I had to right click the link and save as to get the pano. It won't show in the browser but thats because the file is coming in at 12k a sec which is VERY slow.
This was shot with a Sigma 8mm which is a circular fishey but cropped by the sensor therefor making a "drum" image.
The problem you are having is called "light fall off". This is a common problem with circular lenses. This is happening in this case because of the location of the sun. With the lens pointed strait up the light is diffracting through the lens. This is causing an exposure difference between the horizontal shots and the top shot. In other words some of the light is being bounced around in the lens creating a slightly lighter exposre even when set at the same exposure as the other images. The amount of light reaching the sensor is "less" than what is reaching the horizontal shots. This is causing your top shot to over expose even at the same aperutre and shutter speed.
Secondly, with this lens you will get light fall off at the edges in the horizontal shots but when you do not shoot the top the edges of the images are blended in a different way than if you stitch 4 + Top.
When stitching 4 + top the "top" image is blended with the horizontal shots. The image is actually blended "over" the horizontal shots. So the hole is being coverd by the top shot. You are looking through the hole to see the top. This is why you can see the seams so much.
If you stitch this shot without the top using only the 4 horizontal I bet you get a much better sky but with only a little pin hole a the top which is very easy to "clone" out in photoshop.
General Lee
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