11-14-2012 06:18 PM
Problem is the image circle on a EF-S lens is made to fill a crop sensor not a FF sensor, which is why crop bodies will accept EF and EFS lenses but FF only accept EF lenses. EF lenses project a circle big enough to cover a FF sensor.
Put an EFS lens on a FF and assuming you don't hit the FF mirror against the back of the EFS lens, you'll have heavy vignetting because the EFS lens doesn't cover the entire FF sensor. If you don't crack your mirror, you would have to do some pretty serious cropping.
11-14-2012 09:47 PM
The first reply is correct for a Canon EF-S lens but not necessarily true re damaging the mirror if using Tamron or Sigma lenses. They didn't add the part Canon did that projects back into the mirror box area. That said you can use Sigma & Tamron lenses designed ONLY for crop bodies, but in most cases the photos will look like you shot through a pipe.