How good are "stacked" converters ?

I did a bit of playing around the other night and took a few photos of the moon with an F* 300 (handheld) and then I stacked an A 1.4x-S and a Jessops 1.7Pz/AF to see how the combination coped - I've cropped them both to make the images similar size...

F*300:


F*300 plus converters:



No post processing, handheld, non-scientific, just wanted to see how good the converters were stacked

Not much in it really, I suppose if you pixel peep the non-stacked version is a tad sharper, but it is miniscule.

At the end of the day if the converters are of high enough quality, then there should really be not that much difference. I have a friend who is a (ahem) Canon user with a 500mm Lens and he often puts a pair of 2x converters on the lens. The shots are still superb! He just relies on good light (which is very important of course)