This is something I have had to tackle myself recently and the answer is as interesting as it is sometimes vague.
A cinema lens is sometimes the same recipe optically as a stills lens, such as the IRIX or Sony ranges, but this is only one interpretation of the cinema lens dynamic. If this is the case, generally the needs of cinema users come first, the stills lens gets superior optics, but loses the cine lenses mechanical benefits.
Possibly too perfect from the IRIX cinema macro, but that can be tamed and like other cinema lenses, it shares the desired optical qualities of smoothness, clarity and gentleness.
Regardless they are mechanically divergent, with manual (only) focus throw that is usually a lot longer than a stills lens and a click-less and step-less aperture ring that is measured in “T” stops* often with a lot of rounded blades for smoother focus transitions. Bokeh for these lenses is not an occasional thing, it is as important as the sharpness and contrast of the optic.
The same but very different. The Lumix S is a feather weight which is both good and bad, but the cine lens is a solid metal brick (the Lumix feels like a toy in the other hand), helping with hand holding and general heft. In more specific terms, the Lumix does the job efficiently and reliably, the 7Artisans is more tactile, engaging and refreshingly difficult to use. The Lumix does however have some cinematic traits, being part of a set of matched lenses in a new trend to bridge the gap.
Where they are generally different, is in lens set consistency. The focus and aperture rings all line up and are toothed for attaching to follow focus or aperture rigs, the filter threads are often the same, the colour is close and other optical characteristics are shared. This is even the case with cheaper ones, except that they sometimes vary in colour or flare characteristics, but not by much.
Part of a set of three, these are not all the same in overall length, but the two toothed rings are identically placed and the three lenses are close to the same weight and center of balance for gimbal and rig balancing. These are “budget” lenses so some characteristics are not perfectly matched, such as colour (the 35mm is obviously warmer in tone, but processes to be in line easily enough) and the focus ring resistance is a little different, but this is irrelevant on a follow focus unit.
7 Artisans Vision series and Sirui Nightwalkers are a case in point. Each set is matched mechanically, but colour varies slightly lens to lens, some being neutral, some warmer (they can actually be intermingled for better consistency, but then you lose other consistencies).
Optically, cinema lenses are generally well corrected, with flare, distortions, vignetting, chromatic aberrations and Bokeh (blurring) focussed on, but sharpness can be surprising.
They are sharp, more or less, some even top tier sharp by any measure, but there are some lenses that are deliberately less than ideally sharp as modern stills lenses unless you value beauty over clinical perfection.
The file above was shot on the S5 with the Spectrum 50 T2 (at T2.8) as a test of the quite excellent and cheap Neewer 4 stop ND I bought for my mat box. I used Flat profile and no processing was applied. The combination of the profile and lens has created a gentle looking file with a huge amount of room for post. It is like the lens adds a half a “Flat” profile on its own. Something else evident here is the added stability the heft of the lens adds.
A single frame with a colour grade.
Perfection is not very cinematic, not very characterful, but more importantly, it is not “invisible” or natural looking. Perfection tends to take attention away from the subject and place it back on awareness of the process.
One of the great ironies of modern video creation is the trend to “take the edge off” digital perfection using filtering, post processing or applying lenses with less than perfect sharpness (or sometimes all of these).
The two lenses below, the Lumix S 50 f1.8 (left) and 7 Artisans 50 T2 Spectrum (right) were shot as was, with no post processing and deliberately taken in harsh light at F/T 4 the “cinematic” aperture, to promote invisibility and balance the scene.
The Lumix lens is responding to the high contrast scene with sharpness, high contrast and blown out highlights, but very modern Bokeh. The cinema lens is lower in contrast and has less aggressive blurring, but it is still sharp.
Types of sharpness are important especially when you put a multi thousand dollar price tag on them and this is the thing directors and cinematographers are very aware of.
Anamorphic lenses in particular have issues that would make them unworkable as stills lenses, but these built in problems are actually much sought after in cinematography. Designed to “squeeze” the footage then “de-squeeze” it out again after to allow super wide screen coverage (off film originally), they have unusual flare and Bokeh effects often at the expense of superior sharpness and require “de-squeeze” capabilities in either the camera or processing.
I guess to sum up, a cinema lens is a lens that heroes natural rendering with smooth and non-distracting optical characteristics and an ideal mechanic design for full manual control. Unlike a stills lens, it is not out to impress, but to be an enabler for the overall process.
The lens needs to make the process invisible to promote viewer immersion and above all, it needs to make the image attractive in a gentler sense.
No compromises are reached to achieve this, often meaning they are big, heavy and expensive, but not always. This also means though, that decent lenses can be had for little outlay. Removing weight, high contrast and sharpness and auto focus alone takes away the biggest expenses of a lens.
*”T” stops are basically more accurate than “F” stops for lens to lens and metering consistency.