Hi Aleaxander,
You’re welcome.
I hope I get your question correctly. The Gate needs to be set up first, as that is not an option to be changed in a DSLR for example. The Lens length is not connected to the sensor size and stays always the same. In fact most lens can support even a larger sensor. The resulting projection of a lens is typically round (Some Cine-lenses have a cropping, top and bottom, to avoid flare). This round circle is always a little bit greater than you need it. If not you see some problems. Also here, exceptions are given, like with an 8mm fisheye, where the circle is smaller than 24mm to get the full circle. On the edges you can see a lot of problems. The largest circle is typically given with Tilt Shift lenses, so you can move the lens aways from the sensor and keep the sensor covered anyway. I digress, but I like to give exceptions as well instead of incomplete generalization. Another exceptions is given with lenses created specifically for smaller sensors, same focal length, but the provided coverage (circle) is smaller.
Given the fact that the lens focal length is not changing, the gate or sensor size takes not always full advantage of a lens. As it takes then only a part of the circle it looks like a “longer” lens, hence people name this crop factor. Long story and ask four people you might get even more explanations. I use one lens on my RED, and the resolution that I have set up creates a smaller use of that same lens. Lens and sensor distance aren’t changing. Based on the image size settings, a 200mm shows up like in a 2K image as 650mm “lens, compared to an Full-frame image”. But in 5K as a 260mm, always the same lens.
After all these examples, the focal length of a lens is not equal to the filed of view that the sensor finally provides, the smaller the sensor, the less it uses the lens. Same with the gate. The lens set the maximum angle of view, the gate doesn’t change the options of the lens, but it uses only a little bit of it.
If you take a video with your camera this is not 36x24! It might be rather 16x9 perhaps, which would change the whole set up.
The 24-70 example was given as I imagined you have investigated the best lens options for your camera. The newer version (v2) seems to give a wider range than the label promises. So to assume to just take one end of the lens and have 24 or 70 mm as a stable value might limit your precision.
My advice to match thinks, as far as possible: take always any measurements that you can get on set. The camera high, the view angle, etc. If in a green screen studio, measure the distance of tracking markers, or C-Stands. Measure the distance of anything that is in the picture and how far it is to the camera. The more you know the better you are of. Take a lens grid to match the lens distortion.
OK, I have perhaps provided too much information. ;o)
Good luck with you project
Sassi