I am trying to buy my husband a gift from photojojo. He has a Canon DSLR. It's the EF-55-250mm. I have found a white balance lens cap that needs a size and the photojojo site says to look for the size on the outer rim of the lens. There are two numbers that I think it could be, 55mm and 58mm. My question is- are the thread size and the filter diameter the same thing? It's not clear on the lens because the 55mm actually reads "15-55mm" on one of his lenses and 55-250mm on the other AND there's a spot on BOTH of them that have a no symbol and then a "58mm" in another spot.
Yes. The thread size is the filter diameter.
The other number is the focal length. This corresponds to the angle of view of the lens. The range of numbers in both cases is because they are zoom lenses, and those two "kit" lenses are designed as a sort of matched pair, so one starts where the other one leaves off, and that just happens to be at 55mm.
The "no symbol", by the way, isn't a no symbol. It's a diameter symbol, ⌀.