Photo lens - aperture vs resolving power....

[FarQinell]

For telescope observation it is a well known fact that resolving power is proportional to aperture - all other things being equal of course!

So lenses loose resolving power (ability to resolve fine detail) when they are stopped down. Yes of course in practice there are other factors to consider like colour fringing etc. that force you to stop down a little bit to get the best from your lens.

Just say I am photographing something small like the moon and I am lucky to have the  Canon 400/2.8 and the Canon 400/5.6 to compare side by side with say a 550D.

The /2.8 will have four times the light grasp of the /5.6.

So presumably a much larger image crop is possible with the big lens (ie 4x image area or 2x the moon diameter?) and the /2.8 image crop will still look just as sharp as the smaller crop taken with the /5.6.

As the image of the moon on the sensor is so small than presumably the 550D with its high pixel density will be as good as any other current camera to photograph small images?

I'm sure its not that simple - so if anyone can guide me to a good source of information on this subject I would be much obliged.

PS In practice I would use a fast f4 reflector to get decent sized moon images at a tiny fraction of the cost of a 400/2.8!

[Policar]

There are charts for diffraction limits and how they relate to megapixels, but it's not cut and dry.  Since the airy disk fuzzes detail gradually (it doesn't either resolve or not resolve something, although the point at which you get to 0% mtf is calculable) and a bayer sensor doesn't really resolve its quoted resolution completely there probably will be a difference in detail between f2.8 and f5.6 on the 7D resulting from diffraction, but it would be imperceptible and only the very, very best lenses are diffraction-limited below f5.6 and chances are CA, curvature of field, depth of field, etc. will be much bigger issues than diffraction until you hit f11.

If you had perfect lenses and a 300 megapixel sensor (maybe even more than that) you'd be right, but that's assuming a lens that's way more perfect than any that actually exists.  Although I heard rumors (hype) the Leica Summilux cine lenses were diffraction-limited before f4, and it's possible Canon's super telephotos are, too, but then the sensor is still a limiting factor, and of course its light gathering ability is another concern (more noise or less sensitivity when you crop).

[Mt Spokane Photography]

With camera lenses, there are other factors in play.  at very large apertures, viginetting, distortion, CA, LOCA, etc.  diffraction is always pressent, the airy disk getting larger as you stop down.
 
But, the other bad attributes are often greatly reduced as you stop down.
 
Thus, for each lens / sensor combination, there is a sweet spot where you get the highest resolution and least distortion, ca, etc.  Usually, its about f/5.6 - f/8, but some lenses perform very well at maximum aperture or just 1 stop down.
 
There is no simple formula, its best to know how each of your lenses responds if you are extremely critical.
 
Undoubtedly, astronimical photography will have some different parameters.