Canon 1.4X Choices or sigma 135-400

[chris.sager]

This teleconverter will be used on my XT with 70-200 F4L.

 

Tamron - $110

 

Sigma DG EX - $170

 

Kenko - $195

 

Canon - $285

 

Is there a sweet spot in there? Should i just buy the Canon and stop

fussing about the choice?

 

Would the Sigma 135-400 be a better choice? It seems to be the only

lens even close in price and/or quality to the 70-200 w/ 1.4X.

 

On a side note - does the math work like this? The 70-200 has a

magnification factor of .21X. Can i multiply that by the 1.6x crop

factor for a magnification of .336X? Can i then multiply it by 1.4

with the converter? That would make for a pretty decent magnification

of .47. Does it work this way?

 

I know the 1.4X is not as good as a true 300ish mm lens. It does seem

that most of the complaints about them come from the corner sharpness,

something that should be reduced on a crop body. Let me know what you

guys think!

[kin_lau]

What are you shooting? The 135-400 is not a fast focusing lense, and neither will be the 70-200 with a 1.4x.

 

KEH has a few Sigma 400/5.6's that should be compatible with the Rebel XT. The ones that are not compatible are listed as "film only", but there's a few in the $400- range that should be okay.

[robin_sibson1]

Concerning magnification, you can indeed multiply the maximum magnification of the lens by the magnification of any TC you are using with it. The choice of format does not change the magnification, but it does change what magnification you need to use to produce an image in the same proportion to the frame dimensions, and the factor is 1.6. Exactly the same applies to focal length and angle of view.

 

The 70~200/4L is widely regarded as working well on the Extender 1.4x, and my own rather limited experience of occasionally using my son's copy in that combination certainly supports that view. AF is retained although AF speed is (deliberately) reduced to help focus to lock reliably. The main limitation is lack of AF, and on a 1.6-factor camera that's a significant issue at 280mm.

 

Can't comment on the Sigma, but I can tell you that at 400mm you will generally be limited to a tripod unless you have IS, and the two good Canon options (100~400IS, which I have, and 300/4IS optionally with the Extender 1.4x) both cost serious money and weigh quite a bit. If you are thinking of an alternative lens, the recently introduced Canon 70~300IS sounds to be pretty good optically, although the mechanical side is nothing to write home about.

[robin_sibson1]

For "the main limitation is lack of AF" read "the main limitation is lack of IS". Time for my coffee.

[mark u]

If your concern is about achieving maximum magnification at close focus distances (a "butterfly lens"), then you should consider using a diopter such as the Canon 250D or Nikon 5T rather than a TC - it's a cheaper solution. That won't allow you to focus very far away though. At any given focal length (as adjusted by any TC), magnification is inversely proportional to subject distance. Have a play with this calculator to understand the differences:

 

http://ca.geocities.com/lokejul/jlcalc.htm