1. Improves AF accuracy when used in combination with the EF 16-35mm f/2.8L USM lens or EF-S 17-55mm f/2.8 IS USM lens.
2. Fixes a phenomenon in which, in very rare cases, the Peripheral Illumination Correction (PIC) is applied inconsistently.
3. Fixes a phenomenon in which, while performing an EF lens firmware update, the progress bar displayed on the LCD monitor may freeze at 100% even though the update has been completed.
4. Fixes a processing error in relation to the leap second for the 'Geotag information appended to image' and 'Log data' GPS functions.
5. Fixes a phenomenon, where at the time of shooting in very rare cases 'Err 70' occurs, or the shutter does not release.
After updating the firmware, you should reconfirm any AF Microadjustment you may have set for your EF 16-35mm f/2.8L USM or EF-S 17-55mm f/2.8 IS USM lens.
To download the firmware update visit the Canon website
For a full guide to updating your EOS firmware, see EOS magazine October-December 2012 issue, (available to buy as a back issue for £4.95 from EOS magazine shop). You can also download a copy of this article for free.