Lenses: About “parfocal” lenses [Updated]
[See update below.]
Unfortunately, most DSLR-type zoom lenses are not parfocal. In fully-manual mode a non-parfocal lens tends to go out of focus as you adjust focal length (zoom in and out).
A parfocal lens enables you, for example, to zoom-in to a CU to allow you to easily adjust for sharp focus, and then zoom-out to a wide shot — without affecting focus. Using a parfocal lens enables the scene to stay in-focus despite focal-length changes, as long as the distance between the camera and subject remains constant. It makes for a very fast and accurate way to shoot.
This can be very important especially when shooting video. For example, a non-parfocal zoom lens such as a typical DSLR lens isn’t particularly efficient for following continuous, fast-changing action during a live event or performance. After every focal-length adjustment the operator must also re-focus the lens. Not impossible, but can be very difficult to do quickly and accurately, even if you use a high-resolution monitor or EVF with focus “peaking” or magnify focus aids.
I recently called B&H to ask them if they sell any parfocal DSLR-type lenses, and the person I spoke with said, “No”. He double-checked with other people in the photo dept there, and the answer was the same. So, if there is such a thing as a parfocal DSLR lens, it’s not a common item.
UPDATE: This video by “Raitank” show that the Sigma 17-50mm f2.8 EX DC OS HSM zoom lens and Sigma 10-20mm f3.5 zoom lens “hold focus” while zooming. I’ve read conflicting reports as to whether they are actually parfocal or not. Because the 17-50 also has IS, I may buy one for use with the Blackmagic Production Camera 4K. The very sharp and fast Sigma 18-35mm f1.8 zoom lens may be an even better choice (not parfocal, but very very good). Note: YMMV. (Ignore the confusing title of the video; these lenses do not appear to have an infinity focus issue on the BMCC):
Some cameras have very fast continuous auto-focus systems such that when used with a DSLR-type lens featuring very fast auto-focus, the hardware automatically “tracks” focus as the lens is zoomed in & out. So, even if the DSLR lens isn’t parfocal, the result looks mostly in-focus, most of the time. However, it’s not uncommon for the lens to periodically “hunt” for focus during a take, and audiences may find this very objectionable. Canon claims their new 70D DSLR is capable of very fast auto-focus without “focus hunting” in video mode.
Professional ENG-type servo-zoom lenses designed for use with broadcast video cameras with 2/3″ sensors are typically parfocal. In fact, this is one of the main reasons why ENG cameras are so popular for shooting run ‘n gun and b-roll footage. Note: 2/3″ B4-mount lenses will vignette severely if used with cams with larger sensors unless used with a built-in or add-on focal length extender/doubler (and B4 lens mount adapter). For best results, B4 lenses are designed for use with cameras with buit-in prism optics, or with an appropriate add-on prism adapter (for example). FYI: Marco Solorio has configured his BMCC-MFT with a 2/3″ B4 ENG zoom lens via an adapter and appears to love it.
Most cine zoom lenses are parfocal.
I recently spoke with Canon tech support, and they confirmed that the Canon cinema zoom lenses (>$24K) designed for use with S35 size sensors are parfocal. The Canon cinema lenses are available with EF or PL mounts, so the EF version should work great with the S35 size sensor in the new Blackmagic BMPC-4K camera.
Fujinon sells their wonderful “Cabrio” PL-mount servo-zoom lenses designed for S35 sensors (~$40K). Fujinon tech support recently confirmed to me that Cabrio zoom lenses are parfocal.
Note: The PL-mount Fujinon lenses aren’t a solution for the BMPC-4K unless BMD releases a version of the camera with a PL mount, which BMD has not promised to do (yet).
Various cine zoom PL-mount lenses can be used with the BMCC-MFT model camera via a PL-to-MFT lens mount adapter such as available from Hot Rod Cameras, jinfinance, and others. Because the sensor in the BMCC-MFT is smaller than S35, most PL mount lenses designed for the S35 format are compatible. The photo at the top of this blog post shows a Angeniuex 45-120mm PL zoom lens mounted on a BMCC-MFT via a HRC adapter.
Cine zoom lenses designed for use with S16 film cameras can be adapted for use on the BMPCC Pocket Cinema Camera, which has a S16-size sensor. Most of these lenses are fully manual and parfocal.
Speaking of long cinema lenses, including long zoom lenses, Cinematographer John Brawley wrote a detailed post about the advantages of shooting with them.
My list of related links, short films, and resources for Blackmagic Design cameras.
Photo credit: John Brawley
For my words only: ©2013 Peter J. DeCrescenzo. All rights reserved. http://www.peterdv.com
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