sigma 24-70 vs nikon optics question

Ok heres the deal... I just bought a sigma 24-70 fixed 2.8 from

B&H...The problem is that when switching from my nikon lenses

(shooting at F8 and 1/200th on both the sigma and the nikon at 50mm,

again fixed on both)to the new sigma lense I need twice the strobe

output (another stop).

 

In other words my sigma lense seems to need one more stop of light at

F5.6 / 1/200 and fixed 50mm than my nikon lenses. I tried stopping up

and down with the same results all through the apereture range, again

using a fixed shutter speed and fixed focal legnth.

 

I am wondering if I should return the lense for a new one or are sigma

lenses all like that??? Not really looking foward to spending 1500

dollars on a nikon 28-70 2.8.

 

This is the first non nikon lense that I have purchased.

 

any help would be appretiated...

Forgot to mention that the sigma apereture range goes from 32-2.8 and one nikon goes from 22 to 1.8 the other from 22 to 3.5... not sure if this helps or not...

I have the sigma 24-70. no problems. Sharp at F8-11.