Lenses Demystified

lenses
Editor’s Note: This piece first appeared in our Summer issue and reflects the products available at the time of publication. To see our coverage of new lenses, visit our Gear/Lenses and News sections.

A camera’s lens, arguably, is more important in shaping the final look of an image than is its sensor. Perhaps this wasn’t exactly true in the era of film, where the selection of different emulsions and the handling of film could have a radical effect on the final image quality. But with today’s cameras, where digital imaging sensors have advanced to a point where it’s hard to find one that produces a bad image, the lens can change the final photograph in an infinite number of ways.

Like a pair of glasses modifies the light that hits your eye, a camera’s lens simply modifies the light that hits the sensor, with the aim of creating either a more accurate image or of creating an image that’s more pleasing than the human eye can see.

The amount of engineering and manufacturing acumen that goes into building a lens is staggering, and along the way designers need to make many choices that affect size, weight, performance and price. There’s also no such thing as a “perfect” lens, one that’s perfectly free of distortion. All lenses are a compromise.

Lens shopping can be fraught with peril, and there are an astonishing number of lenses available, most of which have almost indecipherable names like SP 70-300mm F/4-5.6 Di VC USD and 150-600mm F/5-6.3 DG OS HSM, names you’d never see on a camera. Can you imagine if you had to choose between the Nikon D5 24MP ISO HI 14FPS WP body or the Canon EOS 1DX Mark II MP24 CFAST USB 3 4K 16F? While each of the terms in the alphabet soup that makes up the names of these lenses means something, their meaning isn’t always clear and is often based on marketing jargon.

For example, both of the lenses I listed in my example have image stabilization, with the first one referring to it as “VC” (Vibration Compensation) and the second one referring to it as “OS” (Optical Stabilization). Both USD and HSM refer to the focusing motor systems in the lenses, but even as a professional product reviewer I find that the focusing performance of a lens seems based on the overall design, not just the motor.

There are some terms in lens marketing that are standardized, but they’re often listed in a way that’s designed to make any lens sound optically superior. The optical quality of the glass elements inside a lens are graded based on their dispersion abilities, and they have acronyms according to their design. LD (low dispersion) glass is inferior to ELD (Extra Low Dispersion), which in turn isn’t as good as Extraordinary Low Dispersion glass. While these all refer to the way that light passes through the glass, some of these are the names different manufacturers give to similar-quality elements.

Product listings will often say something like “the lens has one XA element, two aspherical elements, one ED element and one Super ED element,” but is that better than a lens that has three aspherical elements, two ED elements and an XA element, or is it worse? Even we professional reviewers can’t always tell, and ultimately the proof is in the pudding. A lens can have all the low-dispersion elements in the world, but if it doesn’t focus well, it’s not worth owning. The general rule is that a lens that lists all of its high-end optical elements is trying to send the message that the optical quality will be good as a result. More acronyms don’t mean more performance, but it’s a good sign that the pedigree is on display.

While some aspects of a lens’ construction might be up for debate, there are a fundamental set of things to look for when selecting a lens, including the focal length range, the aperture, the weather sealing and more. Here are some of the most important considerations when selecting your next lens.

Naturally, the more advanced the construction of the optical elements, the higher the price of the lens, generally speaking.

Optimal Prime

Photographers often refer to a lens that has only one focal length as a prime lens, regardless of the quality of that lens. Prime doesn’t indicate that a lens is a better quality than any other, simply that it has a single fixed focal length. A lens with a range of focal lengths is called a zoom lens, regardless of the range. You might have a 35mm ƒ/1.4, which is a prime, or an 85mm ƒ/2.8, which is a prime, or a 70-200mm ƒ/4, which is a zoom. Most manufacturers will refer in their marketing to a zoom lens with the term “zoom,” but not all will refer to a fixed focal length as a prime.

The Glass Is Half Full

Many photographers who started shooting in the digital era are unaware of how many different film formats were available over the years. High-end shooters used sheet film (available in sizes like 4×5” and 8×10”) and medium-format film in the 120 format and then larger-capacity 220 rolls. Amateurs had a bevy of choices, ranging in size from the teeny, dreadful disc film through 110 film and including some historically overlooked formats such as 828, 126 and finally APS.

Every format required a different camera system and different lenses, with almost no interchangeability. You couldn’t put a 35mm film (also called 135) lens on a 110-format camera; there was just no place to put it and no connector for attachments.

Today’s variety of sensor sizes is tame in comparison, and actually allows for some overlapping of technology. The common 35mm format has come to be known as “full frame” though that’s a bit of a misnomer since a medium-format sensor is every bit as “full” frame as a 35mm sensor.

