First - you can get more spacial detail from adding TCs, even to already-slow optics. This is a $60 Bayer-sensor webcam, not some high-end astronomical sensor. Pixel size is 5.6 microns - about the same as the 40D. f/30 on the left, f/15 on the right. According to you, the f/30 shot couldn't possibly be better, but it is. I took these:http://photos.imageevent.com/sipphoto/samplepictures/Jupiter%20f30%20versus%20f15%20comparison.jpg
Second, and this is going to be a little hard to accept for you, but it's fact so I suggest you listen carefully. You're thinking of a TC as a device that increases focal length and decreases aperture. First of all, it doesn't decrease aperture. f-stop = focal length / aperture diameter. A TC can be thought to increase focal length while keeping aperture the same, thus increasing f-stop. However, and this is important, this is only true from the camera's point of view. From the lens' point of view, its focal length, aperture and f-stop remain the same. The TC is, after all, mounted behind it. From the lens' point of view, the TC has changed the camera. How, you might ask? By shrinking the sensor and the pixels on it. If you don't believe me, try this little experiment yourself.http://photos.imageevent.com/sipphoto/samplepictures/Teleconverter%20optical%20reduction.jpg
The point is, increasing focal length while preserving aperture (and thus increasing f-stop) and decreasing pixel size are equivalent. Here's an example of that:http://photos.imageevent.com/sipphoto/samplepictures/Pixel%20density%20versus%20teleconverters.jpg
Finally, if you want to see the effect of diffraction while shrinking pixel size, have a look at the link below. If you prefer, you can think of these as APS-C sensors with 8MP, 16MP, 32MP, and 64MP, all at f/11. The one on the bottom is for reference when using a larger aperture that isn't diffraction-limited. As you can see, even at f/11, resolving power goes up in each case, by ever-decreasing amounts (the so-called law of diminishing returns), just as theory would indicate. I've tested this all the way to oblivion (1.1 micron pixels at f/11), and the MTF 0 spatial cutoff formula I gave you from Wikipedia matches well with real-world testing.http://photos.imageevent.com/sipphoto/samplepictures/Diffraction%20pixel%20size%20test%202.jpg
As for how good our optics are, I tested my version 1 70-200/2.8L IS at different apertures by mounting telescope eye pieces to it and trying to split double stars. Essentially, this is a test of the Rayleigh criterion (MTF 9). I found that it isn't diffraction-limited at f/2.8 but, amazingly, it is diffraction-limited at f/4 - I could split a double at exactly the Rayleigh equivalent separation angle for a 50mm aperture with that lens given sufficient optical magnification. I think you'd agree that we have several lenses in the line-up that are better at faster f-stops than f/4 than the version 1 70-200 is.
For a little more evidence of that, compare the Jupiter shot I posted above, taken with 125mm of aperture of diffraction-limited f/15 Maksutov-Cassegrain telescope, with one posted yesterday also taken with 125mm of aperture this time in the from of a wide-open 500/4. The detail retained is very, very similar providing further evidence that the 500/4 is diffraction-limited wide open.http://forums.dpreview.com/forums/read.asp?forum=1029&message=40928248
The take-home lessons are:
- We can extract more detail at finer resolutions than the MTF50 diffraction limit even with Bayer sensors with AA filters.
- We have optics that are diffraction-limited at f-stops much faster than f/8.
- Because of those two facts, we can make use of sensors much more densely packed than current 18MP APS-C sensors.