Another example is if I focus on something and then zoom in and out some lenses have to refocus and others maintain focus. This is an important feature for video and any kind of fast shooting. Many lenses have much more "focus breathing" than others. That is, the change of the effective focal lens as the lens focuses closer and closer. I had a lens that was 200mm but it was really only at infinity. At close focus is was closer to 135mm since the focus breathing was so pronounced.

Bokeh, chromatic/comatic abberations, geometric distortion are other things I can go on and on about.

It's been a long time since I paid any attention to pro photographic equipment so I don't know if fast lenses are important with modern sensors performing so well in low light. But back in the analog/early digital days 2-3 stops was a huge advantage.

Also something to consider, the pro lenses also focus faster and more accurately. I was never sure if this was down to the software in the lens, which is proprietary and why 3rd party lenses are never as good as manufacturer. I could easily see Canon or Nikon tweaking the firmware on prosumer lenses to focus a fraction slower.

IIRC, the pro lenses (white Canons) focused faster as a result of them letting more light in, as well as having faster motors (USM), though mid-range lenses had these as well.

Around that time, the body also made a big difference since the 1D-series had ~45 AF points, and the lower and mid-range bodes had 9-20 AF points. Not sure if this is still relevant but I think even low end bodies have a ton of AF points these days.

Especially when it doesn't have a nice manual focus ring à la the Olympus m43 lenses.

More importantly, those things combine to make a certain je nais sai quoi that makes photos taken with it seem to have much higher quality. I took a quick test shot of my dog when I got it and my sister remarked that she could practically feel the dog with her eyes, just looking at the photo. Nobody has ever said that about pictures of the dog that I took with the old lens, even if I spent considerable time trying to get lighting right and the dog to hold still.

My old lens ruined a bunch of shots I took in a forest that required running at the extreme edge of its settings. This new lens doesn't look like it will do that.

Actual light transfer (e.g. t-stops in cine, but less quantitatively in SLRs, say - some lenses are brighter than others), sharpness, chromatic aberration, spatial aberration, focal speed and accuracy (assuming autofocus), consistency across the telephoto range (not for primes, obviously) - some lenses will have not just differet f-stops at different lengths, but really different characteristics. This doesn't even get into the subjective aspects (e.g. "bokeh") that some people care a lot about.

Is it brighter for the same camera body, aperture and depth of field?

If so, what would one look for in a lens's specifications to describe this difference?

Which makes it bigger and heavier, harder to manufacture, and smaller market - all of this pushes prices up (an order of magnitude is common)

As for relative aperture adjusted for actual, measured light transmission, that's a T-stop, but these are typically only marked and specified for cine lenses, e.g.,

https://www.zeiss.com/consumer-products/int/cinematography/m...

(IIRC, the Master Primes are geometrically f/1.2 at T1.3)

the maximum f-stop of the lens describes the ratio of sensor length (1D, diagonally is best comparison probably), to the diameter of lens aperture. because the opening and sensor are two dimensional, every sqrt(2) increase in f-stop describes halving the light. for example, a f 1.4 lens can allow ~twice as much light as an f 2.0 lens. Every doubling of light is considered a "stop", like {1.4, 2.0, 2.8, 4.0, 5.6, 8.0}. An important consideration is that this number describes a ratio, not the absolute opening in the lens without respect to sensor size.

One possible conversion method is to consider the sensor size of the system against some popular standard. For example, "aps-c" sized sensors are about 1 stop "slower" than full frame sensors, due to being 1/(2.25) the size of full frame sensors in area, or 1/(1.5) linearly. This means that many lenses can be converted not only on focal length, but also for f-stop, to estimate the total light gathering power of the system, and the output "look" of the image. Full frame sensors (~36mm x 24mm) are most frequently used in the photography world as a standard comparison point. For example, a 50mm 1.4 lens gives 50mm 1.4 on full frame, but that same lens on an aps-c sensor (like a fuji X, sony 6xxx, canon 7d, etc.) would not only give a cropped 75mm [full frame] focal length equivalence, it would also give the depth of field of a 75mm _2.0_ lens [on full frame], not 75mm 1.4. At the same time, the exposure would still benefit from the light gathered at f1.4. This is why many say that smaller sensors give a larger depth of field (more in focus, less out of focus background). In reality, this is due to the lens aperture being measured only as a ratio to the sensor size. Finally, the smaller aps-c sensor will give a larger depth of field at the same f stop, but may also give higher noise at the same ISO (to a similarly advanced full frame sensor). This means that an aps-c camera takes the exact same image when it has ~1 more f stop, ~33% less focal length, ~1 stop lower ISO to the full frame system. A big confusion around any of this "equivalence" discussion is that sometimes people mean equivalence for lens "look", and other times, they mean equivalence for exposure, while others may be talking about absolute image quality and noise. They're actually all related, though much of this discussion isn't important to real world photography.