The bokeh of a lens is by some described as an esoteric lens trait. I’m not one of those.
I rather select lenses with ‘good’ bokeh over ones with ‘bad’ bokeh and try to find out the aperture settings where my lenses exhibit the better kind. This lead to the finding that my cheap old Tamron AF 90mm/2.8 Macro I picked up used for €139,- renders out-of-focus highlights more pleasant from f/4 to f/8 than my Nikkor 85/1.4G at that apertures and battles it for sharpness as well. Tamrons shouldn’t be that good if you ask me. 🙂
Now if you look into photography online forums and read topics concerning bokeh you realise the term ‘good bokeh’ and ‘bad bokeh’ are sometimes disputed.
Paul van Walree offers an extensive technical explanation of bokeh on his site. In layman’s term, lenses with ‘good bokeh’ melt out-of-focus parts of a picture into soft tones like a watercolour painting and show light sources or reflections in a uniform fashion without outlining.
Some lenses where specially designed for smooth bokeh like the Nikkor AF 105mm/2D DC, Nikkor AF 135mm/2 D DC or the phenomenal Minolta 135mm/2.8 [T4.5] STF.
While Nikon opted for adjustable lens elements in the Nikkor DC lenses to alter the lens spherical aberration correction for controlling foreground and background bokeh, Minolta used an apodisation filter in the 135mm STF. Minolta/Sony won the bokeh-king title with the 135mm STF but this lens is manual focus only. To make up for that and leave Nikon and Canon shooters green with envy Sony offers the Zeiss Sonnar T* AF 135mm/1.8 ZA for a mere €1900,-.
Sure as a Canon or Nikon user can choose the EF 200mm/2L IS USM and Nikkor AF-S 200mm/2G ED VR known for their great out-of-focus rendering but apart from their substatial size and weight you might have to sell your car and take the bus to fund one.
Less well-heeled photographers longing for pastel like background rendering in their images might look for a used Canon EF 200mm/2.8L USM or Nikkor AF 180mm/2.8D that give much of the glory for a fraction of the cost, size and weight.
Fast 85mm lenses
Fast sharp 85mm lenses also known for their great bokeh like the Nikkor AF 85mm/1.4D, the newer Nikkor AF-S 85mm/1.4G and of course the Canon EF 85mm/1.2 USM LII or Sony’s flavor of the Zeiss Planar T* 85mm/1.4 ZA might be equally out of reach of most hobbyists budget but their slightly slower and way more affordable brethren like the Nikkor AF 85mm/1.8D, AF-S 85mm/1.8G and Canon EF 85mm/1.8 USM are not much behind. At 1/3 of the price you lose 2/3 of a stop while still getting 95% of the overall quality. In case of the Canon the cheaper lens will also offer significant faster AF.
I haven’t used the new Nikkor AF-S 85mm/1.8G or the Canons but the Nikkor AF 85mm/1.8D stopped down between f/2.5 and f/4 leave not much to desire. At wider apertures it can’t compete with their faster counterparts though.
Lenses under 85mm focal length
Lenses with shorter focal length like most 50mm lenses struggle a bit to archive a good bokeh in some situations and you can even make the ‘mighty’ Noct Nikkor 58mm/1.2 or the Nikkor AF-S 35mm/1.4G look bad if you don’t take some care of the background. Lenses improve not only in resolution and contrast if we stop down a little but also their out-of-focus rendering gets more smooth. I found that especially true for fast wide-angle lenses.