Zoom lenses: good vs. not good vs. both

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This simple feeling… is beyond V’ger’s comprehension. No meaning… no hope… and, Jim, no answers. It’s asking questions: ‘Is my kit zoom lens good enough?’

In reality, we actually know little about zoom lenses except that the best ones (from a numerical standpoint) are very large, heavy, and expensive. Once you move into the enthusiast and kit versions, the question of whether or not they are good (or, more to the point, useful) is complex, subjective, and somehow optimistic.

General limitations

The struggle of zoom lenses, since basically forever, is designing a multifocal, focus-maintaining lens that is at least as good as any lens of the focal lengths covered, without being massively heavy or unimaginably expensive. This struggle is driven by four conditions of design, manufacturing, physics, and software.

  1. Design. Fixed focal length lenses have an inherent advantage because they are always going deliver high performance at low prices. Such lenses require computations at one focal length, have fewer parts, need less assembly labor, and require less glass. A zoom has to be good at a theoretically unlimited number of focal lengths between two extremes and has to maintain focus as it focal length changes.
  2. Manufacturing. The difference between a good lens and a great lens can be 0.01mm. Zoom lenses have numerous glass and precision molded plastic elements that have to work in formation at an infinite number of focal lengths between two extremes (say 14mm and 24mm). It is more difficult to make larger-diameter glass elements with great precision, and the more mechanical linkages exist in a lens (for example, ones that maintain focus through focal length changes), the more tolerances add up. Sometimes low moving mass and “slop” is built in to make lenses focus faster.
  3. Physics. More glass means more flare and dispersion, and zoom lenses have tons of glass. Flare can be mostly tamed via muticoating, but even so, dispersion adds up with the element counts. A 13-element lens with modern multicoating (losing ~1% per air-glass surface) can have a total loss of 25% of all the light coming into it.
  4. Software. This enters the picture in two ways: focus correction and image correction. On DSLRs and some mirrorless cameras, the AF Fine Tune function helps correct focus errors that occur with particular lenses and phase-detect autofocus. The difficulty with zooms is that the nature of focusing errors can change with each focal length, and dialing in a correction for one focal length for a lens can greatly improve images there but degrade images shot at other focal lengths. The second limitation arises in software correction of lens aberrations (distortion, vignetting/falloff, and sharpness). One cheat (or innovation, depending on how you look at it) is to let the camera make corrections that the lens design itself does not permit. This provides more freedom to design smaller, lighter, and cheaper lenses. But you can’t really reconstruct data that isn’t there – or bend it infinitely.

Why many enthusiasts have been suspicious of zooms

It mainly seems to be a thing with people 40 years old and up, who remember the bad old days. As to the history, in the 1970s, optical correction was not what it is today, and zooms got a really bad rap because things like the 43–86mm Nikkor were convenient but not optical superstars. The original zooms were two-touch, which allowed the easy setting of focal length and focus with two separate rings. If a lens mostly held focus as you changed focal length, it was a true “zoom;” if not, it was a varifocal (Nikon is, and long has been, an offender in calling varifocal lenses “zooms”). Zooms of that era were difficult to design, and it was a time where lens design was transitioning to more computerized methods. They worked for a lot of purposes, but given the natural male inclination to over-spec and compete with equipment, they were not taken very seriously.

In the late 1970s, manufacturers went to one-touch, where you could adjust focus and focal length with the same grip. The temptation was to conclude that you could just re-zoom, re-compose, and fire away, but the reality was that focus drift followed focal length changes, and if you didn’t bother to refocus, you could get slightly soft pictures. One-touch zooms also suffered from zoom creep: eventually, as the lens loosened up, pointing the camera up or down would cause the zoom mechanism (governed by front-back movement of the ring) to move on its own. This too, did not help perceptions, though there are some very good zooms of the 1970s and 1980s, including some third-party offerings like the Vivitar Series 1.

SLR manufacturers rediscovered the two-touch in the 1990s, when it became an advantageous design for autofocus lenses (an AF motor could turn a focusing ring but not also a zoom ring). And that is when they all backslid into selling varifocal lenses as “zooms;” the assumption being that the camera’s AF would correct the focus anyway. Although questionable from a marketing standpoint, autofocus helped assure that the new “zooms” would  be

The rise and fall of zooms (1999-2005)

If zooms had a heyday, it was from the late 1990s to the mid-2000s. Several things came together to make this happen:

  • Vis-a-vis prime lenses, zooms were more heavily telecentric than prime lenses. In simple terms, their design created the straight-on light rays that digital sensors like.
  • Advances in lens coating took away some of the performance penalties of using a large number of lens elements for image correction.
  • Prime lenses were getting little in the way of updates. This meant that the best ones were standing still, and others did not work as well as zooms with digital. Consider that it took Nikon 50 years to update the formula of its 50/1.4.

