Allright, here's what I've done:
Same sort of setup, 45mm and a 30mm lens, but now I've rendered both lens views to 720x480 (35mm = 36x24mm so I took 20 pixels per mm) to represent the image the fullsize sensor/film would receive. Then I cropped the 30 to 474x314 (nikon sensor = 23.7 x 15.7). Then I scaled that image up to 720x477 (yes, there's a 3px difference I know it's because of the sensor size) to represent a print or what you'd see through the viewfinder of a nikon dslr.

I think that's basically what you meant. As you can see the fullsize 45mm version and the 30mm with the cropfactor will give you the same object size in your print... and your viewfinder. Here's the images:

 

 

 

 

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