tele converter lens

im looking in getting a teleplus/ tamron converter lens , i would like to know if they are worth bothering with or would i be better getting a larger lens...
very many thanks.........

Maybe. Depends on a lot of factors though - some things to keep in mind are that a 2x teleconverter (TC) will cost you two stops of light - IOW, your f4 lens is now twice the focal length but f8. Other than specific lenses designed for use with specific TC's (like Nikon does, possibly others) you'll lose some sharpness too. Maximum print size will be smaller because of this.

Generally, TC's and zooms don't work well together, and autofocus systems don't work well with anything slower than a maximum aperture of f5.6. Aside from this, they're not all horrible - some can give pretty decent results and are much less expensive than another lens.

It comes down to what's acceptable to you, and what you want to do with the lens/TC combination.