Tamron SP 35mm VC f1.8 v Sigma 35mm f1.4 Art DG HSM
At maximum aperture, and a focal distance of 10m, the Sigma has distinctly sharper corners, with resolution dropping off rapidly in Zone D, but substantially more vignette. The Tamron has a more gradually tapering performance both in terms of vignette and resolution, but isn’t as sharp in Zone D. Both have moderate purple/green CA wide open. The Tamron is slightly sharper in Zones A-C up to f4.
At f4, both lenses comparable across the frame, but the Sigma has a sharper Zone D with more vignette. CA less, but still present, especially with Tamron.
At f8-11, Tamron acceptable across the frame, with no vignette. Sigma equally sharp, but stronger vignette and extreme corners visibly wilting. CA more of a problem for Tamron here. Field curvature different.
Bokeh is very good with both lenses, but Tamron is slightly onion ring. Sigma is textbook and vice free. On balance, Sigma slightly smoother rendering overall.
Win for Tamron by virtue of a larger image circle. Certainly more suitable for GFX as a viable 28mm FOV-equivalent – plus bonus VC* – at the cost of half a stop. Sigma superior at wide apertures in Zones A-C if vignette permissible, or 35mm crop activated.
Tamron SP 45mm f1.8 VC v Sigma 50mm f1.4 Art DG HSM
At maximum aperture and a focal distance of 10m, vignette is similar and definitely present. Tamron slightly brighter and resolves better in Zone D, with less spherical aberration. It is also somewhat sharper across the frame.
At f4, the gap is similar, with more CA evident in the Tamron SP. Correctable. Resolution unacceptable in extreme corners (Zone D outer) but Tamron still superior overall.
By f11 the Tamron reaches a barely acceptable standard in Zone D (corner), whereas the Sigma Art falls slightly behind it. Vignetting is now similar.
The larger aperture and longer focal length of the Sigma Art combine to give a more attractive bokeh rendering from a similar shooting position. However, this is a function of specification, not design. For instance, any 58mm f1.2 would be correspondingly more attractive than the 501/4. However, like-for-like, bokeh of these two lenses is of similar smoothness and character.
Win for Tamron by virtue of a larger image circle showing less spherical aberration in the GFX corners.
*: Effect of combining adapted lens VC and GFX IBIS are TBC