Performance in terms of acutance of any lens varies in a lot of ways, but throwing in the variable focal lengths a zoom lens is capable of adds to the complexity of things. Even a prime lens with a fixed focal length can vary in terms of center sharpness from one aperture setting to the next. How much that sharpness is degraded from the center to the edges can also change at different apertures. Adding the extra variables of a zoom lens creates an even more complex comparison.

In general the gist of what you have been told is basically true, especially when comparing prime and zoom lenses that are both in the same or similar price ranges. If you compare cheaper prime lenses with high end zoom lenses, though, all bets are off in terms of this generalization. And there are some very high end prime lenses that are specifically designed to soften up on the edges and in the corners. The unique look of the Canon EF 85mm f/1.2 L is due to uncorrected spherical aberration that makes the edges of a flat target soft when the center is correctly in focus.

With consumer grade telephoto zoom lenses, the longest focal lengths typically tend to be the least sharp, even at the center of the frame. As the lens is zoomed to the maximum focal length all of the aberrations of the lens are magnified. So if the center sharpness decreases a little as the focal length is lengthened, then the edges will usually soften up even more.

Are prime lenses sharp across the whole frame while zoom lenses are soft in the corners?
I'm trying to setup a squid proxy that will accept multiple inbound connections on one IPv4 across a range of ports, and for each port connection to connect out on a unique IPv6 address.

I've named each port connection, and designated an ACL based on the connection name, and then I've tried to define an outgoing IPv6 for each. The script below works, in that it accepts connections on the different ports and routes out via IPv6 however, all outbound connections use the same IPv6 (user5) even though there are different ipv6 defined per user - the wierd bit is that even if I comment-out that user5 IPv6 from squid.conf, it still gets used for outbound connections.

Can anyone see whats wrong here? (using Squid 3.1.1 on Centos 6 64bit)

## declare an acl that is true for all ipv6 destinations
acl to_ipv6 dst ipv6

##tell Squid to listen on sequential ports and to designate a name for each inbound     connection. 
http_port 241.xxx.xxx.101:3128 name=3128
http_port 241.xxx.xxx.101:3129 name=3129
http_port 241.xxx.xxx.101:3130 name=3130
http_port 241.xxx.xxx.101:3131 name=3131
http_port 241.xxx.xxx.101:3132 name=3132

## designate acl based on inbound connection name
acl user1 myportname 3128 
acl user2 myportname 3129 
acl user3 myportname 3130 
acl user4 myportname 3131 
acl user5 myportname 3132 

## define outgoing IPv6 per user
tcp_outgoing_address [2001:::::::ipv61] user1
tcp_outgoing_address [2001:::::::ipv62] user2
tcp_outgoing_address [2001:::::::ipv63] user3
tcp_outgoing_address [2001:::::::ipv64] user4
##tcp_outgoing_address [2001:::::ipv65] user5
##this last IPv6 always gets used for all outbound connections, even if commented out

