Underexposed images

Hi I’m new to SGP.
ISSUE: My T3i shutter closes too soon (I think) when the exposure setting is much longer. SGP continues to countdown from the exposure I set but the resulting frames are very underexposed as a result.
What’s wierd is the much shorter auto focus frames are are brighter than the light frames.
Am I missing something?
Tony Licata

Maybe similar to this:

Yes that’s exactly what’s happening! So I will check my mirror lock up settings.
Thanks
Tony