Fresnel Lens / DO Lens may be light and good (but not cheap!), but there's a reason why it doesn't take off e.g. the canon 400mm F4.
No doubt the images from these fresnel/DO lenses have "L" quality contrast, color and sharpness, they suffer from bokeh which is not as smooth and creamy as the traditional lenses. Previously when I shoot sports using the 400mm F4, I kena many "queries" from my clients asking why the bokeh of my images look kinda "unusual". Thankfully, they only made queries but did not screw me. But after 3 months with the 400mm F4 I sold it away - I don't want one day to encounter a customer who can't accept the images with the "unusual" bokeh. But I must emphasize that the color, contrast, sharpness, etc are there.
Thus when I was looking at the sample images in this thread from the nikon 300mm f4 fresnel, my eyes are not zooming in on the color, contrast, sharpness (it's expected to be great). Instead I am checking out the bokeh quality - unfortunately the samples images are taken in places with clean/non-distractive backgrounds e.g. the image of the goat in the highlands.
I will wait for sample images of subjects in "messy" background, and only then can I judge and make a call on whether this Nikon 300mm fresnel lens is good or not.
My advice to folks who are thinking of getting this fresnel lens: Hold your horses. Wait for images with "messy" backgrounds before you make your decision.
