24-70 2.8 UNDEREXPOSURE

I, hope some one can help me.

I was making a test with the 24-70 2.8 g against 24 f2 Ais. an i´ve found the following.

Both series were shot in A mode, 2.8, 4, 5.6, 8, 11, 16, and 22. and Ev +1 to bring up whites

I dont know why the exposure changes in the 24-70. As you can see, the first shoot at 2.8, seem to be ok an match with the series of the 24 Ais,

When I stopped down in tends to underexpose like 1 stop.

Thanks for your help and apologise for my poor English.

24-70 2.8 G

24 Ais