70-300mm Lens or Doubler?

[Kudos2Fly]

I just got a 20D, with a Sigma 18-125mm lens. So far I'm very happy with the lens. But I'm thinking I may want to get a Tamron 70-300 lens. But also have been told about getting a double for the the lens I have now.

Will either be okay? If the double works. I would prefer doing that. Just for the reason of not caring a lot around. But also want to know the quality of the pictures on a double as a pose to a 70-300 lens?

Please let me know.

Kim

[Citizensmith]

The doubler (more commonly known as a teleconverter) has 3 effects.

It doubles your focal length. The lens becomes a 36-250 (or 58-400 equivalent on your 20D)

It reduces the amount of light hitting the lens by two stops. The f/3.5-5.6 Sigma becomes an effective f/6.7-11. So say goodbye to autofocus and say hello to manual focus with a really dark viewfinder. Also say goodbye to any ideas about using the lens in low light.

It also makes things fuzzy as crap. A decent quality 1.4x TC can be used without too much trouble. A cheapo 2x TC can look like you smeared an assload of vaseline on your lens. Particularly with this lens as TCs tend to be optimised for longer focal lengths.

Basically, you are much better of getting the Tamron 70-300. I've no clue about the reputation of that particulat lens, but Tamron do make some very good lenses.