Originally posted by WorksAsIntended About usage of fresnel lenses: Using fresnel lenses allows for less mass and volume but you get diffraction issues at the lenses costing you optical resolution.
You are speaking of Diffractive optics? Canon and Nikon have such lenses in their lens line-ups and while you do have savings in terms of weight and mass, the losses in optical quality can be made up by simply sharpening your images more than you ordinarily would. However Diffractive optics have a disruptive effect on Bokeh
OOF Highlight Bokeh from the Canon 400mm f/4 DO IS Lens [From the
digital picture.com]
Nikon have their own variant of the optical technology referred to as Phase Fresnel, which has similar effects on bokeh rendering.
OOF Highlight detail from the Nikkor AF-S 300mm f/4 PF ED VR Bokeh [Image from
PhotogapyLife]
The upshot of using diffractive optics is that it eliminates chromatic aberration which is inherently problematic with long lenses and makes the optical design need fewer corrective elements than what would ordinarily be needed and therefore reduces the weight of the lens. The drawbacks are images from these lenses are softer than classic refractive lenses and the potential for bokeh artifacts to show up in highlights.