Variable focal length vs. zoom lens...

[rpcrowe]

Way back when I was a Navy Cinematographer - that was when Noah (of the Ark fame) was one of my shipmates... We only called a lens a zoom lens if it retained focus at all focal lengths. An example of this type of lens was the 10-120mm Angenieux for 16mm cinema cameras. This was the standard lens that I used on the 16mm Arriflex when I wasn't using a three lens turret with a set of "T" Stop primes. The advantage to this lens was that I could zoom in to maximum focal length (around 120 mm) focus and then zoom out to my shooting focal length. Of course, this was in the days prior to any auto focus capability.

A lens that could zoom in focal length but, which did not hold its focus from one focal length to another was termed a variable focal length lens. The problem with this type of lens for motion picture or video shooting is that you cannot shoot while zooming from one focal length to another. However, since, I almost never zoomed while shooting, that would not have bothered me.

Today, with the advent of auto focus, the need to focus at the maximum focal length (to get a sharp image) is pretty well negated. I wonder how much my 100-400mm f/4.5-5.6 IS lens would cost and how heavy it would be if it could hold focus through the entire range of its focal length and if it had a consistent aperture throughout the focal range. How about adding internal focus to that equation... The cost and weight/size would be enormous

However, I am really happy with my 100-400 II lens because its sharpness and auto focus (especially when used with my 7D2) is marvelous. I love to be ble to count the individual hairs in a person's beard

[proseak]

I remember in the early 80s zooms came in two flavours, varifocal zooms and constant focus. At a given price point, the varifocal lenses delivered better quality, but lacked the facility to zoom in, focus and zoom out while retaining focus. I had a 28-80 VF which I replaced with a constant focus 28-80 from the same manufacturer. The physical sizes were similar, but it was quite a step down in IQ.

Later that decade, constant focus became the norm.

[Manfred M]

Richard - lenses designed for video work are completely different from those used in still photography. The still camera lenses are varifocal, and one has to re-focus when zooming in and out on a subject. Video / film lenses are a parfocal design which means they stay focused on the subject as you zoom in and out on your subject. Video lenses are also designed to minimize "focus breathing", where the image size changes as the focal point changes. A third key difference, which is shared by high end camera lenses is a fixed aperture throughout the zoom range. A videographer does not want to see the image darken or lighten as one zooms in and out of a scene. They are also feature "clickless", apertures i.e. the aperture is infinitely variable and there are no fixed clicks as one goes through various aperture settings.

[William W]

. . . The still camera lenses are varifocal, and one has to re-focus when zooming in and out on a subject. Video / film lenses are a parfocal design which means they stay focused on the subject as you zoom in and out on your subject.

To my knowledge Nikon has made very few, if any, comments regarding parfocal zoom lenses in their stable of SLR DSLR lenses; certainly third party users, such a Lens Rentals, have made comment that none are parfocal.

However, apropos the Canon EF Series of lenses, (for still cameras) some lenses are considered to be parfocal.

Below is an extract of correspondence from Chuck Westfall, dated 29th May 2003:

“. . . parfocal zooms in the Canon EF lens line-up is essentially limited to the fixed-aperture L-series category and front-focusing zoom lenses. The current products include:

EF 16-35mm f/2.8L USM
EF 17-40mm f/4L USM
EF 24-70mm f/2.8L USM
EF 70-200mm f/2.8L USM
EF 70-200mm f/2.8L IS USM
EF 70-200mm f/4L USM
EF 75-300mm f/4-5.6 IS USM
EF 75-300mm f/4-5.6 III USM
EF 75-300mm f/4-5.6 III
EF 90-300mm f/4.5-5.6 USM
EF 90-300mm f/4.5-5.6

Discontinued lenses in this category include:
EF 17-35mm f/2.8L USM
EF 20-35mm f/2.8L
EF 28-70mm f/2.8L USM
EF 28-80mm f/2.8-4L USM
EF 50-200mm f/3.5-4.5 USM
EF 50-200mm f/3.5-4.5
EF 70-210mm f/4
EF 75-300mm f/4-5.6 II USM
EF 75-300mm f/4-5.6 II
EF 75-300mm f/4-5.6 USM
EF 75-300mm f/4-5.6
EF 80-200mm f/2.8L
EF 100-300mm f/5.6L
EF 100-300mm f/5.6


[The] EF zoom lenses with Super Inner Cam focusing, which include most of the non-L zooms introduced from 1990 onwards as well as the EF 35-350mm f/3.5-5.6L USM and the EF 100-400mm f/4.5-5.6L IS USM, are not parfocal.
Chuck Westfall
Director/Technical Information Dept.
Camera Division/Canon U.S.A., Inc.

REF: Canon Professional Services Newsletter (archived, recovered file, WMW)

Additionally, my view is that it is logical to assume that current EF Lenses which conform the general parameters that Chuck outlined in 2003, will also be parfocal: as for one example, the EF 16 to 35 F/2.8L MkII USM.

***

As a comment:

It is interesting to note that Canon, has not to my knowledge made any recent comment apropos parfocal zoom lenses.

My thinking is that any comment in the affirmative would/could be misconstrued as being definitive technical data and so the TR&D and/or the Marketing Department is playing a “straight bat” (source ‘Cricket Terminology’ for a translation of the meaning)

*

Dr Bob Atkins, has made a few comments about (Canon) parfocal zoom lenses: the most appropriate/relevant to this conversation is probably that parameters or ‘parfocalness’ need to be defined

– i.e. in simple terms the salient consideration is –

a lens will be considered parfocal if there is less than [X] degree of measured deviation of the Focus Distance over [Y] degree of the zoom’s compass.

*

. . . I wonder how much my 100-400mm f/4.5-5.6 IS lens would cost and how heavy it would be if it could hold focus through the entire range of its focal length . . . However, I am really happy with my 100-400 II lens because its sharpness and auto focus (especially when used with my 7D2) is marvelous. . .

In respect of Richard’s EF 100 to 400 F/4.5~5.6L IS MkII USM, this review might be of interest as it specifically mentions that the lens in NOT parfocal, but recognises that it is very close to being so, an extract here:

“As with all of Canon's higher grade auto focus lenses, a focus distance window is provided. This lens is not parfocal, meaning that refocusing is required after zooming. This is especially true at short focus distances where the subjects very obviously go into and out of focus during zooming. However, if focusing at 400mm at longer focus distances, subjects remain in very good focus over the majority of the focal length range when zooming out, and under 135mm, the f/5.6 depth of field covers much of the focus distance change needed.”

“Using the near-parfocal feature of this lens (as just described) is helpful for video recording while adjusting focal lengths. Video shooters will definitely appreciate the fact that subjects do not significantly change in size as they go in and out of focus.”

REF:thedigitalpicture.com; Canon EF 100-400mm f/4.5-5.6L IS II USM Lens Review; 2003-2017 The Digital Picture, LLC; Bryan Carnathan [LINK]

WW

[Manfred M]

There was quite a lot of "buzz" in the video field when this inexpensive Fujinon lens was announced ($USD 3800.00) rather than the five and six figures these lenses usually run at:

https://www.bhphotovideo.com/c/produ...t2_9_lens.html


The big brother sells for $USD 87,300.00

https://www.bhphotovideo.com/c/produ...remier_PL.html

[Glenn P]

I wonder how much my 100-400mm f/4.5-5.6 IS lens would cost and how heavy it would be if it could hold focus through the entire range of its focal length and if it had a consistent aperture throughout the focal range.

Canon CN-E 30-300mm T2.95-3.7 L SP PL Mount Cinema Zoom Lens - $44,650.00

B & H offers free expedited shipping.