Saturday, 5 February 2011

Chromatic Aberration and lens correction

Chromatic Aberration (CA) is a type of lens distortion. It is caused by light of different colours being refracted differently by the glass lens elements.

This is one reason why lenses often consist of pairs of lenses grouped together: The two lens elements in the pair are made of different glass types, and have different optical properties. The aim of the construction is to neutralize the effect of chromatic aberrations, so that all visible colours focus in the same place.


Here is an example image illustrating CA. The image was taken with a Panasonic GH1, using the Olympus Zuiko 9-18mm f/4-5.6 wide angle zoom lens at 9mm f/4:


An enlargement of the lower left corner reveals the CA artifacts:


You'll see that there is red and green fringing off the high contrast areas, where the white and black paint meet. These artifacts are typically seen in the corner of the frame, while the centre of the image is generally free from them.

In the extreme corner, the artifacts take up about 2-3 pixels, which is a moderate effect.

CA artifacts are generally found in the corner, especially when using wide angle lenses.  The lack of CA artifacts indicate a high quality lens design.

Automatic lens correction

Panasonic Lumix G lenses are automatically corrected for CA artifacts by the camera. In the JPEG output files, the camera has adjusted the images with the intention to remove these distortions.

When using Panasonic Micro Four Thirds lenses on Olympus cameras, the CA corrections are not done with current camera models. Future cameras from Olympus may employ the same technology as Panasonic uses, and adjust for these effects.

We can still see the original exposures by opening the RAW image files in a converter which allows for not implementing the CA adjustments. One such RAW converter is UFRAW. By using this converter, we can compare the original exposure with the out of camera JPEG output, to see what kind of adjustments were done.

I am fully aware that there are RAW converters which will do the CA corrections as well.  So this is not a test of the RAW converter, but rather a look at the image before the CA correction, to see what kind of CA artifacts the lenses generate.

I have done these comparisons for three lenses. The images were taken with the Panasonic GH1 at ISO 100:

Lumix G 8mm f/3.5 fisheye

Here is the full image, take with a wide open aperture at f/3.5:


And the upper right corner at f/3.5 and f/5.6:


In this example, we can see that the fisheye lens is very sharp in the corner, even wide open at f/3.5.

In the extreme corner, there is some red and green fringing where white meets black.  Perhaps around 2-3 pixels of colour artifacts.  They are corrected well in the JPEG output, though.

Lumix G 14mm f/2.5 pancake

Here is the full image, take with a wide open aperture at f/2.5:


And the lower right corner at f/2.5 and f/5.6:


We see here that when using the lens wide open, there is some vignetting in the corner, and also, the sharpness is not optimal. This is quite common for any lens, really, and I wouldn't say it is a problem.

Looking at the CAs, it looks like there is about 1-2 pixels of red and green fringing in high contrast areas in the original RAW image. In the adjusted JPEG image, there is still some purple fringing.

Lumix G 20mm f/1.7 pancake

Here is the full image, wide open at f/1.7:


And the lower right corner at f/1.7 and f/5.6:


My comments here are mostly the same as for the Lumix G 14mm lens: There is vignetting, and there is dullness in the corner at f/1.7. Again, this is not uncommon, and especially so for a low light lens with a large aperture.

Before conversion to JPEG, there is some small amount of green and red fringing. In the converted JPEG image, the green fringing is gone, but there is still some purple fringing.

Olympus Zuiko Digital 50mm f/2 1:2 macro (Four Thirds lens)

This lens is rather well known for it's CA artifacts. Here is a video showing the lens being used on a Panasonic GH1 camera (with an adapter). When focusing manually close to the minimum focus distance, you can see clearly black text on white background is black only when in perfect focus. Focusing a bit longer gives a green outline. And focusing closer gives a red outline. Doubleclick on the video to get a larger view, up to 720p is possible.



Here is a 100% view of a photo taken using the Olympus 50mm f/2 macro at 1:2 magnification, and f/2 aperture. As you see, text which is beyond the focus point has green fringing, while text nearer has red fringing.




The Panasonic-Leica 45mm f/2.8 macro lens for Micro Four Thirds does not exhibit these kinds of artifacts.

Conclusions

When using Panasonic lenses on Panasonic cameras, some CA artifacts are corrected automatically. However, there are still some purple fringing left in the corners.  It is generally restricted to around one pixel width, which is not much.

Even before the CA correction, the CA artifacts are very moderate on the Lumix G 20mm f/1.7 and the Lumix G 14mm f/2.5 pancake lenses. The Lumix G 8mm f/3.5 fisheye lens has somewhat more CA effects in the extreme corner, but it is well corrected by software.

All in all, I think that CA artifacts are nothing to worry about with these lenses. Even when using the lenses on Olympus cameras, without in camera CA correction, this should not bother you much.

2 comments:

  1. I took some pictures with Olympus kit lens 14-42mm 3.5-5.6 IIR and the 40-150mm 4-5.6R using Oly E-PM2 camera and compared the results with Canon G12. I was shocked to discover that both Oly lenses show shockingly high corner chromatic aberration while G12 compact has almost none.
    Presumably Canon removes CA in software, while Oly does not.. Both jpg images Result: I am not sure if it makes sense to switch to micro 4/3 - the results with G12 are v. good.

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    1. Yes, among Olympus cameras, it is so far only the very expensive Olympus E-M1 which does the CA correction automatically.

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