As regards Swarovski's eyepieces, the 20x, the 30x and the 45x are the common ones. 'x' stands for magnification. Changeable magnification is possible and the 20-60x is the one for that. There is also a 70x but it is said to be for astronomical purposes only.
To begin with, it is better to use the 30x as the angle of coverage, though very narrow by nature, is still about right for beginners to track down targets when shooting in the field. Another concern is the less vignetting you may come across when using the eyepiece with prosumers which have the wide end wider than 35mm. You will also find that the 30x is a bright lens and because of that, you have a better chance to produce sharp and contrasty images.
A point to mention about magnification. If you fix your prosumer at 80mm in focal length and you are using a 30x eyepiece, then the resultant focal length of your set up will become 2400mm. This length is unprecedented in my DSLR experience.
The 45x is also excellent, providing an even farther reach than the 30x. A pin is never sharp at both ends. You will find the advantages in the 30x coming off the balance in the 45x.
The zoom eyepiece is optically as good as the 30x and 45x. You have to preset the focal length you want before making the hinging and to change focal length in the field, you have to dismantle the set up. Few people recommend the 20x. I think that is understandable taking into consideration the reach we want to get to from digiscoping.
About the point you raised attaching a DSLR to the fieldscope, in the case of Swarovski's, you can use its TLS800. A Canon adaptal mount is needed to go between the camera and the TLS800. The fieldscope will then become a 800mm lens with an f10 aperture. The cropping on your DSLR will further extend the lens to 1200mm. You can further add an extender to your camera, e.g. a 1.4x which then changes the lens to be a f11 1680mm lens.
Some digiscoping fans do try attaching a standard lens on a fieldscope mounted with an eyepiece, e.g. the 30x. However, making the mounting substantive to prevent dislocation is a major concern as the DSLR itself is pretty substantive in weight. Another concern is how you can alight the standard lens properly on the central axis of the fieldscope; once off the axis, vignetting will happen. Another major blow is the absence of zooming ability on the whole set up. That will make it difficult for you to zoom out for finding the target you want to shoot; at 1500mm, experience tells that it is difficult to track down targets. Even worse, you may have to move to and fro in order to come up with a framing you like for your target.
What were mentioned are just a few points that I consider important for beginners. You are recommended to follow the threads in the digiscoping corner of this website. You will find lots of useful information there, e.g. mounts, metering and choice of cameras.
Regards,
Peter
