Canon 35mm f/2 IS vs Sigma 35mm f/1.4 Art bokeh comparison

kotmj wrote:

Peter Karbe of Leica said in an interview on the 50/2 M APO that what they tried to achieve with it that was different from most manufacturers is that the MTF needs to fall off steeply away from the plane of focus. That is, the object in focus has great macro and microcontrast -- very high and level MTF curves -- but that the objects infront of and behind the plane of focus have intentionally bad MTF curves. The out-of-focus areas have low contrast and resolution. This difference in contrast between the object in focus and those OOF is what makes for the pop.

He further said the Japanese would never think of doing this.

Leica has had different philosophies in this regard over the decades. Mandler had a piece published where he stated that in what he called "pictorial" photography, even the OOF areas should have high contrast.

Maybe Sigma caught on to Karbe's thinking, or arrived at it on their own.

...the MTF curves measure resolution on the plane at the focal point that runs perpendicular to the line of sight.  If the MTF curves show lower resolution from the center, this could mean the lens sharpness drops off as you describe above (which is a good thing, in my opinion, for central composition wide open) or it could indicate strong field curvature.

Either way, the MTF curves should flatten out as one stops down, becoming sharper across the frame and at top form by f/4 (f/5.6).  The Sigma 35 / 1.4A is a wonderful example of such a curve, although one might argue that lower resolution at the wider apertures in the periphery would have been more desirable.  The 35 / 2 IS, however, would have benefited from greater sharpness in the center at the wider apertures, and, like the Sigma, perhaps a bit lower periphery resolution at the wider apertures.  However, the strong vignetting of the 35 / 2 IS at the wider apertures may serve to give a similar effect.

So, to recap:  an ideal situation with regards to shallow DOF, sharpness, and pleasing bokeh would be near peak sharpness in the central portion of the frame right from wide open, with resolution falling off from the central portion of the frame to the corners as a function of the lens sharpness rather than strong field curvature.  As one stops down, the corner performance steadily improves until it is at top form by f/4-f/5.6 and close to the center performance.