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This I guess is the main reason I prefer the equation that I use, although it (redundantly?) uses image scale factors, it gives a better indication as to what you're actually getting.
It is all well and good to do some calculations on a few different optical systems and cameras to find out which is going to be the best for you just to find that your resolution is too heavily under sampled.
I had a discussion with someone a couple of months ago that continually mentioned that the f/ratio is king and rules over all. On my 1.5 hour trip home stuck in a traffic jam I calculated that a 10" F/6.3 (my Meade with a focal reducer) could slaughter a 12" F/3 astrograph with near the same image scale. It all comes down to the cameras being used.
He was using the KAF-8300 which would give a scale of 1.24 arcsec. If I was to through an Apogee F77 on the back of my Meade I would get 1.55 arcsec BUT twice the efficiency.
Of course, the FOV of these two systems is not comparable! In saying that, by putting the F77 on my 100 ED F/9 it would just barely outshine a 12" f/3 astrograph with a third of the image scale, a vast improvement on the 10" F/6.3.
What is the point of all this? It is one thing to say that a 50mm F/1.8 lens on an FLI-16803 is faster than the same camera on a CDK-17, which would you rather use though? The 50mm may be ~14x faster, the CDK slaughters it though in the real world.
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