Sigma 70-300 slow FPS

I recently bought a Sigma 70-300mm f/4-5.6 APO DG Macro
and while the auto focus switch is on, my frame per sec drop to about half what my 40D can do normaly, and when in manual focus position, FPS is back to normal
anyone having the same problem?
is it normal for Sigma to have that problem?
can they rechip it to solve this?

thanks in advance for any info, I will try to infos on my own to...

Daniel

EDIT: I just realized that its not the FPS thats dropping its definitely the shutter speed thats changing. If you shoot in Av, the body will keep the aperture constant, but will change the shutter speed to properly expose the image regardless if you;re pointing straight in the same place.


what mode are you shooting in?

if you're shooting a mode like Av then the camera is automatically metering for changing light. If you shoot manual say...at something thats in the shade of a tree maybe you'd shoot it at...

1/640, f/5.6 ASA400 on drive then it should burst all at the same speeds.


Hope this helps.

I shoot in P mode often, my focus has be moved to the " * " button and set on AI mode
in manual at 1/250, it didn't changed the rate, only when switching to manual focus did it shoot at full 6.5 shot/sec

Daniel2000 wrote in post #5712668
I shoot in P mode often, my focus has be moved to the " * " button and set on AI mode
in manual at 1/250, it didn't changed the rate, only when switching to manual focus did it shoot at full 6.5 shot/sec

It doesnt matter if your focus is set to AI Servo. If in any mode other than M the camera will automatically compensate for the proper exposure.

if i shoot wide open in Av at f/2.8 pointing at someone indoors....at ASA3200 on a 1Dmk2N my shtter speed will probably be 1/400s if i shift the lens just a little and the camera reads from a darker area the camera will then compensate by changing the shutter speed to say 1/205s.

Which is why i was talking about shooting in M and asked what mode you were shooting in. I hope this helps. If not maybe someone else can explain what i'm trying to get at.