The 17-40L can take photos indoor handheld without flash, but it will often make the camera jump up to ISO 12,800. I'm using a 5D3. If I were shooting indoors a lot and needed a zoom I would consider the 16-35L II instead. The same ISO 12,800 shot at f/4 on the 17-40 would be ISO 6,400 shot at f/2.8 on the 16-35. If you are almost exclusively indoors and still want to shoot handheld with no flash, and don't mind giving up zoom, you'll want to go with a fast aperture prime lens such as the 24mm f/1.4L. The same ISO 12,800 shot at f/4 on the 17-40 would be ISO 1600 on the 24 prime at f/1.4. In this situation you're trading sensor noise for lower depth of field though.