The prototypical lens of this era was the AF-s 17-35mm f/2.8 Nikkor, which was designed for the D1 cameras but was usable with contemporary film cameras too. This lens outperformed most primes within its focal length range, was solid, fast-focusing, and very  popular.

But just as every pendulum swings, the 2010s to the present are where the optical (but not necessarily total) performance level of cheap zooms took a little bit of a dive.

  • In a market with softening demand, maintaining competitive MSRPs for entry-level cameras and lenses required simpler and cheaper designs.
  • The processing power of digital cameras increased to the point where it became possible to correct for distortion, light falloff, and sharpness in-camera.
  • Increased emphasis on video, especially from mirrorless, demanded lenses that could focus quietly and continuously, driving toward lower moving mass.
  • The move by the market toward camera phones meant that the “burden” associated with separate cameras had to be minimized.

In other words, the ethic was (and is) using technology to make cheaper, lighter, and easier-to-make lenses acceptable, not so much to make good lenses better.

At the high-end of the lens lines, updated primes also began to exert pressure on the more expensive zoom lenses, especially where trends push toward small and light.

What is the quick and dirty way to identify “good” zooms?

When shopping for a zoom lens with high potential, this is the general hierarchy to predict (with some but not total certainty), where a lens fits on the performance curve.

— By effective aperture

  • Constant f/2.8 aperture – this generally means a pro-level lens. It also means big, heavy, and expensive.
  • Constant f/4 aperture – this is the high-end amateur or lower-end pro zoom. It takes a lot of engineering to keep the aperture constant on a zoom, and this type of lens generally has the best balance of performance, weight, and cost. Canon and Nikon both make this style of lens. These are not cheap, but they are much easier to live with than monster pro zooms.
  • Variable f/2.8-4  or 3.5-4.5 (1 stop aperture shift) – this lens type of lens generally has been optimized for compactness over speed. F/2.8 is really a bragging right; it’s only half a stop faster than f/3.5.
  • Variable f/3.5-5.6 (or f/6.8) (1.5+ stops) – when a manufacturer lets the maximum aperture float this much, it is generally an indicator that you should not be expecting world-beating performance. But you will get a lens that will do well in most circumstances and not break your back or budget in the process. These are actually the lenses that are fun to shoot with.

— By zoom range

This may sound deceptively simple, and maybe it oversimplifies, but it is a fairly good bet that where a modern lens has a zoom range of greater than 2.5-3x from short to long, it is is probably a “convenience” zoom rather than one oriented toward absolute performance.

Can “not-good” be good?

Yes. Despite having performance a notch or two below pro lenses, kit zooms can be quite good within some limits. First, in an era where photos are overwhelmingly likely to be shared on social media, and not printed, kit zooms are actually complete overkill. In fact, anything beyond an 8mp iPhone 6 might be complete overkill – the same way that a lot of expensive pro equipment in the 1970s was used by amateurs to generate 3.5×5 inch prints.

Second, even if you print, you only need about 6mp of real-world performance out of a lens to print a nice 8×10, which again is bigger than most prints made today. “Real world performance” means type of system resolution that DxOMark measures (lens performance plus body performance, moderated through focus accuracy). That might be a 12mp body and a midrange lens.

Third, you will still get really good results – though optimum performance may be ~3 f/stops from wide-open (f/8 vs. f/3.5 maximum) as opposed to (f/4 vs. 2.8 maximum). There is an old adage, “f/8 and be there,” but in reality, once a lens is stopped down even to f/6.3 (let alone 8), even a plastic meniscus lens will have performance approaching an expensive coated lens. Like vampire tears, the pro-lens advantage evaporates in bright sunlight.

Finally, especially for mirrorless cameras, some lenses have video performance that vastly outweighs whatever perceived deficiencies they have for other purposes. For example, you might conclude that the 16-50mm f/3.5-5.6 power zoom lens that comes with a Sony a6300 is terrible. It’s not terrible for still pictures (in no small part because it is one of the fastest-focusing lenses ever invented), but it really shines in video, where it can silently and reliably track moving subjects without introducing noise. And it’s also tiny.

Conclusion

To wrap up: the performance, utility, and fun factor of zoom lenses is actually pretty subjective. Try as many as you can. Pick the one that you like the best, so long as it does what you need it to do.

 

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