I purchased a Sigma 1.4APO teleconverter for my f/4 300mm.  Now I can get 420mm and better get out the tripod!   I am not so steady as years before and at about 8+X in binocular terms it is not bad magnification.

Danob suggested trying the converter first since speed of autofocus can be a factor. Around here, there is nothing available to try out!  Anyway, with the lens it is mated with, autofocus seems just as quick as before and servo auto works fine, too.

The drawback is having a max of f/5.6 compared to the original f/4 without the converter. I can live with that.   

I haven't done enough "tele worthy" shots with it to post, mostly trial runs at all sorts of images.  The optics appear quite good and photos look sharp and free of any noticed abberations.  The Sigma teleconverters appear a popular item and the quality of mine tells me why.      TomDart.

Can't wait to see the results Tom!!!

Congrats Tom yes the 1.4 converter is often money well spent as the results can be very good, and could save the possible extra expenditure of a telephoto lens that would cost considerably more. The loss of 1 stop seems to be the key the x2 I  have found is not worth the expense as the loss of a further stop makes them only suitable more often than not to be tripod only.. And very very good light  required to get anything decent..

Danny, I did avoid the 2x for that very reason.  I could not afford to loose yet another f stop.

Congrats Tom it is an excellent piece of kit - I have one to use with the 150mm F2.8 gives me a 210mm prime at F4. 
No noticeable loss of image quality and auto focus is fine. 

happy you got one
I got one on my old film camera that is 30 years old and it works very good but  I too have to have very good lighting and a very hight speed of film. I guess ditital cameras is the same

Yeppie, digital is the same. I tried to bait a chipmunk to get a shot this morning. I used sunflower seed at a place the critter shows in the mornings.  He showed up.   The problem was low light.  The sun was behind clouds and behind trees.  To take a shot of a chipmunk at about 1/25 sec is not so good...they do not stay still long and even if look still, a quiver in those little guys is enough to make it blur.  My shots are ok but with blur on the chipmunk.  

I will try for good lighting and will get more shots. I  find a tripod is needed with my steadiness being a problem at the mm of the lens with the converter.   Then again, sometimes we are lucky.

Tom.

Jocko, you have made an artform from glowing body paint!

Congrats Tom. Sheesh, I should get a DSLR so I can join in all of this add-on fun. ;']

Ok, here is a shot...a chipmunk from about 33 feet(about 10 m) away at 400mm.  DOF is close to 9 inches(23 cm).   Lighting was lousy, the sun behind coulds and all overcast.  As it was, the shot was like this: ISO 800, F/5.6, 400mm, 1/25 sec.   Who can take a critter at 1/25 sec? Not me!  Not a figgity chipmunk for certain.

Regardless of the slight movement of the critter and terrible lighting, this does show the converter works pretty well.  The image is brighter than it looked at the time in all the overcast.

I will look for more light next time...perhaps the sun will show where it generally does on this spot and where this fellow generally shows up at about 7 AM for a moment or two.

Color looks fine as does sharpness except for the slight movement of the chipmunk.  So much for a trial shot at a bad time to try it.      I do like the teleconverter.       Tom.