Lens for Video: Focusing and Focal Length change issues

I never noticed this phenomenon until I started to play around with DSLR Video. When focusing, it seems like the focal length actually changes somewhat as if it's zooming in/or out. I've read this is one of the major difference between Still Photography Lens vs. high end Cine Lens (lack of focal length drift). I've played around with 4 lenses but I plan to test it on all of my lenses and more. Here are the videos. Bear with me...these are quick videos of focus ring turning back and forth.

1. Canon 50mm f1.8: Pretty significant Focal length drift

http://www.youtube.com​/watch?v=V7EEpaGZlXs

2. Canon 80mm f1.8: Pretty significant focal length drift

http://www.youtube.com​/watch?v=q_v-LvlYN9k

3. Tamron 24-75mm f2.8: Pretty significant focal length drift.

http://www.youtube.com​/watch?v=cjWvyBvx0Wk

4. Canon 17-40mm f4: Quiet at last.

http://www.youtube.com​/watch?v=2U4nVcxsO4o

Of course, I would rather use lenses with minimal focal length drift while focusing. I'm curious how 24mm f2.8 and 50mm f1.4 behave. I'll post more videos as I test them.

Thanks,

FYI: Zoom lenses where the focus does not change with FL are termed "parfocal"; those where it does change are termed "varifocal".

Parfocal lenses were the norm when 135 format film ruled the Earth. With the advent of AF systems and a shift towards marketing SLRs predominantly to amateurs, they have gone somewhat out of vogue in favour of the cost reductions possible with varifocal designs.

xarqi wrote in post #12202425
FYI: Zoom lenses where the focus does not change with FL are termed "parfocal"; those where it does change are termed "varifocal".

Parfocal lenses were the norm when 135 format film ruled the Earth. With the advent of AF systems and a shift towards marketing SLRs predominantly to amateurs, they have gone somewhat out of vogue in favour of the cost reductions possible with varifocal designs.

But changing of FL was occuring with my prime lenses also..

My apologies. Many of the same sort of posts come up time and again, and I'm guilty of jumping to a hasty conclusion.

Indeed - this is also a well established phenomenon. FL does change with focus distance for most, if not all lenses. The nominal specification is that at infinity focus.

Also, with very wide maximum aperture lenses, the focus point can shift with changes in "aperture". This is commonly raised as an issue with the 50L.

Does it occur with Primo Primes??

thats normal for all lenses. although the insanely expensive cinema lenses dont show it nearly as much as what you have there. i have worked with cinema glass and i would guesstimate its like 1/4 as bad maybe less than what you see in the videos.

Wow I've never noticed that with any of my lenses before. I'm for sure not saying it doesn't do it but I've never noticed it. I don't really shoot video with my 24-70 so other than that the others, I believe..... don't do that?

I believe it's irrelevant in still photography as it only captures a split second of "still." It affects video..somewhat.

I'm going to try it on 24-70L, 16-35L, 50mm f1.4 FD, 70-200 f2.8IS, 100-400, and 100mm F2.8.

The search continues...as I'm trying to find a few "perfect" video glass... I'm going to venture out to some Nikkor lenses... if I ever decide to get an adapter for it.