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You are here: Home / The Last Word / NEX-7 — purple corner testing summary

NEX-7 — purple corner testing summary

January 5, 2012 JimK 5 Comments

Ignoring the brightness falloff towards the corners, which is easy to deal with in image editing and may prove to be esthetically appealing, the most important measure of the purple-corner effect is the change in chrominance (that portion of color that is unrelated to luminance). CIELab provides a formula for calculating chrominance:  (a^2 + b^2)^.5. Measuring the maximum chrominance error of the four corners of the test images will give us a pretty good idea of the level of the problem.

WC chrominance error
Sony 16mm f/2.8 E mount 5.0
Leica Tri-Elmar 16/18/21mm f/4 @ 16mm 6.0
Leica Tri-Elmar 16/18/21mm f/4 @ 18mm 6.0
Leica Tri-Elmar 16/18/21mm f/4 @ 21mm 5.0
Leica Super-Elmar 18mm f/3.8 ASPH 7.8
Leica Elmar 24mm f/3.8 ASPH 9.4
Leica Elmarit 24mm f/2.8 ASPH 5.1
Zeiss Biogon 35mm f/2 3.6
Leica Summilux ASPH 50mm f/1.4 2.2

So, if you think the Sony 16mm is adequately corrected for the purple corner effect, you’d definitely be happy with the Zeiss 35mm and the Summilux 50, which are better. The Leica Elmarit 24 mm is the same, and the Tri-Elmar is the same at 21mm and a bit worse at 18mm and 16mm. The 18mm and 24mm Elmars are clearly worse.

One problem with photographic measurements is that you can identify problems that you were happy ignoring. Some people, after they’ve see the numbers for the Sony 16mm, will look critically at pictures taken with it and want to fix the corners.

Another thing to consider is, although I’ve been calling it the purple corner effect, the errors aren’t always the same hue. Look at the results in the previous post; you can see hues from red to blue, and you can have different hues at different parts of the image.

 

The Last Word

← NEX-7 — Lens testing for purple corners NEX-7 — correcting images with purple corners →

Comments

  1. Rich says

    January 7, 2012 at 2:06 pm

    Great work on this! Just wondering what f-stops you used to determine the WC chrominance error figures.

    Reply
  2. Jim says

    January 7, 2012 at 8:38 pm

    Rich,

    Thanks for the support. And thanks for telling me about Cornerfix. It’s really useful.

    Short answer: f/8.

    Long answer: complete testing methodology is at http://blog.kasson.com/?p=1233

    As you’ll see in later posts, wide apertures make the purple corner effect worse.

    Jim

    Reply

Trackbacks

  1. UWA magenta cast on the NEX7? Even from Sony's own lens? says:
    January 31, 2012 at 10:08 am

    […] […]

    Reply
  2. NEX-7 and the “purple corners” « A Lens a Week Blog says:
    February 16, 2012 at 12:53 pm

    […] NEX-7 — purple corner testing summary […]

    Reply
  3. Sony NEX-7 and the use of legacy ultra wide lenses | PanoTwins says:
    April 10, 2012 at 10:40 am

    […] But there is still a problem when you use ultra wide lenses originally designed for the Leica-M camera system. Depending on the lens you get a strong coloured cast in the corners of your image. Meanwhile this problem is well described throughout the web. For example in an article of Luminous Landscape, the blog of Steve Huff or the blog of Jim Kasson. […]

    Reply

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