Is the Sigma 18-35 par focal on your BMPCC?
 

I have a Nikon mount Sigma 18-35 zoom, it is parfocal on my Nikon D300 – in fact impressively so.

However when used on my BMPCC it is seriously NOT parfocal i.e. will not hold focus during a zoom. This lack of parfocality is present when used my Metabones Speed Booster or with a simple Nikon to MFT adapter.

I would appreciate hearing from anyone using this lens on a BMPCC. All I want to know is whether you have found it to be parfocal or not.

Re: Is the Sigma 18-35 par focal on your BMPCC?
 

Since no one has replied to you, I thought I might at least make a guess as to what is going on. I am thinking that for a lens to be parafocal, you have configure it based on an an exact distance from the sensor. and the elements of the lens are all made to work together. Fitted into closely filling Nikon mount on a Nikon camera, those tolerances are like kept. But when you add an adapter to move things to a EF sensor, I am guessing that the adapter is not giving you same exact distance, and thus the parafocal capability is thown out of whack. Also when you ad an element as the speed booster does, that will likely upset the delicate parafocal set up in the lens.

Just a guess.

Re: Is the Sigma 18-35 par focal on your BMPCC?
 

Thanks Chris,

I have a very fair idea of what is going on and I was wondering whether anyone else has the same problem. Since making this post I have checked the demonstration BMPCC at my dealer. Theirs is out too but by not as much as mine. Based on my observations of two cameras I suspect quality control issues.

When using either the Speed Booster or my optics free adapter my Sigma does not hold focus during a zoom. At full aperture this is quite noticeable on the BMPCC finder screen and more obvious on my computer.

At one stage I rebuilt a couple of 16 mm cameras. To accurately check the lens to film distance one really needs an autocollimator and I still have mine. With a short focal length lens (10mm say) I could readily detect errors of about 0.01 mm in flange depth measurements. Although, unfortunately one cannot use all the features of an autocollimator with digital sensors it still provides a bench top illuminated target that appears to a camera or an observer to be at infinity. When I attach my Sigma to my Nikon D300 and get the camera to focus on the infinity target the reading on the lens barrel is either right on the infinity mark or a line’s width away. This I find very impressive and it is true for all the marked focal lengths on the Sigma.

When I repeat the experiment using my adapters I get the following. Note, when all is well the lens should read infinity when focused on an object at infinity.

My results: -
• With the plain adapter (i.e.no optics) and Sigma set to 18 mm, infinity was at ~ 0.5m.
• With the plain adapter and Sigma set to 35 mm, infinity was at ~ 1.5m
• With the Metabones SB adapter and Sigma set to 18 mm, infinity was at ~ 1m
• With the Metabones SB adapter and Sigma set to 18 mm, infinity was at ~ 1.8m.
•
As there are very few graduations on the lens barrel resolution is poor, however the readings are well short of infinity and are consistent with a short lens to sensor distance.

Re: Is the Sigma 18-35 par focal on your BMPCC?
 

Since my last post here 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.