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What EXIF data is needed to make an accurate lens profile?

May 10, 2012 9:00 AM

Tags: #profile #tags #format #manual #creation #exif #lens #medium

Hi,

 

I've been working on making lens profiles for some manual lenses with medium format cameras and have been wondering what EXIF data is needed by ALPC.  I believe I may be missing some necessary information, as the vignette model is definitely not accurate when graphed, which also makes me question the accuracy of the distortion model (seems to be ok).  What are all of the EXIF tags used by ALPC to create the distortion and vignette models?

 

Thanks!

 
Replies
  • Currently Being Moderated
    May 10, 2012 10:40 AM   in reply to jasonoats

    ALPC would be looking at camera make, model, lens model, lens ID, sensor format factor (crop factor) to auto-match the lens profiles. For sub-profile matching, ALPC looks at FocalLength, ApertureValue and focus distance info.

     

    Some these are in the standard EXIF tags; while others are parsed from the EXIF private MakerNote.  You can open one of those lens profiles in a text editor to see what is inside the file.

     

    -Simon

     
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    May 11, 2012 2:30 PM   in reply to jasonoats

    Hi Jason,

     

    At creation time, ALPC also need the camera make, model and lens model, lens ID to group chart images into sets. These info are also propagated into the resulting lens profile.

     

    As far as impacting the resulting lens profile data, the FocalLength, FNumber are required. The actual SubjectDistance is estimated by the APLC. So that is optional. You need FocalLengthin35mmFormat or (FocalPlaneXResolution,FocalPlaneYResolution,FocalPlaneResolutionUnit ), but you don't need both.

     

    Hope this is helpful,

     

    -Simon

     
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    May 30, 2012 12:52 PM   in reply to jasonoats

    The v.1 of the Adobe Vignetting Model does have known limitation for a combination of large format sensor and wide angle lenses. A later update to the vignetting model (included in LR4/ACR7) supports a much general model that is more accurate for these cases. But we currently have no upadate to ALPC that generates the general model from image based calibrations. The new model is only utilized for lens profiles created from the lens design data.

     

    The other thing that you could try when you calibrate your lenses. Try using a checker chart with smaller squares and frame the checker board image as close to the image frame as possible when you do the shooting (but still not get clipped). This will allow APLC get a better read of the actual light falloff at the edge, intead of just trying to extrapolate based on the data available only at the image center.

     

    Hope this helps,

     

    -Simon

     
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    May 31, 2012 7:57 AM   in reply to simonsaith

    jasonoats:  How are you calculating the vignette curve?  One option is to optimize for the edges (e.g., constrain the optimization to match the corner falloff exactly) so that you don't have under/overcorrection at the farthest corner. 

     

    The other thing to consider during the application of the profile is the appropriate sensor size (you touched on this issue).  ACR/LR needs to know the pixel geometry, i.e., the size of a pixel, so that it can translate physical measurements (e.g., in the lens profile) to pixel measurements for the purposes of applying the correction. 

     
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