Help needed: Sigma 18-35 ART not keeping focus

[bluefonia]

I just discovered that my copy of the Sigma 18-35mm F1.8 ART does not keep focus when zooming. Slowly zooming out from 35 to 18mm the lens gradually looses focus. The same happens the other way around, - focus at 18mm gets lost at 35mm. I can't remember the last time I have zoomed during recording, but anyways - I suppose a lens in this price range should not suffer from such an issue.

Has anybody else had the same problem? As I guess this not supposed to be so, how/where do I get the problem fixed?

[Jonesy Jones]

I have this lens and love it, but unfortunately it does not keep focus when zooming. It is not a parfocal lens. I wish it was, but it's not. There's nothing wrong with your copy. It's just the way most still zooms are designed. 

[bluefonia]

Wow, fast reply I have to say ;-).

Well okay, - nice to know nothing is wrong with my copy. I have never zoomed with this lens while filming. I just got curious when I did some testing up against some Zeiss zooms to night. Normally I use the focus zoom on my cameras monitor to check critical focus, but for some reason to night I just used the lens zoom for that and found out it didn't worked the way I thought it would.

Thanks

[BenEricson]

22 minutes ago, bluefonia said:

I just discovered that my copy of the Sigma 18-35mm F1.8 ART does not keep focus when zooming. Slowly zooming out from 35 to 18mm the lens gradually looses focus. The same happens the other way around, - focus at 18mm gets lost at 35mm. I can't remember the last time I have zoomed during recording, but anyways - I suppose a lens in this price range should not suffer from such an issue.

Has anybody else had the same problem? As I guess this not supposed to be so, how/where do I get the problem fixed?

Are you using the Black Magic Pocket or a camera with the metabones? if so, you need to make sure the infinity focus is set properly on the speed booster.

I have owned canon and Nikon versions, both held focus through the zoom range, however the Nikon version with speed booster flickered when zooming on any aperture but f1.8... The canon version was fine at any f stop. 

[bluefonia]

Yes, I use speed boosters (on Panasonic GH4) and I was aware of the focus issues these can cause at infinity, so I made the test without speedbooster.

Anyways, - as you can see from the post above - there is nothing wrong with the lens, it simply comes down to the way it is constructed.

[Brian Caldwell]

42 minutes ago, bluefonia said:

Yes, I use speed boosters (on Panasonic GH4) and I was aware of the focus issues these can cause at infinity, so I made the test without speedbooster.

Anyways, - as you can see from the post above - there is nothing wrong with the lens, it simply comes down to the way it is constructed.

The linear portion non-parfocal behavior can be fixed with a back focus adjustment.  This is easy to do with a Speed Booster.  With a plain adapter you'll have to luck out and be able to do it by shimming the flange instead of having to remove metal.  This will give the long and short focal length positions the same focal position.  What happens in-between is the non-linear portion, and is controlled by the cam that moves the compensating lens group inside the lens.  If you have a significant lack of parfocality in the in-between region the standard way of fixing it is to re-machine the compensator cam, but only super high-end cine lenses get this treatment.

[bluefonia]

@Brian Caldwell

Thanks, - in fact I'm already aware that Metabones SB¨s can be adjusted if needed. I have one in CY/MFT mount, but have had none of these backfocus issues so far. I just made the final zoom/focus test without SB to exclude all possible error sources.

[BenEricson]

1 hour ago, bluefonia said:

@Brian Caldwell

Thanks, - in fact I'm already aware that Metabones SB¨s can be adjusted if needed. I have one in CY/MFT mount, but have had none of these backfocus issues so far. I just made the final zoom/focus test without SB to exclude all possible error sources.

You sure all of your lenses properly focus to infinity? I adjusted for the sigma and everything was perfect after that, this includes fisheye lenses and non native zoom lenses with adapters.

[jonpais]

To OP: here's an article you might find interesting.

[bluefonia]

8 hours ago, BenEricson said:

You sure all of your lenses properly focus to infinity? I adjusted for the sigma and everything was perfect after that, this includes fisheye lenses and non native zoom lenses with adapters.

I´m pretty sure they focus to infinity, but I think I´ll double check all of them, - both with and without SB.

[bluefonia]

5 hours ago, jonpais said:

To OP: here's an article you might find interesting.

Thanks, the article added so mileage to my limited knowledge about zoom lenses.

Besides owing the Sigma, I recently got two Zeiss Vario Sonar zooms, - F3,5-5.6 and F4 respectively. The Sigma is constant F1.8. The two Zeiss zooms stays in focus aprox. 75-80% of the zoom range while the Sigma stays in focus less than 50%. Wonder if the quite big difference in F-stops have some importance on the matter too?

[Axel]

1 hour ago, bluefonia said:

Besides owing the Sigma, I recently got two Zeiss Vario Sonar zooms, - F3,5-5.6 and F4 respectively. The Sigma is constant F1.8. The two Zeiss zooms stays in focus aprox. 75-80% of the zoom range while the Sigma stays in focus less than 50%. Wonder if the quite big difference in F-stops have some importance on the matter too?

That goes without saying. All tests that proved the Sigma to be not parfocal had their motifs relatively close. Six feet distance @f1.8? But if your backfocus is correct, the distance is more than 15 feet and the aperture f3,5 and above it will look parfocal enough. The same is true for the breathing behavior. That is when you make a focus transition with open aperture @ 18mm to an object very close to the lens, it will be slightly zoomed in during focussing.