Let me say straightaway... Post-flashing is just not feasible with Super-8 in cartridges. This is because their design does not permit backwinding more than a few frames at a time. However, most other films can be easily treated... Double Super-8, Single-8, Standard-8, 9.5mm, 16mm, and 35mm in both movie and still versions. 120 roll-film is not really suitable, although 5x4 cut-film is. Nearly all types of emulsion can be flashed, and even the not so contrasty colour-negative could benefit on occasions.
I am sure there are many different ways of flashing. This is the one I use. It may seem a bit hit-and-miss, but it has always worked for me. Here goes !
First I select a nice sunny day. This is perhaps the most difficult part of the exercise, here in Britain. We just need a few minutes of absolutely clear unbroken sunshine. I prefer the sun instead of artificial light, simply because it's not prone to sudden power-cuts or lamp failure which could ruin your precious footage. And dull or cloudy/bright conditions have a habit of changing their exposure alarmingly... our eyes don't always notice it.
OK. The exposed film has been backwound either in a darkroom or the camera (a lens cap might be handy !) With 35mm film the first frame must exactly synchronise for the second pass, otherwise you'll get a dark bar in the picture. So before you start, you must mark the frame in the gate. Still-35mm film has 8 perforations per frame and cine has 3 or 4... lots of possibilities for error here. All other cine-film sizes have only one perf per frame, therefore no danger of mis-framing.
The camera is set up near a window. In the SHADE, I very securely fix a sheet of matt black card. The camera is angled a bit so that no sheen on the card is visible in the viewfinder. Only a small portion is filmed in close-up so it should look evenly lit. The lens is now defocused to infinity giving just a blurred void. Now I take a meter-reading off a GREY card, then open up an extra half-stop to one and a quarter stops according to the dose intended. Now start filming the BLACK card, with one eye on that next cloud approaching !
Using this method with different coloured cards, it's possible to get some interesting tone effects. Sometimes I may do a bit of fading of the flash within the shot if I remembered to note the frame-number reading. (Footage counters are notoriously inaccurate.) And if I'm flashing odd bits of footage, I usually allow a bit more at both ends, perhaps fading the flash in and out, using the iris of the lens.
