This situation is a perfect example of when the AF with back button (autofocus) is very useful. It is essentially the reason why Nikon calls his AF back button the AF-L & # 39; (autofocus To block) Button on many of your most recent cameras.
When AF is attached to the shutter button each time the shutter button returns to the mid-pressure position, either from the full pressure position or from the non-pressed position, or Each time a frame is exposed, the camera will "reboot" and try to confirm that the focus has been achieved. In other words, it will refocus. If the scene has changed, then the focus distance will be changed. That's a pretty universal behavior with all the digital SLR / DSLR cameras I've used.
In the absence of using the back button …
You say it like it's a bad thing!
The focus of the Back button is a tool that many photographers find very useful. It only takes a little bit of use to get used to remembering forever Press the Back button before releasing the shutter (fully pressing the shutter button all the way down) if the shutter button is set to do not start AF.
But that's not the only way to use an AF back button with most cameras that have options for the back button AF. In general, it can also be configured to to block AF when pressed and kept pressed.
Depending on the camera and the menu options, there is usually a way to use a back button to to block, instead of starting AF. In this scenario, pressing the shutter button halfway starts the AF along with the measurement. When you press the "back button" (usually the AF-L button on Nikon cameras that have one, or the AE lock button reassigned to function as an AF-L button on cameras that do not have a dedicated AF button) L), keeps AF in the current position until it is released. If the button is held over several frames, the focus distance will be the same for all of them.
This is the default setting for most Nikon cameras with an AF-L (auto focus lock) button. With Canon cameras, it's called the "AF-ON & # 39; because the default behavior is to start AF. Using the menu options, the & # 39; AF-ON & # 39; On Canon cameras you can also set it to be a button & # 39; AF-Off / AF-Lock & # 39; and the & # 39; AF-L & # 39; in Nikon cameras it can also be configured to work as a Button & # 39; AF-ON & # 39 ;.
the Nikon D200 It's an older model that is a bit different from the more recent Nikon bodies I've seen. It has a button & # 39; AE-L / AF-L & # 39; (Auto Exposure and Autofocus Lock), as well as a & # 39; AF-On & # 39; separated. If you want to use a back button to to block The AF that is started by pressing the shutter button halfway, then you must press and hold the button & # 39; AE-L / AF-L & # 39; on the back of the D200.
It is covered on pages 51-59 and 147-152 of the Nikon D200 User's Manual. The description of the button & # 39; AE-L / AF-L & # 39; and custom settings for the function of the & # 39; AE-L / AF-L & # 39; They are found on pages 56-57 and 156, respectively.
You can try changing the custom settings & # 39; a2: Priority selection of AF-S & # 39; of the focus priority & # 39; default to the & # 39; priority of release & # 39; It could allow continuous driving mode without confirming the AF between frames, but I would not bet on it.
You can also try changing the custom settings & # 39; a5: Focus tracking with activated lock & # 39; from the default & # 39; Normal & # 39; A & # 39; Long & # 39; to increase the amount of time before the camera refocuses if it detects that the distance of the subject has changed drastically.
If you are using the MB-D200 battery grip with the D200, there is also a custom setting a10 that allows you to re-assign the & # 39; AF-On & # 39; in the grip to work as the & # 39; AE-L / AF-L & # 39; The position of the selector & # 39; AE-L / AF-L & # 39; AE-L / AF-L & # 39; in the body of the camera it would determine if it blocks AE, AF or both.
If the camera keeps insisting on refocusing even when you press the & # 39; AF-L & # 39; and the camera feels that the subject's distance has changed, its only option would be to select & # 39; AF-On Only & # 39; for Custom Setting a6: Activating AF and then pressing the AF button to start the AF and then release it once AF has reached its intended target to block AF in multiple frames.
I have never filmed with a D200 or D300, but it would not surprise me at all that this is the only way to maintain the same focus position between frames after using AF to initially focus the camera.
For a more complete list of different scenarios regarding how you can set up an AF "backspace" button and in which situations each scenario may be useful, see this answer in What does the AE / lock button do? AF by pressing the button in half? The shutter does not?