Near the start of my NEX-7 postings, a reader asked me if I saw purple corners when the camera is used with wide angle lenses. I replied that I had noticed the effect with the 24 mm Elmar. I’ve had a chance to do some more reading and some more experimentation, and here’s what I’ve found.
The effect is real. It is related to the angle of the focused light to the sensor. In the middle of the image, no matter what the focal length of the lens, that angle is 90°. As you move toward the edges of the image, and especially the corners, the angle becomes more acute. It’s well known in film cameras that this changing angle produces images that get darker towards the corners. Some manufacturers of short focal length view camera lenses have produced filters to place in front of the lens to cancel out this effect.
In digital cameras, not only does the image get darker toward the corners, there are color shifts, usually towards magenta. Leica, in their M9, uses both angled micro-lenses on the sensor and lens specific processing in the camera to correct this effect.
The purple corner effect gets worse as the nodal point of the lens gets closer to the sensor plane. This means the wide angle lenses are more affected than longer focal lengths, and that lenses made for rangefinder cameras are more affected than those made for SLR’s, which have designs that move the nodal point further away from the sensor plane to keep the rear elements from interfering with the swinging mirror. To a small extent (unless we’re talking about macro lenses), the effect is worse when the lenses focused on infinity, and appears to be very slightly worse when the lens is stopped down.
Here’s a picture taken with the NEX-7 and the Leica f/3.8 24 mm Elmar M ASPH. Both the lens falloff and the purple-corner effect are quite visible. For reasons that are unclear to me, the right side of the image is more affected than the left.
Here’s a similar picture taken with the NEX-7 and the 16mm Sony lens.
All else being equal, you would expect more light falloff toward the corners and the corners to be more tinted purple than with the 24 mm lens. This doesn’t happen. There is slight light falloff, and essentially no purple-corner effect. Sony must be doing some processing in the camera that affects even supposedly raw files. That’s nice for people who are happy with the quality of the Sony 16mm lens, but it doesn’t help those of us who are looking for lens quality commensurate with the NEX-7 sensor quality.
I investigated what can be done about fixing images like the above in post-processing. I picked a day when the sky was flat overcast and took some pictures of the sky with the NEX-7 and the Leica 24 mm Elmar.
Here’s an example of what happens when you place the sky on Zone V:
I made exposures at various apertures and various Zone placements, analyzing the results in Photoshop. The amount of light falloff toward the corners as measured in CIELab varied with Zone placement, but the color shift as measured by the a* and b*values was remarkably consistent. From the center to the upper right corner, where the worst color shifts occur, a* went from 0 to 10, and b* went from zero to -6.
That led me to a way to make corrections to real images: create a correction image in the CIELab color space to be subtracted from a CIELab version of the image to be corrected. While mathematically fairly pure, I doubted that I’d actually use such a correction image, since I don’t normally work in CIELab. Instead, I tried to come up with an approximation which would allow me to stay in ProPhoto RGB.
I took the Zone V image of the sky and applied a Gaussian blur with 100 pixel radius to get rid of any fine structure that might exist in the image. I then inverted it, and, by moving the black point slider in the Levels control, brought the black point up to the darkest value in the image. I saved that image for further use as the corrector image. The corrector image looks like this:
I tested the corrector image by pasting it is a layer over the original sky image, setting the blending mode to “Linear Dodge”, and adjusting the opacity until the corrected image was consistent across the whole screen.
I then tried the correction process with a real image and got this:
Too flat. I like some light falloff towards the edges of wide angle pictures. I added some:
The good news is the purple corners can be fixed in post processing. The bad news is it takes about a minute an image to apply the fix (this could be improved greatly by creating a Photoshop Action), and you have to leave Lightroom to do it. There may be other Photoshop blending modes that work better.
I couldn’t figure out how to make the corrections with Lightroom’s Camera Calibration tool, but there may be a way to do that. It’s also worth looking at DxO. I could have spent more effort on getting a picture of something that’s really flat white to serve as a basis for the correction process; that’s not as easy as it sounds. I’m suspicious of the accuracy of my quick and dirty sky picture, especially since it’s darker towards the top. It worked well enough, though.