setting UIImageView content mode after applying a CIFIlter

Thanks for looking.

Here's my code

 CIImage *result = _vignette.outputImage;
self.mainImageView.image = nil;
//self.mainImageView.contentMode = UIViewContentModeScaleAspectFit;
self.mainImageView.image = [UIImage imageWithCIImage:result];
self.mainImageView.contentMode = UIViewContentModeScaleAspectFit;

in here _vignette is correctly set up filter and image effect is applying to the image correctly.

I'm using a source image with resolution 500x375. My imageView has almost iPhone screen's resolution. So to avoid stretching I'm using AspectFit.

But after applying effect when I'm assigning the result image back to my imageView it streches. No matter which UIViewContentMode I use. It doesn't work. It seems it always applies ScaleToFill regardless the filter I've given.

Any idea why is this happening? Any suggestion is highly appreciated.