Does anyone have a remedie to avoid the nasty porthole effect in the XA20 yet?

Porhole Effect? Do you mean vignetting or something else??

I've been told, that it was known as the porthole effect. Under brighter conditions, the corners of the image are somewhat gey.

I own an XA20 and I've heard the effect occurs sometimes on full zoom, bright light, ND on, stabilisation on, handheld. Kudos to those who found it and noticed it, I never have and I very much doubt a typical viewer would notice. Why are you asking? If you are considering buying an XA20, I seriously wouldn't be put off by these reports.

That would be corner brightness fall-off type of vignetting, common to ALL lenses, both video and photo, and is one of the lens design considerations..

It is more apparent with flat images with large expanses of feature-less image, like blank walls, or a clear or uniformly overcast sky. It is less apparent in images with a lot of varied content. You can control it to some degree by adjusting the zoom and aperture settings.

See Canon Lens Vignetting (Light Fall-off) (http://www.the-digital-picture.com/Canon-Lenses/Canon-Lens-Vignetting.aspx) for more discussion.

The Paul van Walree link at the end of the above article gives more optical theory with math on the subject.

Thanks for your explanation Don. Fine to find out, that there is something I can do about it.

The articles suggest avoiding wide open apertures and wide zoom to minimize the corner fall off.

Ensure no physical items such as filter stacks or lens hoods obstruct the light path at the edges/corner of the frame.

If you can, avoid wide expanses of feature-less background and avoid lighting that causes hot spots mid frame (unless that you your intent) such as narrow beam lights on the camcorder that can make it worse.

Your recomendations in regard of the feature-less background and lighting midframe showed to be useless at the saltflats of Uyuni, Bolivia, where I sufffered the porthole most some weeks ago. But I will pay more attention to the zoomfactor and the aperture next time.

Jo, it seems you encountered an extreme example - can you post a frame?

These are two frames, that I saved from the actual footage:

As was discussed in the link above the corner fall-off tends to draw the eye to the center of the frame. Not always a bad thing. However, you may be able to apply some form of gradient filter in post to brighten the corners a bit.

Thanks Jo. Like Don says, and if it does bother you, and I don't think it should, there probably are a number of post fixes - I had a quick go in Vegas, just a soft edge mask and increased brightness, I think you'd do better with curves, gradients, secondary color, depends how much time you think is worth spending.

The Adobe Lens Correction filter vignetting settings might work