How much protection do the various shields give [duplicate]

With the new levels of shields available as of 3 June 2013, is there an indication of how much protection each shield will give a portal?

Similarly, do they contribute to the XM drain on the attacker in any way?

For info on the new buffed shields, check out decodeingress:

Brandon Badger published on 31st of May that Portal Shields feel stronger now. But there is a catch, only new obtained portal shields are stronger. If you farmed those shields in last few months they have same stats as old shields. Only new farmed shields have new stats:

Old shields: Common +6 Rare +8 Very Rare +10

New shields: Common +10 Rare +20 Very Rare +30

ingress

Best Answer

The shield mitigation changed ~~once again~~ and again

Common shield    = 30  
Rare shield      = 40  
Very Rare shield = 60  
AXA shield       = 70

~~It was possible to renew all old shields by dropping them on ground, but it's not longer necessary.~~
For more up to date information, and to see mitigation of links see this answer.

What is the formula for damage mitigation in Ingress?