I am using 450d. When using a-def,m,av,tv,p mode, the shutter does not work properly. After pressing the shutter button, you have to press it again to lower the shutter. Do you have a solution? Also, AF is not possible when using the above mode.
Your camera has “Mirror Lockup” enabled. You need to go into the Custom Function menu and disable CF III-9.
Here is a link to a short video that describes this in more detail.