The Sigma 17-40mm f/1.8 DC Art is a fast zoom lens for APS-C cameras. It has a soft bokeh throughout the entire zoom range, which is why I like using it for portraits. I just find the design a bit too large for travel.
Large Aperture
I tested this lens on my Sony A7 III in APS-C mode. This gives my full-frame camera a 1.5x crop, and it works just like an APS-C camera. The lens has a zoom range of 17-40mm, which on an APS-C camera is equivalent to 25.5mm to 60mm. I use this lens for various types of photography, from street to portrait. It has an aperture of f/1.8 throughout the entire zoom range. This is great, because it allows me to take bright photos even at 40mm. I find this especially useful when shooting portraits at night, for example.
Sharp images
Throughout the entire zoom range, the lens produces sharp images with little distortion. Even at 17mm, the edges are sharp, while distortion is normally easily seen with wide-angle lenses. I also find the lens focuses quickly and accurately. I test this with a low aperture: I focus on my cat's eye, and when I zoom in afterward, I can see every individual hair around its eyes. When zoomed out, I use a higher aperture and bring the entire surroundings into focus.
Sturdy design
The lens feels sturdy. At 535 grams, it's not very light, and especially when used with my A7 III, the whole setup feels heavy. If I shoot with it all day, my hands get tired quickly. If you use the lens with a smaller APS-C camera, you'll shoot for longer, but the lens tilts forward easily. The lens has three dials: a focus ring, a zoom ring, and an aperture ring. I find the aperture ring very useful. This allows me to immediately see the aperture I'm shooting at and adjust it quickly.
Silent, smooth zoom
For videography, I also find the 17-40mm zoom range useful. I capture overview shots, but also zoom in on details. The lens has internal zoom, so it doesn't extend as I zoom in. This reduces vibration during zooming, resulting in smooth transitions. The lens also makes no noise when focusing, so I don't hear unwanted background noise in my videos.