Parfocal lens & zoom focus

[arkphotos]

Do you ever find it worthwhile to obtain focus 'zoomed in' & then zoom out to compose?

Saw this mentioned once (can't recall where) & was wondering if this is a technique that many use ...

[Double Negative]

Even though my zooms are parfocal, to be quite honest I'd rather do a quick re-focus once zoomed "just to be sure." Might be a waste of time, I don't know - but it eliminates any focus error due to nudging the focus ring, etc.

I suppose if the timing of the shot was of the utmost importance, then zooming w/o re-focusing might be great. I'm not usually under such constraints and just tap * to focus again.

[JeffreyG]

Even though my zooms are parfocal, to be quite honest I'd rather do a quick re-focus once zoomed "just to be sure." Might be a waste of time, I don't know - but it eliminates any focus error due to nudging the focus ring, etc.

I suppose if the timing of the shot was of the utmost importance, then zooming w/o re-focusing might be great. I'm not usually under such constraints and just tap * to focus again.

And be aware that most Canon zoom lenses are not parfocal. For sure your 70-200/2.8 is not Double Negative, though I cannot comment on your 24-70 or 16-35.

The 24-105L is also not parfocal. Neither is the 17-40L.

One quick way to check is to simply look at the focus distance scale on the lens. Do you see more than one little hash mark for different focal lengths? If yes then the lens is absolutely not parfocal. My 70-200 has three hash marks (labeled 70, 100 and 'blank') for instance. If it was parfocal it would only need one mark for all focal lengths.

[Double Negative]

Actually, they're all (well, the three I have at least) parfocal as per Chuck Westfall of Canon:

http://www.rogercavanagh.com/helpinfo/30_parfocal.stm

[Mike-DT6]

Having read about the zoom-then-focus technique I tried it and not once did it work. It went out of focus every time on both the lenses I tried (17-40mm L and 70-200mm f/4 L).

I presume that there must be some friction that moves the focus setting as the zoom is operated. With this uncertainty I have found that the technique as good as completely useless.

Mike

[SkipD]

I have three Canon "L" zooms - the (original) 16-35 f/2.8L, 28-70 f/2.8L, and 70-200 f/2.8L IS. None of these stays precisely in focus if I change the focal length more than a little bit, even though my understanding is that they are supposed to be parfocal (though the Canon web site does not say that about them).

[SilverHCIC]

Like DoubleNegative, I simply refocus after zooming. ... especially with objects that may move even a bit. I have the 16-35L II, 24-70L, and 70-200 L 2.8IS (which are all listed as parafocal) but I refocus anyway. ... But those lenses focus so fast it's ridiculous, so even if I tap-tap-tap to refocus, it's super fast.

[Double Negative]

To be parfocal, the lens uses an internal cam that adjusts focus as it zooms. Odd that Canon doesn't mention this in any references to these lenses.

With everyone saying how "well" it works... Makes me glad that I never rely on it!

Now consider this; don't video lenses need to be parfocal so that you can zoom in/out smoothly, or do they AF as they zoom?