Should I worry about my camera or lens? at DVinfo.net

Brian Mills · November 12th, 2015, 10:35 AM

Hello All:

Recently I posted about how off I found the witness marks on my Rokinon lenses to be, and the consensus I found was that I shouldn't be too worried about it and shouldn't expect them to be 100% accurate.

But after failing to achieve infinity focus with another lens, I started to worry if my backfocus is off on my camera.

So I took out the Sony 28-135, put up a Siemens star at 8', zoomed in all the way and focused. When I went all the way wide and hit magnification, it was off quite a bit. So I focused wide and then zoomed in. Same thing, by the time I got to the other end of the zoom, focus needed a pretty significant adjustment.

Do the rest of you that have the 28-135 find that it is parfocal?

I don't know how I can test to see if the lens is not parfocal as advertised, or if it is backfocus on camera.

Any thoughts? Thank you.

Simon Denny · November 13th, 2015, 02:15 AM

Hi Brian,

I have the Sony 28-135 on the A7s and have not had any issues. The lens is Parfocal. I'm wondering what f-stop you where using and were you hand held or Tripod?
Can you attach some images as per problem?

Brian Mills · November 13th, 2015, 07:02 PM

That's good to hear that the lens should be parfocal.

I shot my test chart at a F4 at 8 feet on a tripod.

I was actually able to track down another FS7 owner in my city with same setup, so I am now going to test HIS lens on my camera, and my lens on his camera, to nail down where the problem lies. This should be Tuesday.

I will report back with my findings (in case anyone sees similar things in the future).

Brian Mills · November 17th, 2015, 07:16 PM

I don't know what on earth I was doing wrong before, but when I went over to another guy's studio to test my camera/lens combo, they worked fine. The lens is parfocal. I also tested his lens/camera and found his lens to be parfocal as well.

I'm going to go with: I had a gremlin in my camera that day.

That's my story and I am sticking to it!

November 12th, 2015, 10:35 AM	#1
Brian Mills Regular Crew Join Date: Sep 2006 Location: Las Vegas, NV Posts: 126	Should I worry about my camera or lens? Hello All: Recently I posted about how off I found the witness marks on my Rokinon lenses to be, and the consensus I found was that I shouldn't be too worried about it and shouldn't expect them to be 100% accurate. But after failing to achieve infinity focus with another lens, I started to worry if my backfocus is off on my camera. So I took out the Sony 28-135, put up a Siemens star at 8', zoomed in all the way and focused. When I went all the way wide and hit magnification, it was off quite a bit. So I focused wide and then zoomed in. Same thing, by the time I got to the other end of the zoom, focus needed a pretty significant adjustment. Do the rest of you that have the 28-135 find that it is parfocal? I don't know how I can test to see if the lens is not parfocal as advertised, or if it is backfocus on camera. Any thoughts? Thank you. __________________ Brian Mills, Videographer/Editor www.gordovideo.com

November 13th, 2015, 02:15 AM	#2
Simon Denny Trustee Join Date: Jul 2007 Location: Australia Posts: 1,409	Re: Should I worry about my camera or lens? Hi Brian, I have the Sony 28-135 on the A7s and have not had any issues. The lens is Parfocal. I'm wondering what f-stop you where using and were you hand held or Tripod? Can you attach some images as per problem?

November 13th, 2015, 07:02 PM	#3
Brian Mills Regular Crew Join Date: Sep 2006 Location: Las Vegas, NV Posts: 126	Re: Should I worry about my camera or lens? That's good to hear that the lens should be parfocal. I shot my test chart at a F4 at 8 feet on a tripod. I was actually able to track down another FS7 owner in my city with same setup, so I am now going to test HIS lens on my camera, and my lens on his camera, to nail down where the problem lies. This should be Tuesday. I will report back with my findings (in case anyone sees similar things in the future). __________________ Brian Mills, Videographer/Editor www.gordovideo.com

November 17th, 2015, 07:16 PM	#4
Brian Mills Regular Crew Join Date: Sep 2006 Location: Las Vegas, NV Posts: 126	Re: Should I worry about my camera or lens? I don't know what on earth I was doing wrong before, but when I went over to another guy's studio to test my camera/lens combo, they worked fine. The lens is parfocal. I also tested his lens/camera and found his lens to be parfocal as well. I'm going to go with: I had a gremlin in my camera that day. That's my story and I am sticking to it! __________________ Brian Mills, Videographer/Editor www.gordovideo.com