Tuesday, 1 January 2013

Geometric distortion correction

Micro Four Thirds lenses tend to be smaller than lenses from other systems. There are several reasons for this.

One is of course that the sensor is smaller than most other system cameras, with the exception of the Nikon 1 system, which has an even smaller sensor. Another reason is that the register distance is shorter than for DSLR systems.

But one major reason is that the system relies on software correction of the sensor output. This includes correction of Chromatic Aberration (CA) artefacts and vignetting.

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.

In this article, I look at another type of correction employed in two of the newer lenses, geometric distortion correction. By looking at the RAW file in a third party program, I can extract the uncorrected image, and compare it with the out of camera (OOC) JPEGs. For a good reference, I photographed a wall with square tiles:


After processing the RAW image to find the true geometry of the underlying image, I superimposed the two, using red lines for the uncorrected geometry, see below.

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.

Lumix X 12-35mm f/2.8


This lens features some barrel distortion in the wide end, and no distortion correction in the long end. However, even after the in-camera distortion correction, there is some residual barrel distortion in the wide end. This is the most pronounced at close focus distance, but can also be seen with infinity focus, in my experience. It is not uncommon that the geometric properties change slightly with focus for internal focus lens designs.

See the review of the lens for a real life example illustrating the barrel distortion in the wide end.

Also in the long end, the out of camera JPEG images show some distortion. They tend to have a bit of pincushion distortion.


Lumix X 12-35mm f/2.8 @ 12mm: -14%



Lumix X 12-35mm f/2.8 @ 35mm: 0%



Lumix X PZ 14-42mm f/3.5-5.6


It is perhaps unexpected to see that this lens has less barrel distortion correction in the wide end than the basic version of the lens, the Lumix G 14-42mm f/3.5-5.6 kit zoom lens. Given the smaller size of the power zoom version, one would expect that more optical compromises have been made.

Just like with the Lumix X 12-35mm f/2.8 lens, there is some residual barrel distortion even in the corrected image at 14mm. There is also some residual pincushion distortion in the long end.


Lumix X PZ 14-42mm f/3.5-5.6 @ 14mm: -15%



Lumix X PZ 14-42mm f/3.5-5.6 @ 42mm: +5%




Conclusion


I'm a bit disappointed to see that there is noticeable barrel and pincushion distortion in the images produced by the premium Lumix X 12-35mm f/2.8 zoom lens. For such an expensive lens, one would have expected more perfect images.

On the other hand, this is not really any real problem. If you need to have absolutely rectilinear images, then you can apply a bit of lens distortion post processing. Also, I guess this comes down to a compromise between having a traditional focus mechanism, which is slower, but more resistant to distortion at closer focus distances, and internal focus, which is much faster. The Lumix G 20mm f/1.7 pancake lens has the traditional focus mechanism, and quite some people dislike it for the slow and noisy autofocus.

Summary of other lenses


And here is a summary of the adjustments to all the lenses I have tested. The percentage in the table refers to the Gimp image processing Lens Distortion filter value needed to make a rectilinear image: 0% means no correction, a negative value means barrel distortion, and a positive value means pincushion distortion.

LensFocal lengthRelative distortion correction
Leica DG Summilux 25mm f/1.425mm-8%
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 X PZ 14-42mm f/3.5-5.614mm-15%
Lumix X PZ 14-42mm f/3.5-5.642mm+5%
Lumix X 12-35mm f/2.812mm-14%
Lumix X 12-35mm f/2.835mm0%
Lumix G 7-14mm f/47mm-17%
Lumix G HD 14-140mm f/3.5-5.6 II14mm-16%
Lumix G HD 14-140mm f/3.5-5.6 II50mm0%
Lumix G HD 14-140mm f/4-5.814mm-17%
Lumix G HD 14-140mm f/4-5.830mm-4%
Lumix G HD 14-140mm f/4-5.850mm-1%
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/2.830mm0%

3 comments:

  1. You forgot to tell there's no free lunch in optics.
    All that mutilating and stretching of pixels to sweep uncorrected geometry under the rug in software guarantees that image outside center area will never have the highest resolution.
    And when that's combined to further optical undersizing we'll get still expensive lenses with high resolution only in center.

    That's why optically very demanding range Zuiko 12-60mm beats both m.Zuiko 12mm and Lumix X 12-35mm. Even extremely demanding range Leica D 14-150mm has less resolution drop in edges/corners at its wide end.

    And geometric distortion is determined by different optical design of different focal length lenses. And naturally primes are easier for keeping aberrations controlled because of less variance in path of light rays at various "settings" of the lens.

    Here's good article explaining also causes of different geometric distortions:
    http://www.lensrentals.com/blog/2011/09/lens-genealogy-part-2

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  2. It is one thing to get an idea of distortion in a couple of lenses at particular focal lengths and quite another to get a sense across a wide range of lenses for m43. Thank you for all that research. I'd read, here or elsewhere, that pancake design was in part made possible by heavy in camera correction. That seems to be the case! That said, I don't own any of the m43 pancakes but immediately bought the 40mm Canon for my 600D when it came out and love it because it makes the camera much more portable. The 40mm ALLOWS the Canon to compete with my EPL-1 as the camera I actually take with me because it makes it carryable. With any other lens it becomes a chore to take on speculation that I might get a chance to shoot. I don't know what Canon did to make the 40mm such good value but given that they cant hide distortion in the optical viewfinder they have done a nice job bringing this improbable form factor out at a street price of about $150. Although you didn't test the 14-42 kit lens that comes with my EPL-1, I would expect it to liberally use incamera correction. With its plastic mount and kit status I had low expectations of it, but as I began to examine my images I realized it was way better than I had any right to expect it to be. Its light weight and collapsable nature make it easy to always have with me. I team it with the 7.5mm Samyang I learned about here and am about to add the very light weight 40-140 plastic mount Olympus from Amazon. My son has it in Florida and says is producing nice images on his GH2. For me the appeal is that it is light enough to take along as a third lens. It will get me a lot closer to birds and roos and other less accessible subjects without the penalty of too much to carry. I'm doing almost all B&W and post processing in Silver Efex Pro and posting directly to the web at lgude.com which is quite different than printing large prints. Thanks again for an absorbing blog.

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    Replies
    1. I think the Olympus 40-150mm tele zoom lens looks like an interesting lens. It is light, and generally gets good reviews. I wouldn't worry about the plastic mount. It probably saves you some weight, while still being strong enough for such a light lens.

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