Vello Bounce Dome (Diffuser) for Canon 420EX Speedlight

No unfortunately it will not. The flashes have different dimensions. We do not have any options made specifically for the 420EZ flash, but we do have a universal-fit type of option. See the link below for details:http://bhpho.to/19jr53h

The above answer depends on the size of the room in which you will be photographing, and how you frame your image. While the Vello Bounce Dome (Diffuser) for Canon 420EX Speedlight does not list a specific focal length coverage, the unit is designed to spread light omnidirectionally. At 18mm in most rooms, it should provide more than adequate coverage for your needs. At 10mm, in a small- to average-sized room, you should still have adequate coverage compared to using the flash without the diffuser. However, in larger rooms, simply due to light falloff and physics (inverse square law), it would work, but may vignette towards the edges of the frame. As it depends on the framing and size of the room (in addition to the focal length), the best I can recommend would be to purchase and test the unit in the rooms in which you will be photographing. If it does not work for your needs, this item does have a 30-Day Return Policy. In any case, it would be much better than the flash used by itself, which is only designed to cover out to 24mm.

This particular version would not fit on the 540EZ flash but we do offer a similar version made for the 540EZ. See the link below for details:http://bhpho.to/JAKBva