"Because when I look in the eye piece it’s blurry"
What you are saying is it is blurry at only 300mm FL setting? All other FL are OK?
I don't remember if the Rebel T3 has a diopter adjustment but if it does it is probably not adjusted to your eye.
What do you mean by "it won't let me focus?" if the focus ring does not turn in manual focus mode at 300 mm there is some odd mechanical problem with the lens.
You are turning the focusing ring, right?
07/31/2023: New firmware updates are available.
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2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM