Optimising circular fisheye photos
|Fisheye (8mm equivalent)|
Ultra wide angle fisheye lenses produce circular pictures which quite often do not have hard edges, and worse still sometimes have chromatic abberations at the edges. In other words, there may be a colour cast at the edges of the image, and the image might not be perfectly circular because of the way the light catches the edge of the lens. The image left shows this clearly: see the way in which the light has caught the top right of the image and as a result the image is not quite circular.
Today most circular fisheye stitching software is sufficiently sophisticated as to cope with these problems or provides simple editing interfaces. However if you find limitations in your software, manually optimising your images before stitching should help. The remainder of this tutorial assumes you are using Adobe PhotoShop 6.
|Poor alignment caused by stitching software being confused by light catching the edge of a fisheye lens|
Optimising fisheye images in Adobe PhotoShop 6
The first thing to do is to crop the excess black from the image. Use the marquee tool and set a fixed size so you can specify a perfect square. This also means that when you crop the two hemispheres you can easily ensure they are cropped to the same size.
|Square marquee tool and marquee options toolbar in PhotoShop 6|
Next thing to do is switch to the circular marquee tool, fix a smaller size and create a marquee that fits the edges of the image. Obviously you will need to experiment to find the right size of the marquee. Use the menu options Select -> Modify -> Contract or Expand to reduce or increase the marquee size. Make sure you use the same settings on all hemispheres. Note that if your images do have chromatic abberation, it is a good idea to reduce the marquee size slightly so that you are not including the discoloured bit.
Use select -> inverse to select the edge rather than the image.
Press the delete key to delete the excess from around the image circle. If the background goes a different colour than black, you need to change your background colour in the colours palette.
Now compare the images you are going to stitch. You can probably adjust colour, contrast and brightness in your image stitching program, but it might be easier to do so now.
|The right hand image is darker|
|The right hand image brightened to match|
Use your chosen stitching program to stitch the images and you should get a good match (providing of course you shot the pictures carefully too!)
|Finished stitched image|