You still have a very skewed idea, or simply bad terminology, in describing what you are actually experiencing with a TC, though, Lee Jay. Your previous argument in that other thread, that the virtual image of the sensor shrinks when it is observed by looking through the lens into the camera is not indicative of what is really occurring. A teleconverter does not change how many megapixels you have, nor does it change the resolution of the lens.
No, but it has the same effect as doing either one.
The effects are different. If you have an 18mp sensor and a 36mp sensor, and use the same lens on both. The effect of switching from the 18mp sensor to the 36mp sensor has the effect of potentially doubling spatial resolution for the entire area of the object being photographed. (Let's assume for a moment that you have a perfect lens at a very wide aperture, so diffraction is not a problem.) On the other hand, adding a 2x teleconverter has the effect of enlarging the subject, such that a smaller area of that subject is being photographed at the same spatial resolution.
In the first case, you increase the resolution of a much larger subject equally. As such, a sensor with a higher resolution is a hell of a lot better than a teleconverter, since the teleconverter is cropping, for all intents and purposes. It is enforcing that a certain amount of your subject will fall outside of the bounds of the sensor, since it has been enlarged.
Using a teleconverter changes what your resolving by reducing field of view, where as changing the sensor increases your resolving power for the same field of view. A teleconverter can make a smaller FOV better, but a higher resolution can make the same FOV better. Definitely not the same thing, and it is misleading to assume or discuss them as if they are the same thing.
As we discussed in our last debate, the spatial resolution of whatever is projected by the lens, as well as the spatial resolution of the sensor, are pretty limited. If you use a TC or multiple TC's that reduce your aperture to f/8, then according to the laws of physics spatial resolution becomes limited (specifically to around 86lp/mm), which is WELL below the fixed luminance spatial resolution of pretty much any APS-C sensor these days.
f=1/(0.00055* = 227lp/mm at MTF = 0. Using MTF=50, as you did above, is arbitrary and of little value in this context.
The notion that a consumer-grade camera can resolve anything at MTF ZERO is ludicrous. The notion that a camera can usefully resolve anything at MTF 9% (Reighley) is also pretty ridiculous. MTF 50% is of specific value because MTF 50% IS STILL and WILL CONTINUE to be used today as the standard benchmark for image resolution of meaningful sharpness, either by a lens or a sensor. Low MTF image analysis, such as at just above (but not actually at) MTF 0% is used by specialized software to analyze patterns of point light sources (stars) against a black background, in an effort to try and determine if the stars are binary or tertiary. MTF 0% photography is only used in extreme scientific scenarios, it has never held any value in real-world photography. There is little data to indicate that a modern image sensor, even one with microlenses, can really produce any kind of useful output at MTF 9% due to Poisson distribution of photons (photon shot noise)...you could never really know whether two adjacent pixels were different because of meaningful image detail, or simply because of noise.
So I'm sorry, but I beg to differ. MTF 50% is the only valid level of contrast to meaningfully and consistently discuss spatial resolution in the context of consumer-grade lenses and sensors.
Spatial resolution is not increasing, magnification is increasing.
Same thing, since the optics (the lens) didn't change.
That is 100% factually incorrect. The optics DID change...you added a teleconverter. A teleconverter is optics. (Are you sure you are not confusing extension tubes for a teleconverter? If you add extension tubes, then the optics themselves do not change, but they DO move farther from the sensor, which might require a change in focus, which in turn does change the optical configuration and potentially magnification.)
I think what you are doing is accounting for the "entire" size of your subject. If you magnify a part of the moon such that only that one part fits on an 18mp sensor, the "effective size of the whole moon if it were to be measured in megapixels would require a 184mp FF sensor to image in it's entirety." You could look at it that way, but it is extremely confusing, and running about stating "It's like having a 369MP FF sensor" is not really true, and I WILL argue that point whenever you bring it up.
That's fine, and you'll be wrong each time. This is the way people do it in astrophotography, where resolution is what you are after. "Image scale" is determined by arc-second per pixel, and the lens is measured by aperture diameter. TCs leave the aperture unchanged and decrease arc-seconds per pixel. More pixels leave the aperture unchanged and decrease arc-seconds per pixel. Same thing.
The arc seconds per pixel remain the same, but the result is not the same. In one case, arc seconds per pixel decrease for the same number of pixels. In the other case, arc seconds per pixel decrease for MORE PIXELS. They are definitely different things. The only case where they would be the same is if you always and explicitly included the notion that you were CROPPING the larger sensor's image to the same area and dimensions of the smaller sensor. In which case, and only in which case, would the results be exactly the same thing.
In every case, using an better sensor that actually has more resolution will always be better than using a teleconverter, because you can resolve more detail of the same subject. Using a teleconverter on the same lower resolution sensor, you are changing your subject. It doesn't matter if the arc seconds per pixel are the same, the resulting output image from the two systems is very different.