A24 Hasselblad film backs cost a fortune ($819.00 new, $500.00 used). In an attempt to find a cheaper solution, I experimented with using an old "12" back that one can get for about $150.00 or so. I did this after reading that it was possible in some texts I got from the public library.
Here are the instructions as copied from the original text of one of the old Hasselblad manuals. I believe it was "The Hasselblad Way".
This is a special kind of roll film of the same width as No. 120, but without the customary backing paper. Instead of winding a 32 inch length of film with nearly twice as long a strip of backing paper on to a spool-the No. 120 system- the No. 220 roll has twice as long a film, but only two comparatively short strips of paper attached to the beginning and end of the film. This way a spool of the same diameter can hold 24 exposures 2 1/4 X 2 1/4 inches (or 32 exposures 1 5/8 X 2 1/4 inches) instead of the usual 12 or 16 respectively. Using No. 220 film involves two points. Firstly, the film must be advanced into position for the first exposure without watching numbers on the backing paper-there is no backing paper beyond the leader strip. Secondly, we must be able to count frames with the aid of the exposure counter of the film magazine.
There are two ways of going about it. On the one hand the non-automatic No. 12, No. 16 and No. 16S magazines can be adapted for No. 220 film. On the other hand there is also the No. 24 magazine designed especially for this film type.
To adapt the non-automatic magazines for No. 220 film the observation window at the back of the magazine must first be made light-tight. Special plastic plugs are available for this purpose; they go into the rear window from inside the magazine housing. To fit them first remove the magazine slide and the spool holder. Then push the plug, with the lettering "Hasselblad 220" facing outwards, into the rear window from inside. The inner flange of the plug must be flush with the inside of the window, and the outside surface (with the lettering mentioned above) level with the rear surface of the opening. Check that the plug fits smoothly, without gaps all round.
Once the magazine is adapted in this way, here is the loading procedure for the No. 220 film. It differs slightly for early and later models of the magazines. The current models are No. 12 magazines with serial numbers of 64,400 and higher, and No. 16 and 16S magazines of serial numbers above 204,200.
To load these magazines, fit the full film spool and thread the backing paper leader on to the take-up spool in the usual way. Turn the take-up spool by the milled knob, winding up the paper until a row of dots or a two-ended thick arrow across the back of the paper appears in the middle of the take-up spool, as seen from the rear of the spool holder. Insert the spool holder into the magazine housing and lock it. Turn the transport key on the magazine backwards (before advancing the film along) to set the film counter to No. i (see page 69).
Next turn the transport key forward through nine complete turns (18 half turns). With the No. 12 magazine this should bring the figure 7 into the film counter window, with a number 16 and 16S magazine the figure 9. Then turn the winding key backwards to reset the film counter to No. I again.
The magazine is now ready for shooting. Take pictures and advance the film in the usual way until the camera transport and the release both lock (with the magazine on the camera and the magazine slide pulled out-i.e. after the twelfth exposure on a No. 12 magazine and after the sixteenth exposure on the 16 and 16S models). Lift up the transport key again and turn it anti-clockwise to reset the film counter to No. 1 Then carry on until the last exposure is again reached. At that point lift the transport key again and wind it through at least 6 or 7 turns to wind up the backing paper trailer, before opening the magazine and removing the film spool.
The procedure is similar for earlier magazines-types 16 and 16S with serial numbers below 204,199 and with No. 12 magazines of serial numbers between 20,000 and 64,399. The only difference is that the film is advanced ten complete turns or 20 half turns of the transport key to bring the first film frame into position. This brings No. 8 into the film counter window of the 12-exposure magazines or No. 10 of the 16-exposure magazines. With these earlier models the spacing between the film frames tends to get very close at times, with possibly an over lap of a fraction of an inch.
The very first magazines-2 1/4 X 2 1/4 inch models of serial numbers up to 19,999 have no release lock when the film is finished. The loading procedure is still the same but the film is advanced (with the transport key) through 7 complete turns or 14 half turns to prepare the first exposure. Reset the film counter to No. 1 as before. Then make 12 exposures (keeping track on the film counter) in the usual way. After the twelfth shot advance the film once more and reset the film counter to No. 1. Then expose the other 12 frames before unloading.
The automatic magazines cannot be adapted to use 220 film.
The results of following these instructions can be seen below.
It works; but not well. It would probably be OK for print film; but unacceptable for slides as there is, at first, very little and then no space between frames in most of the first 12 frames to allow for cutting and mounting of slides. Many towards the end of the first 12 shots actually overlap; but only by about 2mm.
To see if I could get better results, I modified the above instructions.
(1) I inserted the film and wound to the first frame as directed above.
(2) I set the counter on "1".
(3) I then fired two blank shots.
(4) I reset the counter to "1".
(5) I shot my 12 shots.
(6) I reset the counter to "1".
(7) I shot my second 12 shots
The results can be seen below. (You wont count 24 frames because, once again, I mounted some for projection.) The first 12 have better spacing than I got with the first test roll, and would (just barely) have clearance enough for slide mounts. I ended a little short of 24. The last, or 23rd, frame hit the end tape of the film.
I think my permanent technique will be to:
(1) Insert the film and wind to the first frame as directed above.
(2) Set the counter on "1".
(3) Fire only one blank shot.
(4) Reset the counter to "1".
(5) Begin shooting the first12 shots; but after shot number 2, reset to "1" again and shoot 10 more, for a total of 12 .
(6) Reset the counter to "1".
(8) Shoot the second 12 shots.
Or, just break down and buy an A24 back.
Note: These experiments worked for my camera and old used back, and are presented here only as a reference. Your results may vary with a different camera and/or back.