[quote name='Klaus' timestamp='1350978227' post='20686']
That is the nature of field curvature. Interestingly it (the field curvature) gets worse the more you stop down thus resulting in this roller-coaster ride.
[/quote]
Yeah, I'm pretty sure this isn't a normal behavior... SLRGear review was pretty much in line with your findings and they specifically mentioned field curvature:
http://slrgear.com/reviews/showproduct.php/product/80/cat/10
Interestingly, their graph shows the lens sharpness to decrease slightly when going from f/5.6 to f/8, though it's nowhere near as wicked as what you've found.
That is the nature of field curvature. Interestingly it (the field curvature) gets worse the more you stop down thus resulting in this roller-coaster ride.
[/quote]
Yeah, I'm pretty sure this isn't a normal behavior... SLRGear review was pretty much in line with your findings and they specifically mentioned field curvature:
http://slrgear.com/reviews/showproduct.php/product/80/cat/10
Interestingly, their graph shows the lens sharpness to decrease slightly when going from f/5.6 to f/8, though it's nowhere near as wicked as what you've found.