A larger prism aperture will not necessarily increase the working aperture. That's because a wider prism is attended by a concomitantly longer optical path length through glass. And so while the front opening of the prism system is wider, it's also located farther up the converging light cone, where the cone is also wider. If the proportions are maintained, the longer path length exactly counters the increased prism aperture.
What has to be done in order to increase the working aperture:
- Use an objective if slower f/ratio. This provides a narrower light cone width at the prism entrant aperture, thereby reducing or eliminating clipping of at least the axial light cone.
- Get the eyepiece's field stop closer to the rear prism aperture. This has the effect of moving the prism system farther back and closer to the focus, thereby providing a somewhat narrower light cone width at the prism system entrant aperture. Larger prisms could help here by providing a tad more space beside the side of the first prism where looms up beside the rear prism aperture.
- Related to the immediately previous aspect, one can use a smaller field stop so as to permit it to slip past the adjacent prism's side and hence get closer to the rear prism aperture.
-Use a tapered or stepped first prism. Such a prism has the half that first intercepts the light cone of larger dimensions than the other half. This better accommodates the light cone where widest upon entrance, with the 'normal' sized half being OK after the light cone has tapered down somewhat before getting there. Note that simply installing a larger prism of the usual form (no taper or step up in size on one side) will not by itself help, for the same reason that two such larger prisms will not.
Again, bear in mind the fact of 'conservation of proportionality' as prisms get larger. A scaled up prism comes with a matching increase in glass path length, thus nulling out the advantage ostensibly provided by the increased aperture.
The *primary* determinant for prism size is eyepiece field stop diameter. One does not want the prism aperture to be smaller than the field stop, or at worst get down to about 0.9-0.95 times the field stop diameter. Otherwise notable field edge darkening results. And using a prism of rather larger aperture than the field stop confers essentially no gain, again because of the longer optical path length through the prism system. Indeed, it would be inefficient, in that extra bulk and expense are serving no real purpose.