the last word

Photography meets digital computer technology. Photography wins -- most of the time.

  • site home
  • blog home
  • galleries
  • contact
  • underwater
  • the bleeding edge
You are here: Home / The Last Word / NEX-7 — correcting images with purple corners

NEX-7 — correcting images with purple corners

January 6, 2012 By JimK Leave a Comment

I have been using Cornerfix to correct NEX-7 images that have discolored edges and corners. It does the job, and it does it well. It supports batch processing, which is useful. Originally written to correct deficiencies in Leica M8 images, it works with any camera for which a Adobe DNG file can be generated.

Here’s how to use it. Make correction images of flat white or gray walls as described in the previous posts. Cornerfix suggests that you overexpose these images be two or three stops, but I’ve gotten equivalent results by making the correction images with Zone V centers, which I use for my quick and dirty Photoshop corrections, and using the Lightroom “exposure” control to move the histogram to the right so that it is barely in the no-blocked-highlights range. That way I can use the same correction images for both methods.

Once you’ve got the correction image, export it from Lightroom or your favorite raw converter as a DNG file. Fire up Cornerfix, open the correction image, and tell it to create a profile. Then open any image made with the same lens/camera combination, and apply the profile to get the corrected image.

As with my homegrown correction technique, I’ve found that I don’t want Cornerfix to remove all the corner darkening for any but stitched work. I have a great solution.

Lie to Cornerfix.

Here’s how. Before you save the correction image, apply a set of curves to it that lightens the corners through a circular gradient. That’s right, lightens. Cornerfix will think that the corners aren’t as dark as they are, and won’t lighten them so much. You will have nice controllable darkening towards the corners, instead of the sudden rapid falloff of most wide angle lenses. Any advantage of this technique is that you won’t have as much noise in the corners due to Cornerfix’s lightening the heck out of them.

A picture is worth a thousand words:

Uncorrected (all with the Leica 24mm Elmar, the worst lens for purple corners of all the ones I tested):

Corrected completely:

Modified correction:

 

← NEX-7 — purple corner testing summary NEX-7 — correcting Sony 16mm images →

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

March 2021
S M T W T F S
 123456
78910111213
14151617181920
21222324252627
28293031  
« Feb    

Articles

  • About
    • Patents and papers about color
    • Who am I?
  • How to…
    • Backing up photographic images
    • How to change email providers
  • Lens screening testing
    • Equipment and Software
    • Examples
      • Bad and OK 200-600 at 600
      • Excellent 180-400 zoom
      • Fair 14-30mm zoom
      • Good 100-200 mm MF zoom
      • Good 100-400 zoom
      • Good 100mm lens on P1 P45+
      • Good 120mm MF lens
      • Good 18mm FF lens
      • Good 24-105 mm FF lens
      • Good 24-70 FF zoom
      • Good 35 mm FF lens
      • Good 60 mm lens on IQ3-100
      • Good 63 mm MF lens
      • Good 65 mm FF lens
      • Good 85 mm FF lens
      • Good and bad 25mm FF lenses
      • Good zoom at 24 mm
      • Marginal 18mm lens
      • Marginal 35mm FF lens
      • Mildly problematic 55 mm FF lens
      • OK 16-35mm zoom
      • OK 60mm lens on P1 P45+
      • OK Sony 600mm f/4
      • Pretty good 16-35 FF zoom
      • Pretty good 90mm FF lens
      • Problematic 400 mm FF lens
      • Tilted 20 mm f/1.8 FF lens
      • Tilted 30 mm MF lens
      • Tilted 50 mm FF lens
      • Two 15mm FF lenses
    • Found a problem – now what?
    • Goals for this test
    • Minimum target distances
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Printable Siemens Star targets
    • Target size on sensor
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Test instructions — postproduction
    • Test instructions — reading the images
    • Test instructions – capture
    • Theory of the test
    • What’s wrong with conventional lens screening?
  • Previsualization heresy
  • Privacy Policy
  • Recommended photographic web sites
  • Using in-camera histograms for ETTR
    • Acknowledgments
    • Why ETTR?
    • Normal in-camera histograms
    • Image processing for in-camera histograms
    • Making the in-camera histogram closely represent the raw histogram
    • Shortcuts to UniWB
    • Preparing for monitor-based UniWB
    • A one-step UniWB procedure
    • The math behind the one-step method
    • Iteration using Newton’s Method

Category List

Recent Comments

  • JimK on Zeiss Batis 135 on Nikon Z7
  • Maurin on Zeiss Batis 135 on Nikon Z7
  • Scott Pilla on GFX Natural Live View and raw file histograms
  • Macro Guy on THoS: a NYT infinite loop
  • JimK on Sony 135 mm STF on GFX 50R
  • Alexander Häggström on Sony 135 mm STF on GFX 50R
  • Mike King on Metabones 1.26x Expander on GFX 100 with Otus 55
  • JimK on Diffraction and sensors
  • Barry Benowitz on Diffraction and sensors
  • Raymond on Fuji 45-100/4, 100-200/5.6 on GFX 100

Archives

Unless otherwise noted, all images copyright Jim Kasson.