Back focus issue?

[Garrett Low]

I'm going to be shooting a short movie and the Director wants to use a Canon 7D. On page 32 of the manual it states "If you want to zoom, do it before focusing. Turning the zoom ring after achieving focus may throw off the focus slightly."

Have any of you notice this while shooting video and zooming in. Is there a back focus issue certain lenses?

Thanks,
Garrett

[Renny Hayes]

There's a difference between back-focus problems and non-parfocal lenses. This is a parfocality issue, not a back-focus one.

Still camera lenses are not designed for video, and as such, they are not built with the expectation that users will need constant focus. When you zoom with a non-parfocal photo lens, you do lose focus. Some L lenses are parfocal, but most general lenses aren't. Why would you want to zoom while shooting anyway? The x5 and x10 features are much better for critical focus, and zooming during the scene makes things look kind of cheap. See: Pelham 123

[Garrett Low]

Thanks for your answer Renny. It sounds like the end result is that it really depends on the lens, which is what I thought. I'm being lazy but can someone point me to a reliable list of which lenses will hold focus while zooming?

To answer your question Renny, there are several reasons why you zoom while shooting. It is a technique that is used all the time in cinematography. When you don't have sufficient room or equipment to dolly in you can usually use zoom in to achieve a similar effect. Like any other technique it isn't something you do all the time or the effect looses its meaning.

-Garrett

[Liam Hall]

Originally Posted by Renny Hayes

There's a difference between back-focus problems and non-parfocal lenses. This is a parfocality issue, not a back-focus one.

Still camera lenses are not designed for video, and as such, they are not built with the expectation that users will need constant focus. When you zoom with a non-parfocal photo lens, you do lose focus. Some L lenses are parfocal, but most general lenses aren't. Why would you want to zoom while shooting anyway? The x5 and x10 features are much better for critical focus, and zooming during the scene makes things look kind of cheap. See: Pelham 123

Parfocal lenses can suffer from back-focus issues, just like any other lens. The only difference with a cine or broadcast lens is you can adjust back focus yourself and with a stills lens you'll need to send it in to the repair shop to be adjusted.

Also, you might want to tell Stephen Spielberg that zooming is a no no - he tends to use zooms rather a lot...

[Jonny Norquist]

I believe the GH2 has a very impressive zoom function