Hi folks,

Let's say you have 2 20MP SLRs, one with an APS-C sensor and one with a full-frame sensor, both fitted with the same prime lens, both pointed at the same subject from the same location. Assuming the usual APS-C crop factor of around 1.5 - 1.6, if you take a picture on both cameras then you end up - obviously - with 2 20MP images, with one looking like it came from a lens with around 1.5 - 1.6x the focal length of the other. Let's say that you now crop the image from the full-frame camera so that the two images now look identical.

What I've never understood is if - or at what magnification - there would be an appreciable difference in sharpness between the 2 images. I get that the image from the full-frame camera would now be down to around 13-14MP but wouldn't the APS-C image only come out sharper if the lens can deliver enough detail to let it do so?

Sorry this is a bit muddled... I guess I'm just not sure of whether the sensor's ability to resolve detail is typically greater than the lens's ability, and whether it matters. TIA!

Your image is a function of the lens + the sensor + the camera's firmware/software. Having a crappy lens is almost never something you want, but arguably you might waste money on an amazing lens if you use it on a poor quality camera body. Lots of people with lots more detailed knowledge (and patience) than me have written lots of blog posts/tutorials on this subject:

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
http://neilvn.com/tangents/full-frame-vs-crop-sensor-cameras-comparison-depth-of-field/
http://www.mdavid.com.au/photography/apscversusfullframe.shtml

From the Cambridge in Colour article's conclusion:

"Larger sensors (and correspondingly higher pixel counts) undoubtedly produce more detail if you can afford to sacrifice depth of field. On the other hand, if you wish to maintain the same depth of field, larger sensor sizes do not necessarily have a resolution advantage. Furthermore, the diffraction-limited depth of field is the same for all sensor sizes. In other words, if one were to use the smallest aperture before diffraction became significant, all sensor sizes would produce the same depth of field � even though the diffraction limited aperture will be different."

Are you asking because you have an APS-C camera and are considering whether it's worth it to upgrade to a full frame one?

If you shoot a 20mp frame and crop it down to 12mp, then blow the result back up to 20mp to compare with the output of a 20mp crop sensor camera, then it seems to me the full frame is bound to be poorer quality.
If you crop full frame to 12mp then compare with 12mp crop sensor camera, they'd probably be pretty much alike.
I would think the main advantage of so-called "reach"  is that you  use smaller, lighter gear (but if you use full-frame lenses than a lot of that advantage is lost) and for small subjects that don't fill the frame you are just eliminating wasted space.
Of course, another alternative would be to get a 1.4x teleconverter and add the "reach" to your full frame, which would probably be cheaper than buying a new body.

What I've never understood is if - or at what magnification - there would be an appreciable difference in sharpness between the 2 images. I get that the image from the full-frame camera would now be down to around 13-14MP but wouldn't the APS-C image only come out sharper if the lens can deliver enough detail to let it do so?

If those sensors from two cameras are identical in terms of resolving ability, there will be no difference in terms of sharpness when looked at %100. But there will be a detail difference; 20 MP one will have more data then the 14 MP one. Lets assume that it is a rural landscape photo. In the 20 MP photo (APS-C),  the tree in the middle of your frame will be %42,9 larger than the tree in the 14 MP (FF) one. If it is 100 pixels wide in the FF 14 MP one, it will be 143 pixels in the APS-C one.

If's it's a quality lens, with proper technique, you should see a difference (the higher density APS 20mp would look more detailed than the cropped full frame)

It has the Good Lens Having The Best Quality Glass Should A camera Have for Professional..there is the great Difference and i think You also will feel ..Its Get the Heavy Density Picture also which have the great pixels also ...