对焦和稍微摇摆相机的方式，以实现清晰的对焦并在需要时进行拍照。借助一个增强型的取景器能帮助您更好的看清楚拍摄对象是否对焦好了。

Back button focus can also make for faster compositions. We know taking images with your subject dead center in the frame is usually not the most flattering.  We can move the auto focus points around in the viewfinder, but this method is usually slower than focusing with the center focus point and then recomposing. If using an index finger focus, you have to refocus, half press and recompose for each additional shot. With back button focus you only need to focus once. As long as the distance from the subject does not change you can fire away at will.

后点对焦也有利于快速构图。拍摄过程中，可能大家知道，将对焦中心锁死在中心可能对于拍摄对象不太合理。我们可以在取景器中移动自动对焦点，但是这种方法通常比使用中心对焦点然后进行对焦慢得多。如果用食指对焦则必须重新对焦，半按快门并重新构图。但有了后点对焦，您只需要对焦一次。只要与拍摄目标的距离不变，您就可以随意拍摄。

For super-macro you can pre-focus for the basic composition and magnification and then rock the camera a little back and forth to achieve the sharpest focus.

在拍摄超微距镜头时，您可以预先对焦，确认基本的构图和放大效果，然后来回晃动相机以获得最清晰的对焦。

First, if you're planning to use back button focus underwater, make sure your housing gives you easy access to the back focus control and shutter control with your thumb and index finger at the same time. Thankfully the Nauticam engineers have designed easy to use levers on most housings for back button compatible cameras. If the two controls are still a bit of a reach, you can swap out the standard handles for smaller ones. Next, we need to disable the focus function from the shutter release. This will vary slightly depending on camera make and model, but the setting should be found in your custom control menu. After focus is disabled, the camera will meter with a half press of the shutter button and take an image with a full press. If you have a dedicated back focus button, you're ready to start shooting. If not, you will need to program one of the buttons on the back to meter and focus. Make sure this is the same button on your camera that corresponds to the correct control on your housing. This would also typically be found in the custom control menu, but I would suggest doing a search on your particular camera model on how to set up back button focus.

首先，如果您打算在水下使用后点对焦，请确保您的外壳可以同时使用拇指和食指进行后对焦和快门控制。值得庆幸的是Nauticam工程师在大多数防水壳上设计了易于使用的手柄，借助手柄可以轻松的操作后点对焦，如果这两个控件还够不着，您可以把标准手柄换成较小的尺寸。接下来，我们需要在快门释放按钮中禁用对焦功能。根据相机品牌和型号的不同，此设置可能会略有差别，但应该都能在相机的自定义控制菜单中找到该设置。禁用对焦后，相机将在半按快门按钮时进行测光，并在全按时拍摄图像。如果您相机有一个专用的后点对焦按钮，那就可以开始拍摄了。如果没有，则需要选背面的按钮之一进行编程以进行测光和对焦。请确保这是相机上与外壳上正确控件相对应的相同按钮。通常也可以在相机的自定义控制菜单中找到，建议根据您相机的型号搜索一下了解如何设置后点对焦。

Because the camera will stop focusing any time you are not pressing the thumb button, I typically leave my camera set to [AI Servo] or [AF C]. That way, if you want to continuously focus, just keep your thumb pressed down. You can stop focus anytime by releasing your thumb. You can take photos at any time with the shutter button, regardless if you are holding down the thumb button or have it released. With camera lenses equipped with full time manual focusing like the Canon 100mm macro L and Nikon 105mm macro, you can install a manual focus gear and have manual focus whenever you are not pressing the thumb button.

因为不按拇指按钮的时候相机会停止对焦，所以我通常把相机设置为[AI Servo]或[AF C]。这样一来，如果您想持续对焦，只需要按住拇指即可，还可以随时通过释放拇指来停止对焦。无论您按住拇指按钮还是释放，都可以随时使用快门按钮拍照。像佳能100mm macro L和尼康105mm macro这类配置了手动对焦的相机镜头，可以在镜头上安装支持手动对焦的对焦圈，在不按拇指键的情况下进行手动对焦。

Back button focus can also make for faster compositions. Focus on your subject using a center point, release your thumb and recompose.

后点对焦也可以使构图更快。使用中心点对准您的拍摄对象，松开您的拇指，重新构图。

If you have shot for a long time using the typical index finger focus and shutter release, then back button focus will take a little time to get used to. That being said, I don't know one photographer that has given back button focus a real try and switched back to using a single button focus and release. Once you start using it it, I'm sure you will start to see the advantages for yourself.  So why not give it a try?  It may change the way you shoot forever.

如果您一直使用传统的食指对焦和快门释放的拍摄方式，那么后点对焦将需要一点时间来适应。虽然如此，但目前还没有哪个摄影师在真正尝试过后点对焦后，还会再切换回以前的模式。一旦您开始使用这个功能，我相信您会马上体验到它的优势，那么，为何不尝试一下呢？它可能会永远改变您的拍摄方式。