FS7: Sigma 50-100mm DC Backfocus Changes with Zoom on Metabones EF Adapter

It's a bit of apples and oranges since the Angie is a 2.7x zoom and the sigma is 1.9x so right of the bat the angie is at 35% disadvantage but in general terms I agree with Matt. With lenses the last 10% of performance really means the "last " 80-90% of the effort ;-)
 
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I agree with everything optitek has said. The rest is all wrong.

When the back focus is carefully adjusted, most photographic lenses are parfocal. But native-mount stills lenses seldom give you any way to adjust the back focus. If you just stick the lens on the camera, the back focus is probably a bit off. If you care about parfocalness, you need to try lots of combinations of lenses, adapters, and cameras until you get lucky, or you need to find a way to adjust the back focus. Adapted lenses are helpful because you can shim the adapter or use a helicoid adapter. Screw-mount lenses are easy to shim too.

Parfocalness is just one aspect of lens performances, just like sharpness or distortion or flare resistance. Some are better than others. Some aren't very parfocal at their widest aperture settings. Stop down by a stop and most lenses are parfocal enough for 4k shooting. Like optitek said, there are exceptions. Most focus-by-wire lenses, including almost all Micro Four Thirds lenses, are only electronically parfocal, not mechanically. But most manual focus and gear-driven autofocus SLR lenses are parfocal.
 
I don't want to argue with jacek, but its simply not true that the vast majority of DSLR zoom lens are par focal. It's very possible that that used to be true, but it certainly isn't any longer perhaps because modern DSLRs have much more reliable auto focus than they used to, so most still photographers don't care anymore. Its only us video guys who care and we're a fraction of the market.
I used to have quite a few Nikon Zooms that were largely par focal , the 80-200, 28-70, 17-35. But the newer generation of those same lenses simply weren't and its certainly true for plenty of other zoom lenses. What is true is that its very hard to know whether a lens is truly not-par focal or if your camera or the specific lens is off tolerances. There are plenty of lists on the web however of par focal and varifocal zoom lenses. Those lists seem to be pretty good in my experience.
 
optitek said:
Most lenses I got to check on collimator and projector (probably few thousands in my 30 year career in the business) have been in fact parfocal. Unless they were broken.
The problem 90+% percent of the time is either back focus on lenses or flange depth on camera. That later includes wrong thickness on the adapters.
I'll post a video about it- we just shot it and need to tweak it a little to make it more accessible for general public....
That also seems to be the OP lens problem from his posts....
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Great, please let us know when your video is posted!

Does that that video include a demo of a parfocal Sigma 50-100mm?
 
I think there are two arguments here; (1) using lenses as is and (2) adapting them and making adjustments.

Philosophically, one may believe that many lenses are parfocal (with some adjustments), but another person may question if they are then considered parfocal (with having to make these adjustments).

If I buy a lens with a certain mount for a certain camera and it doesn't hold its focus throughout the zoom then it's not parfocal to me. What anyone else does with this lens on any other camera system to make it parfocal is irrelevant in my view, you know?
 
I guess the appropriate approach for a lens like the Sigma 50 to 100 millimeter, is to treat it as a variable Prime not a zoom lens.
 
Jacek, I would never doubt your expertise, My guess is that because of your position working with professional clients and equipment you just aren't coming in contact with the the more varifocal zooms out there these days.
I do agree with you that the culprit in many cases is the camera mount itself.
My experience with Nikon's on my F3 was that once I adjusted my back focus correctly all my old Nikon's were par focal but new ones weren't. on a collimater though I found my 17-35 held focus at the wide and telephoto end but drifted in the middle.

I am pretty curious about the Sigma 24-105. I have tried 2 of them on a few different Canon DSLR's and my FS7 with metabones adapter and they were wildly out , but some people on this forum have claimed their's were par focal. Poor quality control, operator error? I'd buy one i it was even close to par focal .
 
NorBro said:
I think there are two arguments here; (1) using lenses as is and (2) adapting them and making adjustments.

Philosophically, one may believe that many lenses are parfocal (with some adjustments), but another person may question if they are then considered parfocal (with having to make these adjustments).

If I buy a lens with a certain mount for a certain camera and it doesn't hold its focus throughout the zoom then it's not parfocal to me. What anyone else does with this lens on any other camera system to make it parfocal is irrelevant in my view, you know?
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I tend to agree with this in regards to still lenses or cinevised still lenses used on video cameras, which is what we're talking about.

As far as a lens maybe being parfocal(or "more parfocal") on one body or adapter and not another, I can see that(If I don't get my Canon CN-E's seated just right in the Optitek adapter for my F55 focus is way off, especially on wider glass). That's the way it is on our ENG lenses and why they all have an easily accessible and adjustable back-focus adjustment control. I can set one of my ENG zoom lenses up on my tape VariCam and then move it over to one of my P2 Vari's and it's WAY off and you have to set it again for that camera. Same mount and you'd think same flange distance, but it's not.

But I believe modern still lenses aren't designed and built to be parfocal. They don't have to be. Everyone is shooting with AF on still cams.
 
Liam Hall said:
I'll see if I can seek it out. It was let year when all the zoom lenses were being announced. Emphasis on the word 'truly'.
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If you can't find your source you should go back and delete your original post. I can't believe that anyone at Sony would say such stupid thing. All the Fujinon and Canon broadcast lenses I've used for 35 years have been flawlessly parfocal, not to mention the numerous cine lenses I currently own that are also parfocal. Nobody at Sony in authority would say that "no lenses are parfocal", as you claim. So let's see the source.
 
Doug Jensen said:
If you can't find your source you should go back and delete your original post. I can't believe that anyone at Sony would say such stupid thing. All the Fujinon and Canon broadcast lenses I've used for 35 years have been flawlessly parfocal, not to mention the numerous cine lenses I currently own that are also parfocal. Nobody at Sony in authority would say that "no lenses are parfocal", as you claim. So let's see the source.
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Oh, calm down Doug FFS. The article I recall reading was claiming no zoom lenses are 'truly' par focal. It's about acceptable tolerance. Also, what was acceptable in SD won't necessarily be acceptable in 4K. It's a semantic point and one I've read several times. Pretty sure the article was written when Sony released the 18-110 F4. Can't find it though.
 
Doug Jensen said:
I rest my case. Nothing but BS.
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FFS Doug, you're sounding like Donald Trump.

Even Wikipedia states:

Parfocal lens
A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but small enough to be considered insignificant.

This was the gist of the point being made - it might well have been on Twitter, who knows, who cares...

http://www.newsshooter.com/2016/11/...scribe-their-new-cine-zooms-as-near-parfocal/
 
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Liam Hall said:
FFS Doug, you're sounding like Donald Trump.
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Are you kidding??? You are the one who makes a ridiculous statement with no foundation in truth, and then when is questioned about the source says "well, its something I heard". That is classic Trump!
BTW, thanks for making this personal and calling me a name. That is also classic Trump. Yeah, put the other guy down when I ask for more info. Nicely done.

You attributed a dumb statement to Sony and I asked for your source. Simple as that. Everything else is just you getting defensive.
 
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