Firmware 1.2.3 is required if need any of those features from Canon:
- clean HDMI out
- dual monitor support (internal+external)
- AF at f/8 with teleconverters
If you don't need anything from the above list, please consider downgrading to 1.1.3.
A few ML features are not compatible with 1.2.3 (see the Features tab and the 1.1.3 forum thread).
If you need any of these features, you will have to downgrade to 1.1.3.
Recommended card setup
- install Magic Lantern to an SD card
- use CF card(s) for taking pictures or recording videos
However, any card configuration may be used.
- Format the card (SD or CF) from the camera.
- Make sure you are running Canon firmware 1.2.3.
If you are running 1.3.3, 1.3.4 or 1.3.5, you can downgrade with EOS Utility (instructions).
Downgrading from 1.3.6 is possible, but tricky.
- Once you have the correct Canon firmware:
- copy ML files to the root of your SD or CF card (your choice)
- launch Update Firmware from Canon menu
- follow the on-screen instructions (you will have to reboot the camera before the timeout)
- after installation, copy ROM0.BIN and ROM1.bin to your computer and keep them safe, just in case.
- Run Firmware Update from your ML card.
- Follow the on-screen instructions.