This is only partly true. It's important to note the difference between Canon and non-Canon lenses.Depends on the fitment. the 450d can use EF and EF-S lenses. The 5d will only use EF lenses by design.
There are people who have been able to use some EF-S lenses on the 5d like the sigma 10-20mm but they can't use it at full width of 10mm but can go to 12mm. it is due to the back of the lens sticking out of the lens more on EF-S than on an EF at 10mm the EF-S hits the camera internals.
![]()
Which lenses are you specifically thinking about ?
You wont be able to mount the ef-s lens on the 5D as its full frame and wont have the white dot on the body so as to prevent mirror damage and not filling the frame anyway.
The rest being EF lenses will be fine.
Matt