To maximize the value of their lens inventories, digital camera manufacturers used the same lens mounts for APS cameras as for their 35mm offerings, and so an APS digital camera can use lenses designed for 35mm sensors but only derives the image from a portion of the image circle of that lens. Most manufacturers also make APS-specific lenses, which can be smaller and lighter than those designed for 35mm (all else being equal) because the smaller sensor requires less overall glass. (Think of it this way: Eyeglasses for a rabbit would be much smaller than eyeglasses for an elephant, because the elephant’s eyes are bigger and so more glass is needed to cover the imaging area of the elephant’s eye than the rabbit’s eye.)

When Olympus introduced the Micro Four Thirds sensor, which is about half the size of a full-frame camera, it further shrunk the optical requirements but has required all new lenses. (From here on, I’ll refer to the 35mm sensor size as “full frame” to avoid confusion with lens focal length discussion.)

The variety of sensors available to the average shooter has confused the naming of lenses and complicated discussing their features, for a number of reasons.

The first reason is that the focal length of a lens isn’t a measurement of the length of the lens but a measurement of where the light rays meet up to create a sharp image. The closer the rays of light focus, the “wider” the scene. A lens that focuses at 24mm appears wider than one that focuses at 85mm. You can easily see this effect if you take a magnifying glass and move it away from your eye while looking through it; the subject will appear to get larger relative to the frame the farther away the lens is.

The problem, though, is that most people think of these measurements in terms of a full-frame sensor’s optics. The smaller the sensor is, the longer each of these focal lengths will appear. This is referred to as a focal-length multiplier, and that’s a term we use to standardize lens focal lengths to make them easier to comprehend.

If you tell me that you’re shooting with a 50mm lens, that can mean different things depending on the sensor size. In the full-frame format, that’s a pretty standard focal length for things like portraits and is a length close to what the human eye sees. (The human eye would see around 43mm if it were a full-frame sensor.) This is a “standard” lens (we’ll come back to that in a minute) on full frame, but that focal length on an APS camera is around a 75mm lens in full-frame format because the smaller APS sensor crops out part of the full-frame imaging area. If you imagine taking a crop of an image in Photoshop, you’re doing the same thing, throwing away some of the actual imaging area.

Lenses designed for APS, though, don’t crop out any of the imaging area, so they’re listed in their actual focal lengths but then often referred to in marketing with their “35mm equivalent” focal lengths. For example, a lens that’s designed for APS-C that’s 24mm might say “36mm equivalent in 35mm format.” Confusingly, most manufacturers don’t list what a full-frame lens would function like in APS format—you just need to know the crop factor of the sensor. For most APS sensors, that’s between 1.5 and 1.7, so simply multiply the full-frame length by 1.5 to find out your equivalent focal length.

Micro Four Thirds has a similar naming issue, since the sensor is half the size of a full-frame model. Simply take the focal length of an MFT lens and double it to find its full-frame equivalent.

Open Wide

Another set of terms that describes a lens is related to both the focal length and the way the human eye sees, as I mentioned above. Anything that’s a focal length around what the human eye sees is considered “standard” or “normal.” That’s usually considered around 35mm-70mm in the full-frame format.

A wide-angle lens has focal lengths between around 20mm to 35mm; ultra-wide is from about 14mm to 20mm. On the other end, a medium telephoto lens is from around 70mm to around 135mm and a telephoto from around 135mm to 300mm. Anything “longer” than 300mm is super-telephoto.

The wider the zoom range, the bigger the lens, all things being equal. With lenses, though, they’re often not equal, and some lens-building tricks can be used to keep the size of the lens down, many of which drive up the price.

There isn’t a set rule for how the focal length of a lens affects price, but in lenses you can use the following rule: wide zoom range, high quality, small size, low price. Pick any three.

Fixed Or Variable

There are two more design choices that play into the functionality and price of a lens. The first is the “maximum aperture,” which is the ƒ-stop number that represents the widest aperture possible with the lens. The wider the aperture, the harder it is to construct the lens, all else being equal, so an 85mm ƒ/1.4 would generally cost more than an 85mm ƒ/2.8, if the internals are of the same quality.

The other aperture-related lens design issue that plays a factor in both performance and price is whether the aperture of a zoom lens is fixed or variable—in a variable aperture lens, the widest aperture is only possible at the widest zoom length. It’s more difficult to create a lens that has a fixed, wide aperture across the whole zoom range, so variable aperture lenses tend to be found on the lower end of the price scale.

