Due to the size of the sensor in DX format cameras, an 8mm fisheye lens will not produce a circular image. FX format cameras, on the other hand, have a bigger sensor and therefore will produce a fully circular image.
If you are shooting with an 8mm fisheye lens on a DX format cameras, you will always have dark corners in your picture that require adjusting in order to de-warp the image to a full frame, just as a ‘full frame fisheye’ such as 10.5mm would do. You may simply crop the image but that way you will lose a considerable amount of picture and the image will still remain distorted. Alternatively, you may use the lens correction options in Adobe Photoshop or Lightroom 3, but these offer limited control over the image.
PTLense Photoshop plug-in is a simple and powerful plug-in which also works as standalone software and you will definitely find it useful when working with your fisheye images. The plugin is available to download as a trial version (both Windows and Mac) that has complete functionality, but will only process 10 images. For $25, you can also purchase the licence via PayPal, which is a pretty good deal in my opinion.