The correct replacement hood for the Sigma 28mm f/1.8 EX DG Aspherical Macro is the Sigma hood part number LH825-03. Search for that part number through Sigma parts dealers or camera retailers.

A generic hood is not automatically a good substitute just because it fits the lens mount or filter threads. Lens hoods are shaped for a specific focal length and lens design, so the wrong hood may either:

• vignette (show up in the frame), or
• be too shallow to provide much flare protection.

If you can’t find the original hood, a lens-specific replacement is the best option. On a very tight budget, you can also use a printable paper/cardboard hood pattern made for this lens, making sure it is printed at the correct scale. Use the version matched to your camera format (full frame or APS-C), since sensor size affects how deep the hood can be without vignetting.