9 Replies Latest reply on Nov 28, 2011 11:08 AM by Noel Carboni

    Off-centre lens correction

    Yammer Level 4

      Please excuse me if I'm missing the point. I wanted to ask a question about Lens Correction in Camera Raw which has been bothering me, since I looked into customising LCP files.

       

      One of the parameters in the LCP file format is ImageXCenter (and ImageYCenter). This defaults to 0.5, which I assume means "halfway across the frame", i.e. dead-centre. Is this coordinate the basis for the centre of distortion, or vignette, or chromatic aberration, or all three adjustments?

       

      I appreciate that lenses are not perfect, and that imperfections can be off-centre. But, would such imperfections be identical through many copies of the same lens—that is, could (for example) all 50mm/1.4 lenses have the same X/Y centre coordinates?

       

      On one hand, it's possible that a particular lens's manufacturing process produces equally assymetrical results; on the other hand, the process could produce lenses which are roughly symmetrical, but with small variations on all sides. I don't know—I don't make lenses, and I'm not in the habit of measuring their defects.

       

      So, what's confusing me is—while I can understand the usefulness of centre coordinates in profiling your own lenses—is it wise for Adobe to supply lens profiles with off-centre coordinates to the general public?

        • 1. Re: Off-centre lens correction
          MadManChan2000 Adobe Employee

          Yes, these tags refer to the relative image position.  However, digital image center is not always the same as optical center.  They can be slightly different, not because of an off-centered lens, but simply because the active set of pixels (i.e., light-receiving pixels recorded in the image) is not always symmetric with the optical axis.

           

          It is also true that unit-to-unit variability exists. In general, the projected optical center should be exactly in the same place, but that comes down to a question of tolerances and precision.

          1 person found this helpful
          • 2. Re: Off-centre lens correction
            Yammer Level 4

            But does the "centre" of one lens, used to produce the "master" profile for distribution, necessarily have the same centre as thousands of others?

            • 3. Re: Off-centre lens correction
              MadManChan2000 Adobe Employee

              It is a question of precision. In general, not exactly. There's an article posted recently on Dpreview that explains this in more detail (manufacturing tolerances).

               

              Eric

              • 4. Re: Off-centre lens correction
                Noel Carboni Level 7

                So in real terms, if I have a specific lens for which the CA correction never quite gets all 4 corners just right, and assuming the problem is with the optical center being a bit off center in the image, could I conceivably manually tweak up a lens profile that would then do consistently better than the one I got out-of-box (or via download) then?

                 

                -Noel

                • 5. Re: Off-centre lens correction
                  MadManChan2000 Adobe Employee

                  Yes, possibly.  It depends on the nature of the problem.  Real lenses can also have asymmetry problems that can't be defined simply as an offset-from-center problem.  Zoom lenses can also have asymmetry problems that vary with focal length (e.g., the asymmetry shifts as you zoom in & out because of the physical movements in the lens barrel).

                   

                  Eric

                  • 6. Re: Off-centre lens correction
                    Yammer Level 4

                    MadManChan2000 wrote:

                     

                    It is a question of precision. In general, not exactly. There's an article posted recently on Dpreview that explains this in more detail (manufacturing tolerances).

                    As an engineer, I appreciate inaccuracy and manufacturing tolerances. What I'm unsure of in this case is why a supplied lens profile has an off-centre axis. Is the profile made with a single lens? If so, assuming the lens could easily be at an extreme of tolerances, could that not cause unneccesary inaccuracies at the other end of the scale?

                    Put another way, would it be wiser to supply a profile with a 0.5 image centre? Or is it safer to assume that all lenses of that type are slightly off-centre?

                    • 7. Re: Off-centre lens correction
                      MadManChan2000 Adobe Employee

                      As I explained earlier, the optical center of a lens does not necessarily correspond with the image center (even if a lens is perfectly centered about its own intended optical center).  This is systematic behavior (i.e., not unit-to-unit behavior).  In other words, you could compute the center value for many units of the same lens, average them together, and get something significantly different from 0.5.

                       

                      Eric

                      • 8. Re: Off-centre lens correction
                        Yammer Level 4

                        Right. Thanks for explaining it again, Eric. I understand what you mean now.

                         

                        I've noticed that one of my lenses seems to produce uneven amounts of CA error from left to right; I also noticed quite a significant centre offset in its profile, so I wondered if they were connected. I created a copy of the profile and changed the centre offsets to 0.5 as an experiment. It didn't help.

                         

                        Like I said, I'm an engineer, and I like to know how things work. It's an illness! ;-)

                        • 9. Re: Off-centre lens correction
                          Noel Carboni Level 7

                          Yammer P wrote:

                           

                          I like to know how things work. It's an illness! ;-)

                           

                          It's also getting to be more and more rare, so keep up the good work!

                           

                          -Noel