I'm using a Canon EF-S 18-55mm Ultrasonic lens on a Canon EOS Digital Rebel XT body. The lens works fine from around 30mm to 55mm. I try to take a picture from around 26mm to 18mm the camera does not take a picture and I get an Err 99 message with the lens in ether auto focus or manual focus. Does any have an idea how I fix this problem?
Sorry, for the bad news. It sounds like a loose or bad cable inside the lens.
If I remember correctly, as you adjust the zoom from max to minimum the lens gets shorter. But somewhere around 24mm it begins getting longer again, which coincides with how you are reporting the issue.
The replacement lens more than likely uses an older AF motor called Micro Motor. Your lens has what is called Micro Motor USM. It's not the high end Ring Type USM. That supports very fast AF. Canon has made 3 different types of USM AF Motor lenses.
Actually the OP has the Micro Motor USM version of the 18-55mm kit lens. These lenses were mainly sold in Asia. So this won't fix the problem. Also Micro Motor USM lenses DO NOT support manual focus override. Except for the EF 50mm F/1.4 USM lens. That lens has an additional clutch system to allow manual focus override. Like a Ring Type USM lens with a mechanical focus ring.
"I would just use his but it's not an ultrasonic lens."
Any EF-S lens will work on your camera.
Your XT is approaching 20 years of age. Not that it is bad but it is something to keep in mind. I still have cameras that are even older.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.