Tuesday, 15 May 2012

Geometric distortion correction

Most of the Micro Four Thirds lenses need geometric distortion correction applied for the output images to become rectilinear. This is done totally seamlessly by the camera and software, both for JPEG and RAW images. So the user never notices that the image, as seen by the camera through the lens, is not rectilinear in the first place.

This is in contrast to older DSLR systems. In these systems, there is an optical viewfinder, in which the users sees exactly what the sensor sees, through the lens. With a DSLR system, the lens must be rectilinear, otherwise, the user will be appalled by the geometric distortion when using the camera.

Here is an illustration of two basic kinds of distortion: Pincushion distortion (left) and barrel distortion (right):


In reality, the geometric distortion might very well be more complicated than what is illustrated by these simple models.

I have previously tested the geometric distortion properties of some Micro Four Thirds lenses, and I found that virtually all lenses featured some distortion correction. Especially wide angle lenses, or the wide end of zoom lenses. Since this time, I have acquired some new lenses, and I wanted to test them in the same way.

Again, I have done the tests by taking a pictures of a tiled wall. The images look like this:


Since I am only interested in the geometric distortion, I have increased the contrast so that the images become monochrome. I also superimposed the corrected out of camera JPEG images (black) onto the original RAW uncorrected images (red).

I included the appropriate adjustment needed. The adjustment numbers in percent refer to the "Lens Distortion" filter in The Gimp, an image processing software. Of course, to become rectilinear, some lenses might require more complicated adjustment than the simple model given by the "Lens Distortion" filter. So these figures are just intended to be approximate relative indicators of the degree of distortion. A positive figure indicated barrel distortion, while a negative figure indicates pincushion distortion.

Here is a comparison of the uncorrected and corrected images for some lenses.

Panasonic Lumix 14mm f/2.5 pancake: -16%

There is a significant barrel distortion, which is corrected in the in-camera JPEG image. However, there is some residual barrel distortion even in the corrected image. I have noticed this previously. In fact, the lens is rectilinear (after correction) at long focus distances, but has some barrel distortion at close focus distances. In this example, we see the barrel distortion at close focus distance. This is not an uncommon behavior for lenses that feature internal focus.


Olympus M.ZD 45mm f/1.8: 0%

No distortion correction at all. It looks like there is a small amount of pincushion distortion, though, so perhaps there should have been some in-camera correction done by the camera.


Panasonic X PZ 45-175mm f/4-5.6 @ 45mm: 0%

No distortion correction at 45mm.


Panasonic X PZ 45-175mm f/4-5.6 @ 100mm: +5%

There is some pincushion distortion correction at 100mm, but not a lot.


Panasonic G 100-300mm f/4-5.6 @ 100mm: 0%

No geometric distortion correction at 100mm.


Summary

Including the results from my previous study, I can present a table with the relative distortion corrections of various lenses:

LensFocal lengthRelative distortion correction
Lumix G 20mm f/1.7 Pancake20mm-11%
Lumix G 14mm f/2.5 Pancake14mm-16%
Lumix G 14-42mm f/3.5-5.614mm-18%
Lumix G 14-42mm f/3.5-5.630mm0%
Lumix G 7-14mm f/47mm-17%
Lumix G HD 14-140mm f/4-5.814mm-17%
Lumix G HD 14-140mm f/4-5.830mm-4%
Lumix G 45-200mm f/4-5.645mm+1%
Lumix X PZ 45-175mm f/4-5.645mm0%
Lumix X PZ 45-175mm f/4-5.6100mm+5%
Lumix G 100-300mm f/4-5.6100mm0%
Olympus M.ZD 45mm f/1.845mm0%
Panasonic Leica Lumix DG Macro-Elmarit 45mm f/2.8 1:1 Macro45mm0%
Lumix 8mm f/3.5 fisheye8mm0%
Sigma 30mm f/2.88mm0%

Conclusion

We see that wide angle lenses and zooms typically feature barrel distortion (negative figure) in the wide end. On the other hand, longer lenses are often not corrected, or are corrected for a small amount of pincushion distortion (positive figure). The in-camera distortion correction is some times insufficient, for example we've seen that the Olympus M.ZD 45mm f/1.8 and Lumix G 14mm f/2.5 pancake lenses feature some geometric distortion in the short focus range, while behaving better at infinity focus.

There are some who have speculated that Leica-branded lenses are not subject to any software corrections. I have looked at the Panasonic Leica 45mm f/2.8 1:1 macro lens, and found no indications of software adjustments to the images. However, I still don't believe that statement. One of the first cameras for which the software corrections were widely discussed online, was the Panasonic Lumix LX3 high end pocked camera from 2008. And it does feature a Leica-branded lens, and quite clearly, there is a significant barrel distortion in the wide end of the zoom, which is corrected by software.

3 comments:

  1. I did some comparison shots between uncorrected and corrected distortion, and it's clearly done for the Panaleica 25mm as well.

    http://jg.cl.no/distortion-comparison/

    (Pictures are, from left to right: Converted with dcraw (no correction), converted with Lightroom (corrected) and JPEG out of (Olympus) camera.)

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    Replies
    1. That is quite interesting! I was guessing that with the relatively small size of the Panasonic-Leica 25mm f/1.4, it was probably employing in-camera distortion correction to keep the bulk of the lens down.

      The barrel distortion looks fairly modest, though. So the camera isn't throwing away a major volume of pixels when doing the transformation.

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    2. Yes, I don't mind modest pixel loss if it means I can have a 200g lens instead of a 510g (!) lens.

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