Digital Camera Corner Color Cast and Lens Vignetting: Identification and Flat-Field Correction

First determine whether it is vignetting, edge color cast, or color change of the object itself

Take a picture of a uniform background as a check and watch for a decrease in brightness at the corners and a shift in hue from center to edge. Adobe refers to the asymmetric edge light changes that may be color shifted as shading/lens cast, and points out that it is more common on mirrorless systems and medium-format digital backs using non-reflective lenses. This phenomenon by itself does not prove that the sensor is FSI/BSI, nor can it be used to determine a certain lens structure or sensor defect based solely on the magenta color. 〔G-CAST-01〕

Try the camera’s lens compensation first, but make sure the lens is supported

The Sony α7 IV's Lens Compensation menu contains Shading Comp. (corner brightness) and Chromatic Aberration Comp. (edge chromatic aberration) options. However, this feature is only available for lenses that support automatic compensation; Sony also explains that the corner darkness of different lenses may not be fully corrected by Shading Comp. When using an adapter lens, first check the compatibility between the lens and the body, and shoot RAW / JPEG for comparison. Do not assume that a certain menu will automatically correct all edge color casts. 〔G-CAST-02〕

Produce matching flat-field frames for color shifts and uneven brightness

Flat-Field Correction in Adobe Lightroom Classic analyzes the calibration frame and processes the lens shading / color cast of the corresponding photo. Maintain the same optical configuration when shooting calibration frames, including the same lens, aperture, focal length, and focus distance; Adobe recommends using a plastic diffuser card. Select the scene photos together with the corresponding reference frames in the order they were taken, and ensure that the first or last frame in the selected sequence is the calibration frame. 〔G-CAST-03〕

Perform Flat-Field Correction in Lightroom Classic

In the Library module, select the on-site photos and corresponding calibration frames that are staggered in a natural order, and select Lightroom > Library > Flat-Field Correction. You can choose to correct only the color shift, or both color and brightness falloff; processing produces a DNG file. To keep the source files, do not select Delete originals after success in the conversion dialog box. Check the corner color, noise, and cropping range of the processed results before deciding whether to use them in the final output; the correction effect depends on whether the reference frame matches the on-site optical configuration. 〔G-CAST-03, G-CAST-04〕

It is not recommended to guess the cause based on the thickness of the body glass or the sensor microlens.

Edge color cast may be related to the combination of lens, light incidence, sensor and imaging processing; the degree of color shift of a certain adapter combination cannot be inferred from the specification table alone. The statements made in the original manuscript regarding FSI/BSI wiring structures, microlens offset distances, infrared cutoff filter interference, thin glass modification benefits, and fixed compensation values ​​are not supported by applicable manufacturer data and are therefore not intended as general diagnostic or modification recommendations. For serious work, first do an actual pairing test with the target body, lens, aperture, and processing software.