The single most common way to lose a first roll is to shoot the frames and forget the gate. This is short, and it is the whole thing.
#What you shoot
Per frame: three exposures. One under the red band, one under the green, one under the blue. Same position, same focus — only the light changes.
Once per session: three more, of nothing. The empty gate, with the film out, one under each diode.
That second set is the part people skip, so:
Warning
Without the empty gate a density has no denominator. The app will let you continue without one only if you say so out loud — and the colour it gives you afterwards is not a measurement, it is an estimate.
#Two different measurements, and it is worth keeping them apart
They get confused constantly, including by me:
| What | Measures | How often |
|---|---|---|
| The empty gate | your light — what the bare beam reads under each band | once per session, or whenever the lighting changes |
| The film base | your film — read off the clear rebate around the frames | once per roll |
Density is a ratio: D = −log10(frame / light). The gate gives you the denominator's scale; the rebate gives you what this particular roll's orange mask is doing. Neither substitutes for the other.
#Why three, and not one
One shot under white light makes the camera do the separating, through filters that overlap. Three narrow bands do the separating in the light itself, before the sensor ever sees it. Nothing has to be un-mixed afterwards, which is why a whole roll can agree with itself instead of needing a nudge per frame.
#Leave the rebate in
Crop in camera and you crop away the clear film base — the thing the base measurement is read from. Frame a little loose. You can crop later; you cannot un-crop a measurement.
#The mistakes, in the order people make them
- No empty gate. Covered above. Costs you the denominator's scale.
- Cropping to the image area. Costs you the base.
- Changing exposure between the three bands of one frame. They have to be comparable; the app is reading ratios.
- Moving between the three. Registration will pull them back together, but it can only fix a shift, not a re-focus.
- Re-lighting between rolls and reusing the old gate shots. The gate belongs to the lighting setup. Change the lamp, change the distance, change the gate.
#What good looks like
For a 36-frame roll: 108 frame exposures, plus 3 of the empty gate for the session. Import them in any order — the app works out which three belong together.
Note
If your first roll comes out wrong, post it in Troubleshooting and Bugs with what you shot it on. Nearly every first-roll problem is one of the five above, and they are all cheap to fix on the next roll.