Understand dmin, dmax, and characteristic curves

Before you start

The examples use developed negative film. “Exposure” means the original exposure on film, not the later camera-copy exposure. Reversal curves run differently and do not use these negative-conversion steps.

From density to conversion

1. Higher density means less transmitted light

   Negative and positive tones run in opposite directions: brighter scene areas generally produce denser film, while darker areas are more transparent. Density is not display brightness or a photo file’s pixel value.

   

   1. 1
      dmin: the low-density floor, not zero density

      The density floor includes the film base and developed stain or fog. Clean, unexposed but developed film can provide low-end evidence; it still attenuates light and is not equivalent to an open perforation.

   2. 2
      The curve: the relationship between the ends matters

      Density changes with original film exposure, but not at one constant rate throughout. The characteristic curve describes this relationship, explaining why simple inversion alone does not account for shadows, midtones, and highlights.

   3. 3
      dmax: distinguish the physical limit from a fitted anchor

      Physical Dmax is the maximum density attainable under the relevant conditions; ordinary photographs may not reach it. The software’s dmin+dmax method uses estimated low/high roll references. Its high anchor does not establish the film’s physical limit or a point that must become pure white in the positive.

2. Read the toe, straight-line region, and shoulder

   Rightward means more original exposure; upward means greater developed negative density. These three regions help read the diagram; a roll need not contain the entire response range.

   1. 1
      Toe: the low-exposure end

      Near the density floor, exposure differences may produce little density separation. Brightening later cannot automatically recover underexposed detail buried in the base signal and noise.

   2. 2
      Straight-line region: a more regular density response

      This portion is approximately straight against log exposure; its slope describes the rate of density change. Color-negative channels need not share the same starting level or response, so one curve is not simply copied three times.

   3. 3
      Shoulder: the response flattens at high exposure

      At high exposure, density may rise more slowly, affecting highlight separation. A dense-looking negative alone does not establish that physical Dmax has been reached.

3. How dmin+dmax and Curve differ

   Both methods use information from the roll. One works mainly from low- and high-density references; the other estimates the channel-response curves.

   Two roll-conversion approaches

   Method	Main basis	How to assess it
   dmin+dmax	Establish channel baselines from estimated low/high roll references	Check tonal range and neutrality; these are not ordinary black/white point eyedroppers
   Curve	Estimate channel-response curves using roll evidence	Inspect midtones and color consistency across densities, not just the endpoints

4. Prepare suitable photographs for roll analysis

   Prefer one roll per project. Frames with varied tonal content help estimation, but capture conditions should match. Check Calibration Area, excluding perforations, the light source, and abnormal edges. Include a small clean base region if available. Capture clipping is not a film-density limit and should not be used to set dmax.

5. Compare several frames from the roll

   Compare Curve and dmin+dmax with the same input, CCM, viewing, and adjustments. Inspect usable shadow detail, midtones, highlight separation, and neutral casts across tonal levels. A low-contrast frame need not span the full range or contain pure black and white. For isolated faults, inspect capture and analysis boundaries before switching the roll to the unrecommended independent Per-frame fit.

6. Manual references for a roll or a single frame

   Start with automatic references. In supported color-negative projects, right-click a Contact Sheet frame and choose Use this frame as roll Dmin/Dmax to use its reference for the roll, regardless of the current multi-selection. Pick Dmin/Pick Dmax in the Viewer changes only that frame. If your mode or version does not offer these options, continue with automatic references.

7. Sample a reference and restore automatic settings

   If Pick Dmin/Pick Dmax is available, open the frame in the Viewer, exit other tools, choose the picker, and wait for Preparing sample… to finish. Pick clean unexposed but developed film for Dmin, never a hole or external light. Without a credible high-density reference, retain automatic Dmax rather than choosing an arbitrary darkest pixel. The wheel changes sample diameter; click to submit and wait through Applying sample…. Esc/right-click cancels. Undo a bad pick or use Dmin/Dmax: Follow roll in the Viewer context menu. Restore a manual roll reference with Use automatic reference in the Contact Sheet menu. Endpoints restore separately; inspect several frames and status again.

8. Distinguish characteristic curves from editing curves

   A film characteristic curve describes how film records exposure. Curve is a conversion method; Density Curve and Film curve are editing controls. Their names are similar, but they work at different stages. Choose the conversion first, then make any edits you need.