Auto Focus And Zoom Lenses

[zhendirez]

You know, somehow once I started using autofocus way back when, this question never occured to me.

When I started out in film cameras and manual focus lenses, it was a general practice that when using a zoom lens, you would zoom all the way in, focus, then adjust the zoom to where you want it and take the picture.

Here's my dumb question: Is that still true with current autofocus zoom lenses?

I ask because I got a Tamron 17-50 for my 20D and was playing around with it when I discovered that a shot I focused and took at 17 mm was not as sharp as at 50 (expected) or if I focused at 50 and zoomed back to 17.

I'm not sure why this never came up for me before (crappier lenses previously, I suppose). But should I still "have" to do this with autofocus?

[Jon]

It only worked for some zooms back then, and still only works for some zooms. If a zoom is what's called "parfocal" it will retain its focus as you zoom. In that case, focusing at max. zoom and recomposing will work; if it isn't parfocal, the focus will change as you zoom. Better focus as you're about to shoot, at the working focal length.

[zhendirez]

So is there a place I would go/look to find if a particular lens is parfocal or varifocal?

I guess I'm just trying to see if there's a problem with the lens or a problem with the way I'm using it.

[Glenn NK]

All of my zooms (three) must be re-focused after changing the focal length.

It's not hard to check - zoom to the shortest focal length (wide angle), and focus on something in the scene (I used my monitor), then zoom to the longest focal length and look at the object that was focused on, and hit the focus button again.

In all likelihood, the lens will re-focus.

Mine do, although in the case of the 24/105 L, the correction is very small. Which leads me to believe that it's quite common.

So I use Jon's method.

[SkipD]

Even my three Canon "L" zoom lenses (16-35 f/2.8, 24-70 f/2.8, and 70-200 f/2.8L IS) require a little touch-up to focus if you change the focal length radically.

[Stocky]

I think that when auto focus was less common and less capable then designing lenses so that they maintained focus was much more important. Now I think that lens designers focus more on image quality and consider the focus features less.