EOS T4i will not focus when the AF assist beam is activated. Works fine when AF assist isn't needed

[GregA]

I have a T4i with an 18-135stm lens that absolutely love. My daughter was purchasing a new camera last year and wanted the the same system I have, but settled on the T4i wit the 18-55 EF lens because of the price. Since the camera was new in January 2014 she was always complaining that a lot of times when she would try to use it, the camera wouldn't take a picture. She figured that she just wasn't using it correctly. We finally got together for a mini training session and the first thing I did was clear all the settings. I also don't like the face tracking focus so I changed that to Flexi Zone Multi. Everything worked fine with camera until it started to get just a little darker in the house and the AF assist beam was needed. The beam would pulse, but the focus would not lock on at all and there was no beep or anyway you could take a picture. If you pointed the camera in the light, it focused fine, beeped and the shutter would activate. I used my camera and in both situations it had no problems. I swapped lenses then her camera worked fine as did mine with her lens. Switched lenses back and her camera with her lens worked for a while then the same problem occured. We were focusing on a varity of things at distances between 5 and 10 feet away. Lighting was evening indoor lighting (not dark) that was just barely dim enough for the AF assist to fire. The camera/lens is 13 month old, but she bought the extended warrant at time of purchase. I wish I would have looked at it before it was a year old. Please let me know your thoughts

[ScottyP]

Hi.

There is no reason it would work for a while and stop. It can be frustrating to get a camera to autofocus in dim light, and what the camera considers dim may not seem dim to your eyes.

I edited my reply after reading about the new STM 18-55 vs the older 18-55 (non STM). Evidently the new STM focuses faster, but the T4i shipped with the older non-STM version. Your 18-135 lens Was upgraded to the newer tech before the 18-55 was, so at the time T4i was sold, the 18-135 was the better choice for the kit lens than the 18-55 at that time.

[ScottyP]

Question: how are you shooting? Are you putting your eye to the viewfinder, or are you using the LCD screen on the back? It makes a difference.

[GregA]

Thank you for your reply, but I am really thinking there is a definite hardware/contact problem just not certain if it is the lens or the camera body. Both cameras have the same/latest firmware. Autofusing didn't work with the AF assist on the slightly newer t4i body with the 18-55 EF lens. I shot same subject in the same light with the other t4i with the 18-135 STM lens and the AF assist fired and the camera locked on without a problem. Completely swapped lenses on the two bodies and both cameras were able to lock on without issues even though it was at least 25% darker. Swapped the lenses back and both cameras again were able to lock on and focus with the AF assist beam. We played with the cameras for a while and again the one with the 18-55 EF lense stopped being able to lock on and thus not be able to take a picture whenever the AF assis its beam was needed. I took the lens off and put it back on the same camera body and it started to work again. I am just not sure for how long. Your thought would be appreciated.

[GregA]

I appologize, I just realized that I didn't answer your question. I was using the view finder for capturing the images.