nikon zoom not maintaining focus while zooming

[Mike N]

Hello I own a Nikon 17-35 2.8 and a 70-200 2.8 zoome lenses. Using them both on my RED.
I noticed that when i focus the image at 35mm and then pulling the focus out on 17 the lens does not maintain focus. this is noticeable on the bigger zoom but to a lesser extent.
is this a normal behaviour for a photo lens ?
thanks

[Robert Frank]

Isn't that a back focus issue typically?

[James Wright]

I noticed that when i focus the image at 35mm and then pulling the focus out on 17 the lens does not maintain focus.
Is this a normal behaviour for a photo lens ?

Nail on head :-) Still lenses are not designed to hold focus during a zoom. They are varifocal (http://en.wikipedia.org/wiki/Varifocal_lens). Think of them more as a variable prime than as a zoom lens. That's part of the reason a 17-35 F2.8 still lens is $1500 and a 16-42 F2.8 Optimo is $30,000. A cinema zoom lens will be parfocal (http://en.wikipedia.org/wiki/Parfocal_lens).

[ericyoung]

Some stills lenses are parfocal, but more by accident than design. Being parfocal is not necessary when shooting stills, especially with autofocus!

[Mike N]

ok thanks. didn't notice this until i started using them for shooting.

[Michael Hastings]

Mike,

Check your back focus/flange depth. Many of the higher quality zooms are parfocal, so the earlier poster may be right. Not sure on your exact lenses but the 2.8s are generally the highest quality. Evin grant or another Reduser may know if it should be parfocal.

[Richard Foster]

Yes definitely check your back focus. My Nikon 17-35 f 2.8 holds focus perfectly throughout the range.

[Alexander Ibrahim]

The Nikon 17-35 I use has focus drift in and out.

Its not a backfocus issue... notice I said drifts. It doesn't shift predictably.

The lenses are engineered to be varifocal in operation, but the design was originally parfocal. (Or so I was told by Nikon folks when I bitched)

So, the net is that QA is done for effectively varifocal operation with AF on DSLRs.

That means some of these lenses will hold focus and some won't, and Nikon won't support you (i.e. exchange the lens) for not holding focus through a zoom.

So... fiddle with your backfocus, it may help or even eliminate the issue. Just don't expect the performance of a cinema zoom.

[Lee Saxon]

Some stills lenses are parfocal, but more by accident than design.

I wish I owned one such lens so I could test if they're really parfocal or if the variation is just small enough to be difficult to see under most conditions.

Pretty sure all of mine are clearly varifocal.