7 Replies Latest reply on Aug 10, 2010 7:00 AM by MadManChan2000

    How does ACR compute focus distance?

    markrgli

      Hello folks,

       

      In the recent versions of ACR, I've seen that it can include in the EXIF the focus distance from converted RAW files. I'm just wondering how it determines such information, particularly with Canon CR2 files. The focus distance seems to be accurate only on very long focal lengths (500mm and above). I have a 400mm lens and I noticed that I'm getting only any of 6.5m, 11.5m, or 76.6m as focus/subject distance.

       

      Can you guys provide me an idea as to how the the focus/subject distance is determined? For the type of photography that I do, this would be a very useful piece of information.

       

      Thank you very much.

       

      Mark

        • 1. Re: How does ACR compute focus distance?
          JimHess-DIrcbP Level 3

          I don't believe ACR "computes" focus distance.  That is a metadata field that is filled in with information in the camera.

          • 2. Re: How does ACR compute focus distance?
            markrgli Level 1

            Thanks, Jim. I'm inclined to believe in that too, especially for lenses that have a focus distance scale. If this is the case and no computations are done, then does it mean the camera is embedding incorrect values for this metadata field? In one occasion I took a shot of a subject at ~25m (I verified by checking the lens' focus distance scale after taking the shot), but after converting with ACR, I got 76.6m.

            • 3. Re: How does ACR compute focus distance?
              JimHess-DIrcbP Level 3

              I probably should not have responded to your question because I am by no means an expert in this area.  I know nothing about your camera.  On my Nikon D90I have a number of different focus points, and on any given image several of those points will have an impact.  Some of those focus points are on points of the image that are at a greater distance than the main subject.  Is there a possibility that this is what is happening in your situation?  If not, then I have no idea of what to suggest.

              • 4. Re: How does ACR compute focus distance?
                ssprengel Adobe Community Professional & MVP

                On my Canon DSLR I get somewhat incorrect values for the focus-distance, too.  It seems as though the camera only has a few, rather coarse numbers it stores for this but they are good enough for the lens profiles to work from.

                 

                If you want to see what your camera is putting in the RAW or JPG files, try EXIFtool:

                 

                http://www.sno.phy.queensu.ca/~phil/exiftool/

                 

                After you download and install it, you can drag-and-drop a RAW or JPG file to the version of the EXE with (-k) in its name and it'll give a list of all the information about the camera and photo it can see, which is usually many dozens of items.

                1 person found this helpful
                • 5. Re: How does ACR compute focus distance?
                  MadManChan2000 Adobe Employee

                  They are read from the image metadata, but please note the reported focus distance is a very rough approximation of the true subject-to-focal-plane distance.

                  1 person found this helpful
                  • 6. Re: How does ACR compute focus distance?
                    markrgli Level 1

                    Thanks, guys!

                     

                    @Jim:

                    I'm using a Canon 40D . Indeed some of the focus points are on areas which might be farther than the subject, but they were not activated (since I used only the center point AF). I believe embedded focus distance should be based only on the active focus point/s.

                     

                    @ssprengel:

                    Yes, it seems there is a predefined set of distances that are returned.

                     

                    Thanks for the link. I tried it and I'm still trying to figure out how the focus distance is determined based on the metadata (that was an awful lot!) retrieved. Of interest are the fields FocusDistanceUpper and FocusDistanceLower, but I couldn't find documentation about these fields in the web. The tool will be very useful. When I have the free time I can probably conduct some tests.

                     

                    @MadManChan2000

                    Does it mean the camera embeds a rather rough approximation of this information?

                    • 7. Re: How does ACR compute focus distance?
                      MadManChan2000 Adobe Employee

                      Yes, the approximate metadata is reported by the camera & lens.