image quality of a tele-converter

My most tele lens is a Sigma 180mm macro. Next
weekend I am going to the Dayton air show. My question is do I buy a converter or is image quality bad enough with one that I should buy another lens. Consider I rarely have a need for anything beyond 180mm. Based on past experience I feel 180mm won't be enough to pull it in.
thanks

For not a lot more than a tele converor you could buy yourself a sigma 70-300 DL lens. The image quality would be far better than with the convertor and you have another lens in your arsenal to boot

Richardthesane

its funny you mention that lens because thats one of two I have narrowed it down.
thanks man

I have a Sigma 170-500mm zoom. It works good, but you better have the optional trailer to haul it around.

---Bob Gross---

how is sharpness and auto focus

on your 170-500? this lens was my first consideration.
Is it soft in the 300-500 range around f/8?
thanks

A friend of mine got the sigma 70-300 super DL macro for his kit, and I must say I am fairly impressed with the results he is getting. It is about as sharp and my canon 100-300 USM (but not my 100-400L ). For the money there is a lot of lens there, and if you don't need that focal length often then it comes highly recommended.