The one drawback of using an 850 nm filter over the lens of a DSLR converted with a filter of a lower cut-off (e.g. 720 or 590 nm) is that the viewfinder is completely dark. On a tripod it is an easy matter to align the camera, screw the filter in place, and adjust the exposure.
With later Nikon DSLRs models, like the D90, the solution to using the camera off the tripod is easy: use Liveview, However, ready-converted Nikon D70s and D80s are popular in UK especially for those trying infrared photography for the first time. With no Liveview it would seem there is no way that they could be used with an 850 nm filter for hand-held photographs. But there is a way: put a cheap direct vision finder of the type used for rangefinder cameras in the accessory shoe and use that to line up the shot when the filter is in place—not to frame the shot necessarily but to align the camera to the central point already identified without the filter in place.
The procedure then is simple: Without the 850 nm filter, frame the shot through the normal reflex finder and remember where the central point of the photograph lies in the scene (easy with a central focus point showing in the finder); add the filter and then with the direct finder aim the centre of that finder at the spot identified earlier, and take the photograph.
Provided you aim for the centre it does not matter what focal length the direct vision finder is simulating, although it does help if the frame of a longer focal length lens is outlined. To reiterate the finder is being used as a direction indicator or gunsight, not as a means of framing the shot. This method works, of course, for all manner of zoom and fixed focal length lenses since the frame shown is of no concern.
The photographs show a brightline finder fitted to a Nikon D80. The central point is easy to see because of the marking for a 135 mm focal length lens (on what it was designed for, a full-frame 35 mm camera). Incidentally, I do not recommend the finder shown (Helios, made in Japan); a poorer article it would be hard to imagine since it was poor in both design and construction. I got this some time ago from an eBay seller but only got round to using it more recently. The first thing I noticed was the name ring seem misplaced but then I saw that the frame were moving around inside the finder. I took it apart. It had been crudely glued but the non-original as well as the original adhesive had come unstuck so that the frame (on a piece of film and glued between two bits of glass) was rattling about. Fastening it back in so that the centre of the view was at the centre of the frames was not easy since there was no fixed point for attachment. I assume the factory must have had some sort of jig to drop the film-glass sandwich onto drops of glue. I put the whole thing back together, aligning the frames by eye, and it does now align with the centre of the proper viewfinder (at what is effectively infinity) pretty well spot on.
