PAR focal lenses, what are them?

davide sorasio · December 28, 2015

Hi everybody, can anybody give a little insight on what parfocal lenses are?

Thanks so much in advance,

Davide

Phil Rhodes · December 28, 2015

A zoom lens that retains the same focus distance as the focal length is adjusted is parfocal.

Fully-mechanical zoom lenses intended for film and television work will be parfocal.

Variable-focal-length lenses for stills work are generally not parfocal, and are not really zooms; they're more like variable primes which rely on the autofocus feature of the camera to maintain a sharp picture.

Some low-cost zooms on video cameras have used this approach. You can usually tell by snap-zooming, which can cause the image to go soft as the servos can't keep up. This approach reduces cost and may make it possible to make a sharper, faster lens, if you can stand the caveats. It isn't widely used in high end products.

P

davide sorasio · December 28, 2015

thanks so much! one last thing, what's the difference bewteen zooms and variable focal lenght lenses?

David Mullen ASC · December 28, 2015

It's a fuzzy line between them, either have been called the other term at different points depending on how you use them. Technically, it's what Phil said, a variable focal length lens is not designed to be zoomed through during a live video shot because it won't hold focus or maintain consistent exposure, but people call them zoom lenses all the time.

And there were some cinema zooms by Zeiss called variable focal length lenses that could be zoomed through fine, they were just very short in range.

Mark Dunn · December 28, 2015

Years (at least 50, I'd say) ago parfocalness (parfocality?) was a selling point for a zoom, but my modern low-end DSLR zooms are parfocal. Provided you zoom in to focus, which is standard procedure for cine zooms for DoF reasons, there's no re-focusing on zoom-out. So are my old 80s 35mm. zooms.

I assume Phil's point is that with AF they no longer have to be parfocal. I'd be surprised if some of the extreme 10x and up zooms were. It's an unnecessary refinement when you can save a few quid by not bothering.