I am looking at a Canon 100-400mm 4.5-5.6L lens to help with ceremony footage in big churches. Even with the tripod collar for the lens, I wondered if there was any bouncing when fully zoomed in? Vignetting? What is the longest lens you use for weddings?

Canon 100-400mm f/4.5-5.6L IS USM Autofocus Lens 2577A002 B&H

Dude, just get a 2x extender .. the Canon one. It will turn your 70-200 2.8 into a 140-400 5.6. Much cheaper than a new lens and very compact to pack. No vignetting. It will be hard to keep from bouncing if you try to hand steady the camera/lens, but that will happen with the 100-400 too.

I have shot a few events with my 100-400 and it is razor sharp with no vignetting. As with ANY long focal length you must have a great tripod or every flaw will show up.
On my 7D, the 400mm end turns into a 640mm which is just a touch longer than my XF300 at full zoom.
I shoot aircraft and moving objects frequently and this is what prompted me to get a better tripod. Went from a Manfrotto 503hdv to a Sachtler FSB-6. At full zoom I can move both cameras with no bounce or jitter at all.

The ergos of the 100-400 are a bit weird as it's a pull-push for the zoom as opposed to the ring on the 70-200. You can't record zooms with the 100-400 as it's aperture changes causing flicker. The 70-200 is constant aperture so it works just fine.

I love the 100-400 but the doubler may be a better choice if you already have a 70-200 f2.8

In the past I've used 7D and Canon 70-300mm f4-5.6. A sharp and budget-friendly lens. Combined with the 1.6x crop I get 112-480mm out of it. Just had to stick at f5.6, which is a pain if shooting in poorly lighted church.

I've rented this beauty on a few occasions.

LensRentals.com - Rent a Sigma 120-300 f/2.8 OS (http://www.lensrentals.com/rent/canon/lenses/telephoto/sigma-120-300-f2.8-os)

2.8 aperture with OS. Just gorgeous. Wouldn't use it enough to justify paying 3,000+ to buy it, so renting works well for me.

What are you filming with? This has quite a big weigh in, as far as what focal length to get. As noted above filming with the 7d, is the crop factor. This changes the focal length. I'm using an AF100 which is 2 x crop, so with me using a 70-200, it's actually a 140-400. I find this a bit long on some shoots, so I just picked up the olympus 35-100 which is much more usable up front, or from the sides at the ceremony, but still allows me to zoom right in for some close ups. It's constant f/2.0 throughout which is very helpful in dim settings. I use it as a variable prime instead of a "zooming" lens.

At those longer lengths without IS you may regret your decision. I think Travis hit the nail on the head. A 70-200 with an extender. That way, you have a nice long lens to use with a fantastic f2.8 and with the extender you get in closer when needed.

With the 100-400 you loose that lovely bokeh we all want from our DSLR's. Also, f4.5 in most churches (certainly here in England) would be too dark.

I haven't used it for video yet, but how 'bout the Canon 28-300mm f3.5 L lens. Also a push-pull with incredibly fast auto-focus and great image stabilization. Great on both wide and telephoto end (fairly fast on the wide end but may be a bit slow on full zoom). Very pricey but you may be able to find one to rent at your local camera shop.

As with any video shooting with a DSLR, you'll need a tripod.

Use a 600D(T3i) in 3X HD crop mode with a 70-200mm F/2.8L IS & you have the equivalent of a 210-600mm F/2.8L IS which gathers one more stop of light than the Canon 600mm F/4L IS which retails at about $12000.

Use a 600D(T3i) in 3X HD crop mode with a 70-200mm F/2.8L IS & you have the equivalent of a 210-600mm F/2.8L IS which gathers one more stop of light than the Canon 600mm F/4L IS which retails at about $12000.DHL just delivered us a Canon 2X teleconverter which I have just fitted to the 600D with 70-200mm F/2.8L IS & just filmed activity on the bird feeder with the 3X HD crop mode or the equivalent of using the mythical $120,000 Canon 1200mm F/5.6L Canon EF 1200mm f/5.6 L USM Lens Review (http://www.the-digital-picture.com/Reviews/Canon-EF-1200mm-f-5.6-L-USM-Lens-Review.aspx)