Still got your 30D Malcolm? I was thinking more of a shot from FF and a shot from a smaller format both with the same lens, the subject the same size in the frame and the same DoF in each shot. No easy task! (This would mean shooting from further away and with a wider aperture when using the smaller format)

As a subject I guess a letter in large print would do....newspaper headline....cereal box....cover of AP....anything to hand