Originally posted by Alex645 flare is usually more visible with smaller apertures, not large ones. Earlier I had written the opposite.
With my own personal experience with this lens this is especially true, the probability of flare increases as you stop down. However there are some lenses where using them wide open also increases the probability of flare - Truth be told flare is a complex phenomenon that isn't easily nailed down with a few rules, it varies from one lens design to the next.
Originally posted by Alex645 APO or ED elements don't really affect flare.
ED element's are still elements, they can contribute to flare just as any element can. But there is no such thing as an Apochomatic element, there are always three elements in a classic Apochromatic triplet to correct the focus of Green,Blue and Red wavelengths of light.
Sigma 18-35mm f/1.8 - all at 35mm, starting from the left, f/1.8,f/4,f/8,f/11