Sigma 18-35 not parfocal on my BMPCC

I recently purchased a Sigma 18-35 mm zoom and a Metabones Speed booster. When the lens is used on my BMPCC either with or without the Metabones it does not hold focus during a zoom, i.e the combination is not parfocal.

If it was a film camera one would simply temporally replace the film with a hard flat plate and measure the flange/film distance with a depth gauge. As this is not an option I have used a different approach to find the problem.

I have a portable illuminated test target (autocollimator) that produces an image that appears to be at infinity to an observer or to a camera. It can be used to check infinity settings on camera lenses. When I fit the 18-35 to my Nikon I find that this combination holds focus during a zoom and also that the infinity graduation on the lens is accurate. When I test other Nikon lenses on my Nikon I get similar results so I now trust the Nikon and Sigma both as a combination and individually.

Having eliminated the Sigma as the cause of my problem it means that my problem is either with the Metabones or Nikon /MFT adapter or the BMPCC. To use the Sigma without the Metabones I use a Nikon to MFT adapter. When I use the Sigma with this Nikon/MFT adapter I also find that zooms are not parfocal. To achieve focus on my infinity target I have to focus the Sigma at less than infinity which means that the lens is not far enough from the sensor for either the Metabones/BMPCC combination or the Nikon MFT adaptor/BMPCC combination. The adapters are made by different manufacturers and I would be surprised if they both had similar manufacturing errors which leads me to suspect the BMPCC.

As both the BMPCC and Sigma are both under warranty I do not intend to fiddle with either. However my Nikon/MFT adapter was cheap and furthermore it is easy to remove the Nikon mount and pack it out with shims so that the lens is further from the sensor. I have now tried three thicknesses of packing. My calculations suggested that the flange sensor distance was short by 0.4 – 0.6 mm (given the paucity of lens graduations it was hard to come up with a better figure). In fact even packing out by 0.9 mm was not quite enough. This is a massive error and suggests to me that a shim has been omitted in the manufacture of my BMPCC.

The obvious location for a shim would be between the BMPCC lens mount and body. Could someone tell me if there is a shim in this location – it should be obvious as it would be ~ 1 mm thick?

Who told you the Sigma 18-35 was parfocal?

It is not parfocal.

Jules

Hi Jules,

My 18-35 Sigma has a Nikon mount and it is par focal when I use it on my Nikon D300.

If you set focus on the long end (35MM) you can hold focus while zooming in and out as long as you like. If you set focus at the wide or at an in between point, then the focus will pop out when you zoom. It's by definition a varifocal lens but you can usually cheat it.

Also, even if you set focus off the long end, if the lens is stopped down you can usually get enough dof to hold focus. It doesent pop out of focus nearly as much as other zooms

I'm using the ef bmcc. I don't know if thats making any difference.

I should add to my comment above that when I add 0.9 mm of shims to effectively increase the flange depth of my BMPCC that the combination is then much closer to being par focal.

Focal depth decreases when you zoom in to 35 mm, like with all lenses. Thus it is much easier to maintain focus when you focus at 35 mm and then go back to 18 - yet still this is no parfocal length, even though it does a good job when treated right.

Since my last post I have contacted both Metabones and the technician at the local BMD agency. The technician thought that as the Sigma was not on the recommended lens list for the BMPCC I would not get much sympathy. Metabones suggested making adjustments to the Speed Booster and provided a link to the relevant instructions. The adjustments worked and now my Sigma/Metabones /BMPCC combination is parfocal.

However I still do not know whether it was the Metabones or the BMPCC that was out of adjustment or perhaps both were out. To decide that I need access to an accurately collimated MFT lens. Unfortunately the local BMD agency does not have one.

Metabones suggested making adjustments to the Speed Booster and provided a link to the relevant instructions.

Would you share this information?

Hi Mac,
The link was: -
http://www.metabones.com/article/of/inf ... oster-only
My version of the Metabones is the N/F BMPCC Speed Booster.

The instructions wisely recommend marking the position of the elements as supplied before trying any adjustments. I made what I thought was a suitable mark and loosened the lock screw. Unfortunately my mark was made on a collar that became loose as the screw was undone - so my reference mark was useless. So beware. Also I would recommend just undoing the screw the minimum amount to permit moving the elements.

The portable ‘infinity’ target that my autocollimator provides was very valuable for this adjustment. I could adjust its light intensity to a level that permitted use of the maximum aperture on the Sigma. As I can use it a range of an inch or two there were no problems with atmospheric conditions. To judge whether the adjustment was at infinity I used the BMPCC green focus assist. I was impressed with its sensitivity.

Good luck.

Thank you for the link, and especially for sharing your valuable experience!

It's not parfocal but it can be close depending on the situation


Sent from my iPhone using Tapatalk

I too have the Sigma 18-35 lens on my BMPCC and the Metabones speed booster adapter for Nikon to BMPCC. I also noticed that it did not hold focus if I focused at 35 and then widened out to 18. I adjusted the focus adjustment on the Metabones adapter by loosening the screw. I got it adjusted pretty quickly by using a simple focus chart. The adjustment collar on the metabones is made of plastic and in my opinion is not suitable for use in a professional lens. Becareful with it and don't adjust the screw too tight. Also mark an alignment point on the collar and the housing in case it moves out of adjustment.

I too have the Sigma 18-35 lens on my BMPCC and the Metabones speed booster adapter for Nikon to BMPCC. I also noticed that it did not hold focus if I focused at 35 and then widened out to 18. I adjusted the focus adjustment on the Metabones adapter by loosening the screw. I got it adjusted pretty quickly by using a simple focus chart. The adjustment collar on the metabones is made of plastic and in my opinion is not suitable for use in a professional lens. Becareful with it and don't adjust the screw too tight. Also mark an alignment point on the collar and the housing in case it moves out of adjustment.