If you’re shooting landscapes, it might not matter to you that your zoom has ƒ/4 at the widest setting and ƒ/5.6 at the longest end of the telephoto setting—after all, you probably are fine if all the trees and mountains in a shot are in focus. If you’re shooting portraits, though, it might be problematic to lose some of the background blur at the longer setting.

If there’s a single aperture listed after the focal length, it’s a fixed aperture zoom. A 24-70mm ƒ/2.8 lens would be able to shoot at ƒ/2.8 at any of the focal lengths. A 24-70mm ƒ/4-5.6 would have a widest ƒ/4 setting at the 24mm end and ƒ/5.6 at some point approaching 70mm.

Weather Or Not

One thing that’s of particular importance to photographers working in the field is the amount of weather sealing on the lens. Studio photographers don’t often encounter rain or dust, but for everyone else, inclement weather is part of the gig. Higher-end lenses often have more robust weather and dust protection, and that’s part of what factors into their size and their price. Be sure to check the specifications to see if the lens can stand up to your shooting environment.

All Things Considered

There are a number of little considerations that go into the quality of a lens and that factor into the price. Small things like the design of the focus ring or whether a lens has an external aperture dial can make a lens much more or much less useful in the field.

The market for lenses is crowded and complicated. Armed with an understanding of the functions and features of a lens as well as a good mental picture of the type of shooting the lens will be used for can help ensure the right choice is made and the perfect image is captured.


The Lowdown On Low-Dispersion

  • Low-Dispersion (LD) glass elements, used in Tamron lenses, are made from special proprietary glass materials with extremely low dispersion indices (an inherent tendency for glass to disperse or separate light rays into the colors of the rainbow). LD glass, in all its various incarnations, makes sure the path of light through the lens is concentrated and stable for the best possible color accuracy, image brilliance, sharpness and contrast.
  • Extra-Low-Dispersion (ED) glass elements can be found in lenses by Nikon, Olympus, Pentax and Sony.
  • Ultra-Low-Dispersion (UD) glass is a special type of optical glass developed by Canon whose properties nearly match those of fluorite. Fluorite, which is crystalline, has abnormally low refraction and dispersion characteristics, which optical glass can’t achieve. The effect of two
  • UD glass elements gives almost the same effect as one fluorite element.
  • Extraordinary Low-Dispersion (ELD) glass was developed by Hoya (the glassmaker) and is currently used in various Sigma lenses. It has even lower dispersion characteristics than Special Low-Dispersion (SLD) glass, which Sigma has been using in its APO lenses (and some non-APO lenses as well) for some years now. Like the other types of low-dispersion glass, both types improve color accuracy, image sharpness and contrast by minimizing chromatic aberration.
  • Super-Low-Dispersion (SD) glass is the special proprietary glass used in select Tokina lenses. These elements, with extremely low dispersion characteristics, minimize the secondary spectrum or optical noise caused by chromatic aberration, which can result in color halos or “purple fringing” around details in your photograph.

2 thoughts on “Lenses Demystified

  1. Confused and confusing. “The optical quality [sic] of the glass elements inside a lens are [sic] graded based on their dispersion abilities…” Not true, as the writer makes clear when he states that knowing the dispersion characteristics of a given lens’s elements tells you little or nothing about its overall optical quality.

    Most amateur photographers know virtually nothing about any aspect of photographic technology — especially lenses. The amount of blather they post is amazing. This article could have been a clear, easily understood introduction to how lenses are designed and used. Instead, it is a near-incoherent mess that’s a waste of the readers’ time. If you say “Write something better”, I’ll be glad to. How much do you pay?

    PS: “The proof of the pudding is in the eating,” not “The proof is in the pudding.” Good grief.

  2. Hi Grizzled. Sorry you are confused. The phrase “the proof of the pudding is in the eating” dates back to the 15th century, and the first recorded version of it was in 1605, reading “All the proofe of a pudding, is in the eating.” That phrase lost the old-English “proofe” and the comma sometime later. In the 1920’s the phrase in America became “the proof is in the pudding” and has been in use commonly since the 1950s in that form.

    We feel comfortable using a proverb in the form in which it has been the accepted common usage for ore than sixty years.

    Pudding has also changed meanings since then. When the proverb originated, pudding referred to a meat-based dish, bearing more resemblance to a meat pie (think Yorkshire Pudding) than today’s dessert pudding. We’re also comfortable accepting that pudding today isn’t the same thing as it was in the 1400s.

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