Upload & Sell: On
Being a Sony user I have no horse in this 'race', both appear to be fine lenses indeed - so it speaks volumes that the 25mm does so well against lavishly acclaimed wider brother.
The distortion figures are expectedly worse for the moustachioed 21mm at closer to on-axis, and correction favours the 21mm also, but not optical vignetting, also to be expected. Illuminance at f5.6 is much superior for the 25mm especially into the corners, almost a shallow linear affect.
Denoir, the broken lines are tangential (lateral definition across the frame) and the unbroken lines are sagittal ('arrow' away from frame centre, or longitudinal definition with distance moving away from the camera). There is nothing really in it for the sagittal MTF except for the curvature noted in the 25mm at the long frame edge point - the significant kick upwards which coincides with peak distortion.
The 21mm has a better tangential performance especially for the crucial high frequency 'fine detail' 40 lpmm line, much more parallel to the paired sagittal line, but the 25mm's line levels off at a reasonable level (45%) and will deliver very good cross-field definition at lower contrast. For an overall assessment I think it useful to average the two line pairs, for a resolution comparison. Corner performance should be virtually identical at mid apertures.
Zeiss also have the convention of reprting MTF at wide open and two stops down. By f8 things may be a little different and the f11 referred to above will benefit the 25mm more than the 21mm with DOF, all other things being equal. It is clear from the charts provided above that neither lens needs that near-diffraction aperture - most makers would be delighted to get real world 40 lpmm MTF of 50% in the corners by f5.6, so I am inclined to agree that Lloyd might be zeroing in on close focus with the 25mm and seeing curvature. Otherwise he got a poor sample or he is wrong, simple as that. If you want to see some dramatic curvature and poor corners, take a look at the ZF28 f2 data sheet..then again, Lloyd most likely panned that one also.
These two here are *much* more alike than dissimilar, and if one does not need the extreme AOV of the 21mm, the 25mm looks like a great bargain for its intended usage at half the price, with a better filter size and a weight advantage also. I'm another who would like to see more non-close focus images from it.
All in all, only minor issues for the 25mm which could be expected to be a great buy, IMHO. As for comparisons with a Canon zoom, please..