The GH2 Ex Tele Conv. (ETC) feature effectively uses only a small part of your sensor, achieving something like a digital zoom by using one sensor pixel for one image pixel (normally it uses the full sensor and computes each resulting image pixel by using an area of sensor pixels). This results in:

• Objects appearing closer (focal length changed by a factor 2.6 for full HD)
• More noise
• Depth of field increases (like that of a small-sensor camera)

After having read this article I thought about how this feature would help my filmmaking and then came up with a few possible scenarios:

1. Getting the extra focal length to zoom in on stuff very far away (since I’m not doing documentary stuff, this is not really important to me)
2. Getting extra light e.g. by using something like the SLR Magic 12mm 1.6 lense effectively as a 31mm 1.6 (really great, especially in situations when I do not want a shallow depth of field and light is scarce)
3. Getting an extra prime lense without changing lenses by switching between ETC and non-ETC (again something that is probably of more value for documentary work where changing lenses can be a time problem like during a wedding for example it may be a great thing to switch between 20mm and 52mm with you pancake lense)

I tried 1 just for fun and another thing this did was that a soft lense (e.g. I love the image quality you get from buying cheap old Canon FD lenses and own a 200mm prime which I wanted to use as a 520mm using ETC). The result was that the image became so soft that it was hardly usable. The only reason I can think of is that the lense was made for a full 35mm area and on a MFT it already gets softer because only the center part of the lense is used for the whole image. This is now even increased by another factor of 2.6 and in that case it definitely was too much. For the Panasonic 14-140mm zoom the results were better.

The result of the tests for scenario 2 were that the improvement by being able to use a lense like the SLR magic 12mm at 1.6 as a 31mm in a low light situation were eaten up a lot by the additional noise. So the difference between an image produced by that compared to using the Panasonic Zoom at 31mm and cranking up ISO to the level required to get the same exposure wasn’t really that big a deal. So that was a bit of a disappointment. Must probably be decided on a case-by-case basis.

Tests with scenario 3 showed that it can indeed be of great value to be aware of that possibility. Imagine yourself filming a wedding in a dimly lit chapel with your 20mm 1.7 pancake (not unusual, because this is were that lense shines) and just quickly switch to ETC to get to 52mm for a detail shot of the rings without changing lenses or fiddling with ISO settings because they can stay were they were in non-ETC mode. Not what I do often personally but for people who do, keep that in mind.   

To sum up my opinion: Not knowing this feature is a bit of a waste, because you might run into a situation were it would have been the life-saver but it is not quite the magic bullet I had hoped after reading that article but maybe that was just me getting carried away in wishful thinking.