Au fait, voici les caractéristiques des deux objectifs :

EF-S18-135mm F3.5-5.6 IS STM

- [...] new focus by (STM) stepping motor and six-group zoom
- Greatly improved performance [of] video AF
- AF achieve smooth video recording of a moving subject in combination with the EOS Kiss X6i[Rebel T4i/EOS 650D]
- Adopted a single sheet and high-precision aspherical lens 1 UD lens, achieving high image quality compared with conventional models
- The first with a dynamic IS valid when shooting motion
- Stage 4 minute camera shake correction in terms of shutter speed

EF40mm F2.8 STM

- The thinnest and lightest lens in the EF line-up
- [...] smooth and quiet AF drive [adopting a] (STM) stepping motor
- 22.8mm Length
- 130 grams in weight
- [...] one glass mold aspherical lens
- 7-blade circular aperture
- noise reduction control is [achieved] adopt[ing a] micro-step drive to the diaphragm mechanism
- Minimum focusing distance is 0.3m
- full-time manual focus adjustment is possible to focus one-shot AF

Source : Digicame Info ????EF40mm F2.8 STM?EF-S18-135mm F3.5-5.6 IS STM??????????????????? - ????info via Canon Watch [Rumor] EF 40mm f/2.8 Pancake & 18-135mm STM Specification Leaked | CanonWatch