Purple Edges on Jewellery Photos? Fix Them Free in darktable

A thin purple or green line around a silver ring, polished watch or white bottle is easy to mistake for a bad colour grade. It is often chromatic aberration: the lens has focused different colours in slightly different places along a high-contrast edge.

That small fault matters in ecommerce. It can make metal look plated, contaminate a white background and give an AI editor a false colour to preserve or exaggerate. The safest correction happens before background generation, upscaling or colour variation.

This tutorial uses darktable 5.6, a free, open-source RAW developer and photo workflow application. Its current chromatic aberrations module works with RAW files and rendered files such as JPEGs, although a RAW original gives you more room to correct the image without compounding compression damage.

1. Confirm that the colour is actually a fringe

Open one difficult image in darktable’s darkroom and inspect it at 100% to 200%. Look at boundaries where a bright product meets a dark background, where polished metal catches a hard light, or where the focus falls just in front of or behind a detail.

A lens fringe usually follows an edge as a narrow purple, magenta, green or cyan strip. Real product colour tends to belong to the material or reflection rather than tracing every high-contrast boundary. Gemstone fire can also be real, so do not treat every coloured sparkle as a defect.

Choose a test frame with both the problem edge and an important coloured detail. For jewellery, that might be a silver setting beside a blue stone. The stone becomes your warning: if the correction makes it dull, the settings are too aggressive.

2. Let the lens profile do the first pass

Start with darktable’s lens correction module. It can use correction data embedded in the RAW file or a Lensfun profile to address distortion, vignetting and transverse chromatic aberration. Confirm that darktable has identified the correct camera and lens before trusting the automatic result.

Use the TCA controls to reduce the obvious red-cyan or blue-yellow edge separation. Do not stack every similarly named correction simply because it exists. Darktable warns that enabling TCA correction and the separate raw chromatic aberrations module together can create over-correction artifacts.

The ordinary chromatic aberrations module used in the next step is different from the raw module. It can handle residual colour fringing after the lens pass and does not require RAW input.

3. Correct the remaining fringe gradually

Enable chromatic aberrations. Temporarily raise the strength so the module’s effect is easy to see, then increase the radius until the coloured edge is reduced. Radius is the main control. Use the very large chromatic aberrations option only when the ordinary range cannot reach a wide fringe.

Try the available guide channels and keep the one that removes the fringe with the sharpest edge and fewest artifacts. Once the radius and guide are working, lower the strength again. Darktable specifically recommends reducing strength to avoid washing out genuine colours.

4. Protect the product’s real colours

If the global correction weakens a coloured stone, enamel, fabric print or brand mark, restrict the module with a mask. A drawn mask is often the simplest option for a small product: include the affected metal edge while excluding the colour that must remain untouched.

For more complex images, darktable also supports parametric masks based on colour and luminance values. Its current AI object mask can help select an object, but thin chains, open settings and reflections can be difficult boundaries. Inspect and edit the resulting path instead of assuming the automatic mask is accurate.

Another controlled method is to use two low-strength instances of the chromatic aberrations module. Darktable suggests one in brighten-only mode and one in darken-only mode for difficult cases. Several gentle corrections are usually safer than one heavy pass that removes colour from the whole product.

5. Run a product-truth check

Toggle the correction off and on at high zoom. Check the exact edge that concerned you, then move away from it. Heavy corrections can create grey rims, soft halos or desaturated patches that are less obvious when you stare only at the original fringe.

For jewellery, inspect prongs, engraving, chain gaps, gemstone edges and polished highlights. For packaged products, inspect small type, coloured logos and the boundary between the pack and background. Also view the full image at normal listing size. A correction that looks slightly imperfect at 200% may be completely clean at the customer’s viewing size.

Keep the original RAW file and darktable’s XMP sidecar. The history stack is saved in the library database and sidecar, so the adjustment remains editable instead of being baked permanently into the master.

6. Apply it to a shoot, not blindly to a catalogue

Images made with the same lens, aperture, lighting and camera distance often show a similar fringe. Correct one representative frame, then use selective copy to copy only the relevant correction modules. Select the matching images in the lighttable and use selective paste in append mode.

Selective copy matters because a normal complete-history copy excludes several modules by default, including lens correction. Append mode also preserves the destination image’s existing edit history, while overwrite can remove it.

After pasting, spot-check the sharpest product, the most colourful product and a frame from the edge of the shooting area. Chromatic aberration changes with focus position and image position, so one setting may not fit every photograph even when the equipment is identical. Split the shoot into smaller groups when necessary.

7. Feed the clean master into AI tools

Only after the colour fringe is controlled should you remove the background, build a lifestyle scene, generate colour variants or upscale the image. This prevents a generator from interpreting a lens defect as part of the SKU.

Keep the corrected master conservative. If an AI result later changes the metal tone or invents coloured reflections, compare it with both the corrected master and the untouched RAW file. The correction should remove an optical error, not redefine the product.

When this workflow will not fix the problem

Chromatic aberration controls cannot repair every coloured edge. Sensor blooming, aggressive sharpening, JPEG ringing, colour spill from a backdrop and real reflections need different treatment. If the fringe appears only after export or upscaling, trace the workflow backwards before changing the RAW edit.

The best long-term fix may also happen during capture. Test a smaller aperture, avoid extreme backlighting, keep the important product plane near the lens’s best focus and compare another lens when possible. Software correction is valuable, but a cleaner capture protects fine product detail and reduces the amount of colour reconstruction required.

Sources

darktable 5.6 manual, chromatic aberrations: https://docs.darktable.org/usermanual/5.6/en/module-reference/processing-modules/chromatic-aberrations/

darktable 5.6 manual, lens correction: https://docs.darktable.org/usermanual/5.6/en/module-reference/processing-modules/lens-correction/

darktable manual, copy and paste: https://docs.darktable.org/usermanual/development/en/module-reference/utility-modules/shared/copy-paste/

darktable 5.6 manual, AI masking: https://docs.darktable.org/usermanual/5.6/en/darkroom/masking-and-blending/masks/ai-masking/