Image borders darker than center

[ropel]

I Just noticed that if I render a simple flat white surface illuminated by a sunlight, the image is darker at borders: the wider viewing angle, the bigger is the effect
(The effect is visible in every scene, but a white surface makes it more visible).
This is a real problem for me because i need to join six images with a 90° degree viewing angle (a cube) in a spherical proiection.

Can I avoid this effect without post processing the image and without changing the viewing angle (that for me is mandatory) ?

Thanks

[pixie]

This vignette effect is akin to reality and AFAIK can be naturally removed, otherwise it would appear in all photo. I think it has to do with aperture, can be wrong though

[ropel]

I tried changing aperture: a smaller one isn't useful, a really big one reduces my depth of field and almost everything goes out of focus (i get a uniform blurred white image)
The only way I can imagine is to combine my white vignetted image together with the real one with GIMP, but it's hard to obtain a good result without loosing image quality...

[CTZn]

No, from my experience that's correct pixie, or with a wide camera angle. In the following image, camera angle is above 120°:

[CTZn]

My answer was incomplete... vignetting in Indigo is not an option, although it's a physical effect it can indeed prevent stitching.

There is a tutorial in these forums (maybe it's a scene in a wip) showing how to create a latlong hdr environement output with a virtual chrome strobe, maybe you could do that and convert later to cubemap with a free app like hdrshop (if it can, I believe so) ?

Now, you can request either making vignetting an option, or adding an environment camera, wich by definition don't use vignetting... I guess the second is more popular

[zsouthboy]

Vignetting is literally caused by less light making it to the "film" at oblique angles.

Render to .exr and open in Photoshop, for example, and it's quite easy to "correct" - and unlike real life, Indigo's vignetting is perfectly aligned.

[ropel]

Ok, Thanks to everybody.
I'll surely solve the problem searching for the virtual strobe or generating a white vignetted image and subtracting it from my real images.
Meanwhile I'll request an environment camera, endeed useful for making env_maps