Right now, the highest resolution FF cameras are 36 MP, the highest APS-c are 24 MP. There is absolutely no value in promoting false apologies where you have to bend the real world to fit the analogy. The analogy fits, or you throw it out. If it doesn't fit it's not useful. There is simply more to small sensors verses big sensors than sensors with the same pixel pitch. Since one of the advantages of smaller size sensors is they almost always have a higher pixel counts, talking about different sensors as if that weren't true is completely misleading.
Now if you're talking about a 645z compared to a D810, or a D800 compared to a K-5 series camera you're talking pretty close. But even then you can't just say one is an effective crop of the other.
Other factors that are relevant
Lens size - if you shoot from the same spot, you're using a wider lens on the APS_c and you'll have more DoF.
Distance - If you use the same lens but back up, you will also have more DoF with the smaller sensor
Magnification - If you blow a smaller sensor up to the same size as the larger sensor you must magnify it more. The effects of such magnification are problematic, and the magnification will create different results depending on where the subject is within the DOF, and the size of the circles of confusion.
What leads to confusion is simplifying a very complex subject to one false analogy and trying to fool people into thinking that means something. The more informed person is going to be saying "that hardly tells the whole story, or sheds any light on how the crop factor applies to photography.
Essentially in your analogy, you're assuming same pixel density, same lens, you're just cutting out the middle of the Medium Format sensor to create a smaller image. In which case the crop sensor image would be a small subset of the total image of the Medium Format sensor. And the FF sensor is just as much a crop sensor as the APS_c sensor is. IN real life, no one shoots like that. One backs up, or uses a wider lens. If the Medium Format image is the perfect frame, then lenses and distances will be manipulated with APS-c or Full Frame, to create the same field of view.
That's why when you look at a D610 image or a K-3 image, they are essentially the same image with some variations of DoF and sometimes some different distortion from the different focal lengths of the lenses. There is no apparent "crop" to the image. But that's the real world. Not some analogy dreamed up to make a point.
D750 vs K-3, "where do I see the crop factor". Can you even tell which is which? Hint the best one with the best contrast and clearest focus is the K-3 (APS-c). Whatever advantages the larger sensor has, clearly are not apparent shooting at 100 ISO, where in this case the APS-c has a slight advantage. Despite all the hogwash about total light, cropped images, blah blah blah, the APS-c image is better. It can happen. Learn to live with it.
Last edited by normhead; 11-04-2014 at 07:25 PM.