Any 'fake' zooms from Panasonic for 4/3?

[tribal-warrior]

I was browsing through the Panasonic G2 instruction manual and I was surprised by what I read with regards to zooming and focusing. The manual advised that if you have zoomed after you focused, you should focus again. All true zoom lenses should retain focus while you zoom. There should be no need to refocus after zooming. I believe that there is a cheaper type of lens known as a vari-focal lens which does shift focus during zooming. Apparently, the kit lens of the Canon XL1 video camera was prone to this.

I did some testing with my G2 and the 14-45mm lens, focusing and then zooming in and out multiple times and it looks like the focus setting was retained. I wonder if the same can be said of the other Panny zooms made for the 4/3rds format. There's got to be a reason why Panasonic gave this advice in the manual....though I do find it hard to believe that they would incorporate such cheap lens designs in their 4/3 system. Then again, Canon did with for their flag ship semi-pro video camera.

 

[Godfrey]

The terms are "parfocal" and "varifocal". Zoom lenses can be either.

Parfocal: focus setting is held while zooming
Varifocal: focus setting can drift while zooming

In the manual focus era, most zooms were parfocal (or designed to be), manual focus shooters often used the zoom in, focus, zoom out technique to make focusing at short focal lengths with slow lenses easier.

Most modern zooms are varifocal in the consumer and mid-grade lenses since most people use autofocus rather than manual focus, which makes focusing at the set focal length more convenient. It's easier to design and less expensive to manufacture a high quality lens when you loosen the requirement on parfocal behavior. Higher end zoom lenses are more likely to be parfocal.

Parfocal zoom lenses are particularly important in video use when you might want to use the focal length change in the course of a take without losing your focus setting.

 

[hot]

My (very old - ~ 1978 ?) Vivitar 35-85 / 2.8 is one of the first SLR Variofocus lenses. It was calculated with NASA computers, because all Japanese computers were not strong enough.
You can see white (ft) and green (m) focus lines on lens. If you zoom, you have to move ring to left/right to get focus.

 

[ekoe]

A parfocal zoom lens must be in precise collimation otherwise it will not hold focus through the zoom.

 

[tribal-warrior]

The terms are "parfocal" and "varifocal".

In the manual focus era, most zooms were parfocal (or designed to be), manual focus shooters often used the zoom in, focus, zoom out technique to make focusing at short focal lengths with slow lenses easier.

Yes, this is my standard practise / procedure when shooting with video and cine film cameras. A varifocal lens design just wouldn't cut it for me because it makes sense to focus at the maximum tele end of the zoom where depth of field is shallowest - ensuring the highest accuracy of focusing. However, I guess I don't have anything to worry about anyway since it looks like proper focus was maintained when I tested the 14-45mm lens. Though of course this can be hard to verify when I zoom out considerably wider because depth of field naturally increases. But then it becomes a moot point anyway.

 

[ekoe]

The zoom-in/focus/zoom-out method is referred to as pulling 'critical focus' or 'critical focus method'.

 

[ekoe]

Here's an interesting article on the subject: http://poetzerofilm.com/2011/05/demystifying-tech-parfocal-and-varifocal/

Notice the author states in the last paragraph "poor maintenance, misalignment, and incorrect backfocus (among other problems) can cause a similar effect with parfocal lenses."

Months back, Godfrey disputed this fact when I addressed it in a thread regarding the adjustment of a focus ring, saying: 'A parfocal lens will hold a set focus point regardless of whether the mount is at perfect registration distance or not. Has NOTHING to do with collimation.' He then referred to my point as 'stupidity' and used his authority as moderator to close the thread. http://www.flickr.com/groups/1084614@N23/discuss/72157624881896511/?search=parfocal

That wasn't very cool.

 

[Audii-Dudii]

I wonder if the same can be said of the other Panny zooms made for the 4/3rds format.

Unfortunately, Yes, it can be said, as the otherwise wonderful 4/3-format PanaLeica 14-50/f2.8-3.5 zoom that was shipped with the DMC-L1 does shift focus slightly as it zooms ... or at least the pair that I have do.