lens never sharp at sides and center at same time (infinity)

[Jayme Cowley]

With a couple copies of the Sigma 30mm f1.4 lens, i've found that it it impossible to get a sharp image in the center and sides of the image when shooting distant subjects such as landscapes, even on lower apertures, such as f5.6

using the liveview on the camera, I can adjust the focus so that the corners are very sharp, but then the center goes out of focus, and vise versa. This is a problem at 1.4, 2.8, 4, 5.6 and its not until f8-f10 that i can get a side to side sharp image.

is this a fault, or just how lenses are made? what is the best way to go about shooting landscapes so they are sharp from side to side?

 

[dxqcanada]

Thom agrees with you: 30mm f1.4 Sigma EX DC HSM Lens Review by Thom Hogan

 

[Derrel]

Thom agrees with you: 30mm f1.4 Sigma EX DC HSM Lens Review by Thom Hogan

As Thom wrote, "I'd judge the absolute corners to be soft at every aperture. The central area is very usable from from about f/2 to f/11, though. Indeed, at it's peak it is what I'd regard as excellent in the center. For landscapes and things where you need good edges, this isn't the lens. For indoor shooting of events where the corners may already be dark, this lens performs decently."

 

[Jayme Cowley]

but when one speaks of 'soft corners' do they mean soft always, or soft when the center of the image is in focus.
and my issue is more with the entire right and left sides of the image. i can manually focus and make them tack sharp, but then the center goes oof, or if I set the center to be tack sharp, the sides go off.
is this a common issue with many lenses, or just specifically the sigma 30?

 

[Derrel]

Field curvature can cause what you describe. Depth of field can compensate for field curvature if the DOF is deep enough (at smaller apertures, or at longer distances). You say you have had a couple of copies of this lens, and it suffers from the same problems Thom Hogan reports, and he has tried multiple copies in an effort to get one good enough to test fairly....hmmm...do you see where this is headed? This lens has earned its family nickname of SIGnificant MAlfunction, or SIGMA for short.