another thing you can do (which is kinda helpful) is to create a locator. then in your camera attribute editor, click the checker box next to focus distance and go over to the utilities tab and create a 'distanceBetween' node.
now you need to open your connection editor (window>>general editors>>connection editor). select your camera and click the 'reload left' button. select the distanceBetween node (if you can't find it, open the hypershade and click on the utilities tab) and click the 'reload right' button. now select 'translate' for your camera and 'point1' for the utility node. after that, select the locator and click 'reload left' again. select the translate for the locator and 'point2' for the distanceBetween.
now instead of trying to figure out the value or inputting values every time you want to change your depth of field, all you gotta do is move the locator to where you want everything in focus. it takes a minute or so to set up, but makes life (or rendering, anyway...) easier
edit: this can also be done with a camera with aim, but moving the aim will obviously move the camera's center of interest (and change your composition).
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