08-14-2012, 06:19 PM
To get the film back dimensions on DSLRs you have to know how the CMOS sensor captures the video. There are two ways it can do it. 1) Most DSLR's capture full sensor and then drop lines down to HD, this is the main reason for Moire and Aliasing. 2) The other way is the camera only captures video data from a small portion of the sensor. This is essentially a crop to the sensor which may change the focal length perception of the image.
So with situation one, you have to get the chip dimensions and use those as your film back size. You then crop to 1.78 in post. This because the camera still uses the full chip whether its recording it or not. I shot a film recently on a 5d Mark II and you clearly see that the video is working beyond the crop but only the crop is being recorded. Doing the 1.78 or whatever aspect in post maintains the focal length.
With situation two, you need to determine the physical size of the video capturing area of the chip and use that as your film back settings. This will associate the proper crop to focal length magnification. In this case the crop is usually set by the recording area of the sensor and it will not be needed to place your crop in post.
I hope this clarifies the situation. In most case Nikon and Canon DSLRs fall under situation one but double check with your camera model. I am not an expert on each camera.