Saturday, 12 March 2011

GH2, does ETC affect the video quality?

ETC, or Extended Tele Converter, is an interesting feature with the Panasonic GH2. I've previously looked at how it can be used to get closer video recording of the moon using the Lumix G 45-200mm lens.

Essentially, ETC is a digital zoom for video. During normal video recording, the entire sensor area is scaled down to 1920x1080 pixels. When enabling ETC, the camera crops the 1920x1080 pixels from the centre of the sensor. This achieves full HD resolution, as well as a digital zoom. See the illustration:


Since 2800/1080 = 2.6, the ETC mode gives a tele conversion effect of 2.6x. If you use the 720p video mode, the conversion effect becomes larger still, 2800/720 = 3.9.

A question that remains is to see if the video quality is still good using the crop mode. In theory, it could be somewhat worse, since the camera doesn't have the option of scaling down from a larger number of pixels.

On the other hand, we know that the camera doesn't actually sample all the 16MP of sensor pixels for each frame in the video.  That would require too much bandwidth and processing power.  Just how many are sampled is unknown, but given that the electronic shutter 40fps continous mode has 4MP, one can guess that about that many are sampled during video recording.  This is, of course, just speculation.

To study this, I have used the kit zoom lens, Lumix G 14-42mm f/3.5-5.6. Since it has a zoom ratio of 3x, using ETC in the short end should give the same field of view as the longer end without ETC. This is a good basis for comparing the video with and without ETC.

I used 40mm without ETC, and 15mm with ETC. Since 15mm times 2.6 is approximately 40mm, these two modes give about the same field of view. I recorded video using ISO 160, 640 and 2500. Here are the recordings in sequence:



To better evaluate the quality differences, I have compared still images from the video streams (click for a larger version):


It appears that the video stream recorded using ETC has worse image quality than the normal stream, at all ISO values. There is some lack of sharpness and contrast, and more noise in the ETC video streams.

The reasons for these differences could be the sampling from a higher number of pixels when not using the ETC mode, as discussed above. Or it could also be related to the lens sharpness, see the discussion in the Appendix, below.

This does not mean the the ETC videos are useless, far from it. But it is fair to say that there is some image degradation when using ETC. As a rule of thumb, I think you should avoid using ISO higher than 640 when using ETC.

ETC is still a very good tool to use when you need extra reach during video recording. If you don't have any longer lenses, ETC is the only way to get quality videos at longer reach.

Appendix, some technical aspects

I used the new 1080p, 24fps cinema mode. This is the video mode on the Panasonic GH2 which gives the highest bitrate. It is generally recommended to use this 24ftp progressive mode, except when you have significant movement in the picture frame. When there is movement, people generally recommend to use the 60fps (NTSC) or 50fps (PAL) modes.

When the lens was set to 15mm, I stopped down the aperture to f/4. In ETC mode, the camera only uses the centre of the image, where lenses are generally very sharp. I've previously looked at the sharpness of the Lumix G 14-42mm kit lens, and found it to be good wide open at 14mm. So I don't think the centre sharpness at 15mm f/4 is a significant limiting factor when using the ETC mode.

But there is probably some impact on the sharpness due to the lens performance when using ETC. Remember that the ETC mode is more demanding on the lens sharpness, since there is no downscaling. When video recording without ETC, the image is downscaled from a larger sensor area, and the lens doesn't need to be tack sharp. It is like putting the lens designed for a 16MP sensor on a 2MP sensor camera.  It's not very likely that you can identify a lack of sharpness with a 2MP sensor.

In real life use, it is of course silly to use ETC in the short end of the Lumix G 14-42mm f/3.5-5.6 kit zoom. It is better to just zoom in. But I did this to get comparable images with and without ETC.

It is much more relevant to use ETC with the longer end of the Lumix G 45-200mm tele zoom lens. However, many have concluded that it is not very sharp in the longer end, so it may not be sharp enough to get a good video quality with ETC. You may want to stop down the aperture to f/6.3 or f/7.1 for the best result.

7 comments:

  1. Hi.thanks for the article!
    i agree with you 100%, there is more noise in the ETC mode, and the overall quality is slightly minor.
    one advantage of the ETC is in the Macro!
    check out a short video that i've made with a broken lens! : http://www.vimeo.com/20840788

    : )

    oren

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  2. I suppose that ETC is mainly used to enlarge the reach of tele lenses. The Lumix G 45-200mm lens becomes 520mm in the long end with 2.6x ETC. And that corresponds to 1040mm on a traditional 35mm film camera. Not bad!

    But you are right, it also has interesting applications for macro. Good idea, indeed!

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  3. I use a G2 with a Nokton .95. Using the EZ setting (similar to ETC) lets me get some nice macro/close-up shots.

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  4. Tank you for this nice testing! I´ve also noticed, that the ETC mode is not as sharp as the normal mode and it seems to show slightly less contrast. I was wondering, what causes this slight quality loss, but you´ve answered my questions!

    Daniel

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  5. I wondered about and tested another possible application of ETC and that was possible noise reduction as oppossed to zooming the lens. I wondered if it would be better to use the zoom lens wide open at F:3.5 and Teleconvert using a lower ISO than Zooming the lens in at F5 or so and raising the iso.

    I found that the noise added by using ETC was far greater than the noise introduced by raising the ISO to get the same exposure at a smaller F Stop. It makes sense since alot of sensor noise must be reduced by pixel binning but I had to try for the heck of it.

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    1. This is consistent with my own experience as well. The ETC function is very useful for extending the reach of a lens with video. But it is generally best to avoid using it if you can, especially at high ISO. So use the optical zoom first, then add the ETC for even more reach when needed, that is my conclusion.

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  6. I think it has to do with the aperture of the lens, that determines ultimately the resolving power of the lens (numerical aperture). That is why it doesnt make sense to put on a bigger magnification oculair in a telescope. For 77 mm opening it allows for about 30 or 40 times magnification, beyond this there is no gain in resolving power by using a higer magnification. The ETC mode asks a lot from the resolving power of the lens, it acts just like a loup. It would be best to use a big front lens when recording in ETC mode. Lateral chromatic aberration should me less of a problem, using only the center of the view.
    I think the ETC mode therefore needs a sharp and big lens. Because of atmosferic conditions it would be best on really small subjects not too far away in good lighting conditions and on a good tripod, trying to buy a gh2 at the moment, must be great to use ETC for butterflies, lizards and amphibians and birds.

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