Just to mention, for whomever it might be useful, you can do something like this in the Rhino plugin, using the
Maxwell_PickFilmSizeRectangle command. What it does is to have you pick a rectangle in your viewport, then, provided the picked rectangle is not more than 100% of its own width offset from its center (in either direction), it calculates the film size and lens shift necessary to render just that rectangle, with respect to the perspective being shown in the viewport. Of course, you can also do this yourself, if you are using a different plugin or Studio.
Here is a quick example of how that works. Take a standard viewport, which in the host application (here, Rhino) has an inherently-35mm perspective:
To create a perspective match, the plugin follows along with the 35mm-height film, calculating a width of 38.44. However, using the same camera position, but with a film size of 17.47 x 16.81, resolution of 294 x 283, and lens shift of -35.2/-10.42, the film sees this region:
It is not necessarily easy to calculate those values, but here is a more typical example, showing what you would get by cutting the film size and resolution in half and offsetting the film -50/-50:
In an application which has a more sophisticated film concept, like Studio, it may not be immediately obvious that you can do this, since the viewport actually shows you what you will render (as opposed to the overlay shown above, which is used in the plugin out of necessity; a similar overlay is used in the SolidWorks plugin as well):
Were you to render it, though, you would find that the region matches in size and perspective.