F00 reading on canon len? how to change aperture value?

Question

I have a Canon XTi that works fine with the EF 18-55mm f4-5.6. I was able to change the shutter speed under the Tv mode and the aperture under the Av mode. Also, I was able to adjust both under the M mode. BUT, when I use the EF 75-300mm f4-5.6 len, i have a f00 reading at all time (not blinks). I can't change the aperture (even under the Av mode), only the shutter speed. Is the len defective or is there something i'm missing or not doing it right? Also, how come the camera show a f value greater than the len indicated ie on the EF 18-55mm f4-5.6 len, I could dial it up to f16 and beyond? Is f stop and f value are the same (aka aperture, right?)?

it's a Canon len (no ring)

Answer 1

It Sounds Like The Lens Is Defective Or Has An Aperture Connection That Is Dirty/Damaged.

I'm Assuming Its A Canon Lens And It Has No 'Manual' Aperture Ring? (If It Does, It Needs To Be 'Set' At The Green 'A' For It To Work Correctly)

Check The Metal Connections At The Rear Of The Lens, if One Or More Of The Little Metal Tabs Are Dirty, Use A Pencil Eraser To Clean Them. If They Look Damaged/Missing Or It Still Doesn't Work, It Needs Service/Replacement.

Good-Luck!