Crop image from all sides

I need to add an image size, for WordPress featured image, cropping from all image sides (top, bottom, left and right). The results of the crop should be an image detail of the original image.

Is it possible?

I tried to add this:

add_image_size( 'my-img-size', 40, 30, true );

It results in a complete image (cropping only the enough portion for keep aspect ratio) with low resolution.

Thanks