Friday, 18 May 2012

300mm plus ETC gives an enormous reach!

The Lumix G 100-300mm tele zoom lens is a very long one, useful for birds, wildlife, events, sports, and so on. When recording video with the Panasonic GH2, you can use the ETC, Extended Tele Converter mode, which uses the centre of the sensor only, for an additional 2.6x factor on the effective focal length. At 300mm, this gives a staggering 1560mm equivalent focal length, when comparing with the traditional 35mm film format.

I have made some example video clips at 300mm, including the 2.6x ETC mode. The following video contains four segments, from the May 17th Constitution Day parade in Oslo. Notice that with the 1560mm effective focal length, the perspective is very compressed.



A stable tripod is needed. My tripod is not too stable, and just pressing the button made the video wobble. So the first seconds of footage was always unusable, while the camera and lens settled down.

Notice how the video flickers quite a bit, especially in clip number two and three. This is due to atmospheric disturbances. All the people in the parade generate heat, and it affects the path of the light. The first clip is from the very start of the parade, and the air is much more calm.

This can be seen in still images as well. Here is an image taken from the same point with f=300mm, ISO 160, 1/200s, f/7.1, tripod mounted:


An 1:1 crop of the centre of the image illustrates the blurring caused by the atmospheric disturbances even better:


Please note that this blurring could not have been fixed by the usual measures: Lower ISO, faster shutter speed, smaller aperture, tripod, or even a better lens. It is simply a physical phenomenon which affects an image when the light travels a long distance across air with different temperatures. The only ways to avoid it would be to move closer, or to take the image when the air is more neutral, possibly early morning. But in this case, I wanted the compressed perspective, and rising early was not an option since the parade had not started at that time.

In my review of the tele zoom lens, I noted that it loses a bit of contrast at 300mm in strong sun light, due to flare. I think this happens in my example video as well. It could have been corrected a bit by adjusting the curves in the video, but I left it as is.

Conclusion

The ETC mode, when combined with a long lens, allows for a very significant reach for videos, and a very compressed perspective. However, be sure to have a stable tripod, and beware that atmospheric disturbances may make your video footage less usable.



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.