I could use some advice.

[teamster633]

I haven't had a long telephoto lens since I switched to digital 5 years ago. I had an old f/7.2 500mm Sigma but, that wouldn't work with any of the digital bodies I've had since the switch. Currently, my primary lenses are an f/2.8L 16-35mm Canon, an f/1.4 50mm Canon, and an f/2.8L 70-200mm Canon. I've been looking at these 2 lenses:

http://www.bhphotovideo.com/c/product/1092632-REG/canon_9524b002_ef_100_400mm_f_4_5_5_6l_is.html

http://www.bhphotovideo.com/c/product/1082152-REG/sigma_150_600mm_f_5_6_3_dg_os.html

I'm very much hoping to retire in a couple years, so this may well be the last big lens purchase I'm likely to make. If you had to choose between these 2 which would you pick and why?

[Major Nikon]

1. Do you have a specific purpose you are trying to achieve?

My advice is to consider the type or types of photography you want to do and then buy the best glass for that which fits your budget. This seems simple, but often people buy lenses and then try to figure out what they can do with them afterward.

[ManiacJoe]

4. That is the correct question.

Lenses are a "right tool for the job" kind of item.
What subjects/circumstances are you looking to photograph?

[juxtaposed]

2. I'd go with the canon 100-400mm you already have 3 great lenses I love the 16-35mm

You can always buy extenders if you want more reach 1.4 or 2.0.
Also buying used in lenses has always worked good for me.

[teamster633]

3. That is my first inclination.

I already have a Canon x2 TE that I occasionally use with the 70-200. The 100-400 would allow for hand-held use much more readily than the Sigma (not that I'm averse to using a tripod when appropriate). I've never had an IS lens and each of these has that feature but, it seems like the Canon's would be more effective if for no other reason than shorter focal length.

[juxtaposed]

5. I've never used the 100-400mm, I limit myself to 70-200mm for zooms I like primes beyond 200mm.

The IS is great for hand holding and lower light. Another good thing with going with the canon is if you're not happy with it or you want to move to super telephoto lenses, canons hold their value better for resale. I think canon makes a 100-400mm push-pull zoom and heard from users they weren't happy with it in some situations.

The 70-200mm 2.8 is is a great all around lens, I've used it for everything.
I wish you luck which ever way you go.

[flamin lib]

6. If possible always stick with the manufacturer's glass.

This is particularly true with digital where the camera and lens "talk" to each other.

[teamster633]

7. The inability to communicate between lens and body

is why I had to give up my slow but, otherwise capable 500mm in the first place. While it was by no means a particularly "modern" lens, it was current enough to have jettisoned the manual aperture ring. That left me with no way to stop down the lens even if I could have gotten past the error message that invariably popped up every time I attached the lens.