Can I ask what degree of adjustment you guys made? I'm trying to adjust mine but I can't find a particular adjustment that gives me the desired result?

Very helpful. Thanks!

Hi Cryo,

As mentioned in an earlier post I made a bit of a mess when I adjusted my Metabones SB. Advice from Metabones suggested I would need less than 1 turn of the lens block. I think mine took about a quarter of a turn. I was rather disturbed to find just how sloppy the adjustment thread on the lens block was and I have contacted Metabones about the issue. I would be interested to hear if anyone else thinks the adjustment is sloppy.

I had the luxury of using my autocollimator as an infinity target. I could set the brightness and test my adjustments at full aperture and hence minimum depth of field. An infinity target also gives you gives you the infinity setting on the Sigma to work to. Note that the adjustment is more sensitive at the 18mm end of the range because of the reduced depth of focus.

Alastair,

I have a couple of questions about your methodology:

1. What infinity mark on the Sigma are you using? Are you going all the way to the stop, or are you going to where the L-shaped indicator matches the witness mark?

2. Having adjusted the SB to compensate for your 0.9mm (!!) back focus error, have you tried another lens, preferably one with a hard stop at infinity, like a manual Nikkor? Can you focus such a lens at infinity?

I have no doubt you understand the principles at work here, but I suspect there are some glaring errors being made and I'd hate to see others led astray.

BTW, even under the best conditions the Sigma 18-35 is parfocal only to certain tolerances. If you want critical focus like a landscape stills photographer using a high megapixel sensor, no way can it be adjusted to be parfocal. If you are shooting HD video within a limited range of focus distance or zoom, you might find it adequate. Like someone said above, it depends on your eye.

Hi Steve,

A few points.

My Sigma 18-35mm has a Nikon mount and when used on my Nikon with an infinity target, focus is achieved when the moving and fixed marks form a straight line. The lens can be focused past infinity.

As the Nikon’s autofocus produces more reproducible results than my eye-sight I use autofocus.

When using the BMPCC I use the green focus assist for the same reason.

After adjusting the SB so that both 18 and 35mm focal lengths were ‘on the line’ I checked the other marked focal lengths. They were either on the line or less than a line’s width off. I found that impressive.

I have checked my Nikon prime lenses and all are acceptable. All focus to infinity. The worst was my 20 mm f4. It was also the oldest and shortest of my collection.

I also have a Nikon 28 – 300 mm zoom. It does not hold focus at different focal lengths as well as the Sigma but this is not surprising given the wide range of focal lengths. Its results are acceptable for stills but not for any video camera that I own.

The 0.9 mm error was nothing to do with the Metabones but was in a low-cost (unbranded) Nikon G – MFT adapter. I have since modified this adapter by making it deliberately too long and then shortening it by 0.0005” steps, retesting after each cut until I got the length right.

Alastair,

When shooting stills on your Nikon D300, using the Sigma or the other zoom, you have AF-ON. Is that correct? If so, the AF is simply functioning correctly to keep the image focused. This has nothing to do with the lens being par focal.

Hi Steve,

I prefer to use autofocus when possible as it tends to give more reproducible results than I do and I like the BMPCC focus assist for the same reason. I would agree that autofocus could give you the impression that your zoom lens is parfocal when it is not.

I have stressed that I did my adjustment using my autocollimator as an infinity target. On my BMPCC I can either adjust the focus manually with or with the BMPCC focus assist. In the case of my Nikon I can focus or with or without the autofocus. Having achieved focus on my infinity target by whatever means I can then check the alignment of the witness mark with the infinity mark on the lens under test. I can then repeat the exercise at another focal length and check the witness mark alignment again and so on through the range of focal lengths. When I do this with my Sigma 18-35 -( after adjustment of my Metabones SB) the maximum variation through the zoom range was about a line’s width. I am still impressed.

It was a very different story with the Metabones as supplied and if there is a message to be gained from this thread it is that if your Sigma 18-35 / camera / adapter combination is not parfocal it can probably be made to be.

I needed to adjust my Metabones SB too.

I got the same issue too with my original Metabones Flange Adapter. Doh. With my Speedbooster (i adjusted it for Tokina 11-16) everything is "near to fine". But the amount of defocus on the Metabones adapter is HUGE. VERY HUGE.

I could loose the screws on the adapter and add some shim there - has anyone experience with that?

Any other Idea? (dont want do buy a second Nikon SB - need and EF for the 24 - 105 first )

Rick

Alastair,

I have a couple of questions about your methodology:

1. What infinity mark on the Sigma are you using? Are you going all the way to the stop, or are you going to where the L-shaped indicator matches the witness mark?

2. Having adjusted the SB to compensate for your 0.9mm (!!) back focus error, have you tried another lens, preferably one with a hard stop at infinity, like a manual Nikkor? Can you focus such a lens at infinity?

I have no doubt you understand the principles at work here, but I suspect there are some glaring errors being made and I'd hate to see others led astray.

BTW, even under the best conditions the Sigma 18-35 is parfocal only to certain tolerances. If you want critical focus like a landscape stills photographer using a high megapixel sensor, no way can it be adjusted to be parfocal. If you are shooting HD video within a limited range of focus distance or zoom, you might find it adequate. Like someone said above, it depends on your eye.

I used the Sigma 18-35 on numerous shoots and it held infinity perfectly. I had to spend about an hour adjusting it perfectly… Set it to 1.8, at the widest setting, and take the adapter on and off until infinity is truly infinity. I had no problems with any other lenses after the adjustment. It works with the 8-15 EF fisheye, etc.