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Sigma 20mm T1.5 Collimation

Mark A. Jaeger

Mark A. Jaeger

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I received a new Sigma 20mm T1.5 PL lens from B&H today. I had to wait for ~2 months for backorder due to Covid to clear.

I put the Sigma on my Monstro and it looked pretty good in the monitor. As another 20mm I demo'd was not well collimated I decided to test this lens. I set up at 68 inches and focused with edge mode and visually. Both showed 13.5 feet on the lens body. I then set up at 10 feet and repeated the test. The lens focus distance indication was midway between 90 feet and infinity. The markings are basically worthless.

Yesterday I was shooting paint clouds in water. The clouds can't easily be focused so setting the CP.3 to distance was the answer. It worked well as the Zeiss is well collimated vs my Monstro.

I can have the lens collimated for a fee but that means shipping, and more messing around.

Is this normal for Sigma and/or other lenses? Does one need to buy from vendors like Duclos so that the lens will be useful when received?

Frankly, I thought the Sigma would be good-to-go out-of-the-box but it is NOT.
 
Sorry to hear about your lens. I have a set as well. Get your lens shimmed properly and it should drop at least roughly into place re marks (assuming your back focus on your camera is good)... There are few lens techs on this list who will give you more accurate advice but mine from experience is not to work of an infinity lens collimation as the sigma scales are generic and the cam is pretty radical in its expansion of far and contraction of near distances....

My thoughts are that Sigma Cine prime lenses are basically ‘incredible’ material value for money... cam based, fast, PL, cine housed Lenses start at 3+ x sigma money. I suspected they may be a Little light re QC and therefore I would probably only buy from a retailer with in-house lens techs (like Duclos)... I got mine for Cvp in the Uk and 2 Lenses were out but they sorted them out.

I don’t massivly love the look of them but they pretty amazing considering the price... push to get your lens sorted but 1st double check your camera..

Good luck re the 20mm...

Ps re the Zeiss what focal length is it... If it is a lot longer than the 20mm then it will be less sensitive to your camera being badly set up..
 
Mark - I originally thought the same thing with my set of Sigma PL primes on a Helium. I then adjusted the back focus on the sensor with the 20 mm focused on a target 36" away. The markings then were very close to accurate at varied distances not only for this lens but also for most of the others as well. My PL mount is a Revolva, but I imagine this could be an issue with any mount/lens combination. For Monstro you can do this in camera, if a Ranger then shimming would be required as I understand it. Respectfully suggested.
 
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It's not an absolute guarantee, because something could still be "off" between the lens and camera, but buying from a dealer that's not just a big box store, and honestly that's what B&H is, is totally worth it, especially on glass and cameras, and it doesn't cost you anymore. Abel Cine is my primary dealer and every lens I've bought from them was individually QC'd by a tech before it was shipped to me. Still lenses and still cams, I'll buy from B&H without a second thought. Cine and broadcast lenses and "real" cameras, I'm buying from my rep that I've known for a decade and a half and can text pretty much 24/7 if I have an issue.
 
Thanks for the replies above. I read them with interest.

The CP.3 that I was using is a 50mm. I will check it's collimation again but my first focus distance vs. lens collar indication tests were pretty close to spot on.
Can one safely infer that the Monstro back focus does not need revision if the unmodified Zeiss is "spot on"? (Making the Sigma the target for correction)

It would have been better to buy from Duclos but I thought B&H's purchasing power would be most effective at resolving the lengthy backorder predicted in April. There is some learning going on here. I did speak to Duclos today and they can do the work fairly quickly.

I will start the resolution with B&H. I hope it will not be a challenge to B&H Corporate Sales dedication to taking care of the customer. I have been doing business with them for almost 3 decades.
 
I wonder if Red does set the right back focus distance for the sensor when they sell/upgrade a Monstro with PL.
Or if they assume that the client will do it? My Monstro came with the locking screw not locking the sensor position (it was in the spare hole position)

Anybody knows it?
 
I didn't know how far my cameras' back-foci were until I put a Tokina 11-20 zoom on them. Dollars to donuts your real problem is with the camera, which a 50mm lens can easily hide but which a 20mm lens (or wider) will reveal.
 
Michael Tiemann said:
I didn't know how far my cameras' back-foci were until I put a Tokina 11-20 zoom on them. Dollars to donuts your real problem is with the camera, which a 50mm lens can easily hide but which a 20mm lens (or wider) will reveal.
Click to expand...

Yes, this is what I found which at first was confusing, with an 85mm the marks were correct, but with a 20 mm they were way off. After adjusting the back focus, which is really an amazing capability on the DSMC2 camera, the 20 (and 14) were correct but interestingly the 85 remained correct as well. Even the 135 was perfect. The trick is to use a wide lens up close to adjust the back focus, then the others fall into line.
 
Jon Dishler said:
Yes, this is what I found which at first was confusing, with an 85mm the marks were correct, but with a 20 mm they were way off. After adjusting the back focus, which is really an amazing capability on the DSMC2 camera, the 20 (and 14) were correct but interestingly the 85 remained correct as well. Even the 135 was perfect. The trick is to use a wide lens up close to adjust the back focus, then the others fall into line.
Click to expand...


Back focus reacts exaclty the reverse as "front" focus.

You get critical focus with a long lens in "front" focus but it's less critical with shorter lens, and back focus is critical with short lenses and less critical with long lenses. It's due how the light rays are parallel or not in the entrance pupil to the subject or the exit pupil to the sensor.
 
Well...after a bit of lab work (in the garage) I decided that I owe Michael T a donut or two or maybe some dollars.

He was right that the back focus was off. After 3 adjustments I found the Zeiss CP.3 50mm and the Sigma 20mm were both reasonably well correlated/collimated at 36 and 60 inches and at infinity. At 96 inches the error was a bit larger but I don't expect that to be a problem. I do not know if the lock screw being in the storage location had anything to do with the need for adjustment but I put the screw into lock at the conclusion of my fiddling.

I did not check my Nikon glass as I almost never focus them using the relatively coarse distance markings.

Thanks to all.
 
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