Why Does Correcting Ordinary Lens Vignetting Leave a Colored Gradient Across My White Product Backdrop?

Separate brightness falloff from color nonuniformity

Inspect the unedited RAW photograph of the neutral backdrop. Compare center, left edge, right edge, and corners without relying only on an overall white-balance adjustment.

Vignetting describes spatial variation in brightness. A color cast can also vary across the field, meaning different regions have different relative color-channel responses.

RawTherapee's Flat-Field documentation describes correction operating on linear RAW data and addressing both luminance and color-cast nonuniformity. This is not the same as painting a decorative vignette or adding a global temperature adjustment.

If the backdrop is darker at the corners but remains neutral, brightness correction may be the main issue. If its brightness becomes even while the color gradient remains, investigate spatial color variation and the capture conditions.

Use an appropriately matched flat-field reference

A flat-field reference is intended to characterize nonuniform response across the image field. Its usefulness depends on matching the relevant camera-lens and capture conditions.

Follow RawTherapee's documented Flat-Field workflow for the installed version and inspect the required RAW reference. Record the camera, lens, focal length, aperture, and lighting setup rather than treating any photograph of a white wall as universally suitable.

Compare the correction with the original source and check whether the remaining gradient follows the optical field or a real lighting difference across the backdrop.

If the background itself is illuminated unevenly by different colored sources, a flat-field correction is not proof that the physical scene was neutral. It may compensate numerically for the photograph while leaving the capture conditions unsuitable for color-critical product evidence.

Hypothetical neutral-backdrop product photograph

Hypothetical illustration—not an executed RawTherapee test: a merchant photographs a white package on a neutral backdrop. The center looks neutral, the left side appears slightly green, and the right side appears magenta.

Version A is the untouched RAW. Version B applies an ordinary brightness-oriented vignette correction. Version C evaluates a matched RAW flat-field reference under the documented workflow.

The proposed review checks the same center and edge regions in all three versions. If B makes brightness more even but leaves opposing color casts, the problem is not fully described by luminance falloff.

If C reduces the gradient, the result supports investigating camera-lens field variation. It does not independently establish that the package's physical white is reproduced accurately or that the illumination was uniform.

The example contains no measured RGB values or claims that a particular lens produces this pattern.

Decide whether the correction belongs to capture or processing

Preserve the RAW, flat-field reference, and selected processing settings. Review neutral regions and the actual product label together; a corrected background should not be obtained by distorting the merchandise's defining colors.

If the color gradient follows the camera-lens field and a suitable flat-field reference corrects it consistently, retain that controlled processing route for the relevant capture conditions.

If the gradient changes when the lights or backdrop are rearranged, investigate the physical lighting instead. If a matched reference cannot be established, record the limitation and consider a controlled reshoot.

Do not apply a generic painted gradient across an entire catalog and describe it as optical calibration. The acceptance condition is a repeatable, source-supported correction that preserves product appearance and does not conceal actual mixed illumination.

Record your own observations

The filled rows are illustrative examples. Use the blank row for your own files and observations.

Download the editable CSV worksheet

What the references establish

• RawPedia — Flat-Field: Documents RAW-stage flat-field correction for luminance nonuniformity and lens color cast; this is the current English project documentation.