I noticed that when I enlarge a 35mm negative to 11x14 using a Beseler 23CII enlarger and a Nikkor 50mm enlarging lens at the widest aperture (2.8), the corners of the print seem underexposed. The height of the enlarger head is set to the 35mm level.

Is this a problem with using the enlarger lens at the widest aperture and if so, how much should I step down to avoid this problem. At 2.8, the print requires about 30 seconds. Stepping down would thus increase the exposure time to 1 or more minutes. Since I am new to printing, is 1-2 or more minutes considered typical and am I being too impatient in trying to reduce the exposure times by keeping the lens wide open?