Image scale describes sampling
An image-scale estimate uses focal length and pixel size to express how much sky each pixel represents. It does not promise corresponding resolution: tracking, seeing and optical aberrations still matter.
Field of view describes framing
The sensor dimensions and effective focal length determine how much sky you capture in each dimension. A target that fits a wide-field camera may not fit on a longer focal-length combination.
Account for optical accessories
Reducers and extenders change the effective imaging configuration. Use actual specifications for the assembled system rather than only the telescope’s advertised focal length.
Test your real target
Run multiple camera and telescope combinations through the calculator. Consider how much cropping, guiding and processing work each requires. Select the equipment for your subjects and conditions, not the biggest number on the box.