How do I apply a low-resolution lens shading map to correct a RAW image?

Yes: first resize the low-resolution lens shading map to the RAW image’s resolution using interpolation, not nearest-neighbor.

Then apply the correction per pixel to the RAW data:

• If the map contains correction gains (an inverse shading map), multiply the RAW image by the map.
• If the map contains the actual falloff/shading values (a direct shading map), divide the RAW image by the map.

In formula form, for each pixel:

• corrected = raw * gain_map
• or corrected = raw / shading_map

Which one is correct depends entirely on how the TIFF map is defined. If brightening toward the corners is needed, the map is usually being used as a gain map.

Because this is RAW processing, do the operation in linear data, before any gamma/tone curve steps. If you preview the result in an editor, a linear workflow is preferable for accuracy.

So the practical pipeline is:

1. Load RAW and shading map.
2. Upscale the map to match RAW dimensions with interpolation.
3. Multiply or divide pixelwise depending on whether the map is inverse or direct.
4. Continue with the rest of the RAW pipeline.