Is a parfocal lens necessary?
 

Seems to add a lot the price of a lens. For video, why not just use that zoom/FA button? The big problem with the 7d is holding focus, not getting it initially. Parfocal wouldn't seem to help pulling focus unless you're zoomer?

That's correct. Parfocality doesn't matter unless you're a zoomer.

But it does help you get it correct the first time and lets you set frame off of that.

That's how I always focused zooms, zoom in to focus, zoom out to desired frame, push button. But with that two position zoom button on the 7d, that isn't necessary any more. One way to save some money.

Interesting, I had to look up this term to see what it meant. Being used to video zooms, I sort of assumed that photo zooms all held focus throughout the zoom. Is that not the case? If so, how would the focus marks on the lens work, as they don't change as you zoom? So are most photo zooms not parafocal? I bought the 17-55 2.8 (Canon) with the 7D and am planning to get the 20-700.

Correct, it's not the case. Many still zooms don't even try to be parfocal, and of the ones that do try, there is unit-to-unit variation.

The focus marks are already useless on the non-parfocal lenses for other reasons, such as the very short travel of the focus ring. They would have to add a real manual focus before it would be good enough to have trouble because the focal lengths aren't parfocal. The parfocal lenses tend to have better manual focus features, but still not as good as lenses that are designed for it.

I think these lenses are intended primarily for autofocus.

Correct; most are not. None are guaranteed to be, but some are better than others.

I don't know about the 17-55, but the 70-200's tend to be nearly parfocal, the focus shifts only a tolerable amount on most units.

Good to know, Daniel. Thanks for that info. And thank god for the excellent focus mag feature on the 7D.

I use a Tamron 17-50 f2.8 and a Tamron 70-200 f2.8 and both are Parfocal. They really are great video lenses and are worth every cent spent on them. As far as the true parfocal nature of them they are very close through out the the full zoom range. If you are at a low f they wander a little but not much. YMMV.

On the 24-75 f2.8 Tamron, I've found exactly what Daniel says, short travel of focus ring -- which makes focusing super critical and difficult on the LCD. The interesting thing is, that same lens, when viewed through the viewfinder, is an absolute piece of cake to focus, even at the widest zoom setting. Too bad Canon couldn't figure a way to use that viewfinder during video mode, it'd solve a lot of issues.

what is this two position zoom button you refer too? Where is that exactly?

Upper right corner of camera back.

It's best described as an image magnification function, just to avoid confusing it with zooming as in changing focal length, which of course is possible only via the control ring on the lens barrel.

If it was up to me, I'd call it focus assist. That's what they call it on other cams, but even in the manual, there's no mention of it as a focus assist tool. The manual generically names it "Magnify Button".