What Is a "Good" Lens?

Last updated on June 27, 2012

Introduction

In this article, we'll take a look at what makes a "good" lens and the various aspects one must look at when making that determination. Not just the technical, but also the optical (and thus artistic) characteristics. While the former can be quantified and measured, it's the latter that's a very personal thing and almost entirely subjective.

Depending on how you look at it, a 50 year old lens can be better than the latest, most expensive and highly-corrected lens simply because it has "soul." For some, only the absolute best imaging properties, meaning the sharpest, most even-performing optics will do. Mechanical properties are also a consideration.

Mechanical Construction

The easiest aspects to describe are the mechanical properties of a lens. What is it made of and what is the quality of those materials? How about the fit and finish? Or the smoothness of its controls?

M system shooters have it pretty good in general. Practically all lenses for the platform - dating all the way back to 1954 - are largely made of metal(s). Brass and aluminum as well as magnesium alloys are what most lenses are made of, be they LTM or M mount. And not just from Leica, but also those from Voigtländer and Zeiss, Canon and Nikon, etc. From the most expensive lens to the cheapest, it's a pretty safe bet that the barrels and especially the innards - are composed of some sort of metal.

LTM, or "Leica Thread Mount" lenses can be adapted with an inexpensive adapter ring that basically screws onto the lens on one side and has an M mount bayonet on the other. This opens up a lot more lenses, from a lot more manufacturers. Life is good.

Fit and finish varies a little more between lenses and manufacturers. What is fit and finish? Borrowed from car talk, it's basically how well the lens is designed and assembled. Does anything wobble? Are there any rattles when you shake the lens? How about the controls - do they offer the same resistance from one end to the other? There's no rule that says, "all Leica lenses are tops while so-and-so makes lousy lenses." Again, M system shooters are pretty lucky here - as most lenses from the top to the bottom are generally of a good fit and finish. You could generalize a bit and say that Leica lenses tend to be of the highest level. All controls are smooth, precise. They feel solid, dense. While there are a couple of lenses that have a rattle (a matter of Floating Lens Element mechanics) most do not make a sound (some folks also notice a slight change in drag on those same FLE lenses - again due to the mechanics involved). The focus and aperture controls are smooth and have an even drag across the range. Detents are precise.

Other manufacturers have very high standards as well, though they generally don't feel quite as refined as lenses from Leica. That's not to say they're not excellent (especially if you're used to mass market DSLR lenses). But some lenses have a distinct "clack" when they reach the first and last apertures for example. Some lenses are made of lighter weight materials (e.g. magnesium alloy) while others are made from brass. We tend to equate weight with quality. A chrome finish Summilux compared to say, a typical Zeiss ZM lens feels... Different. In any event, even the most inexpensive lenses generally have a good feel to them.

Hoods vary a great deal. Most use metal, some use plastic - ironically, it's Leica that uses plastic the most while Voigtländer and Zeiss ZM lenses use metal hoods. Though the reasons why may not be immediately obvious. Leica's hoods vary the most. You'll see a lot more rectangular hoods and are designed to be as effective as possible. Which makes sense if you think about it, as the frames - be they film or digital sensors - are also rectangular. Round hoods are cheap and easy to manufacture - rectangular hoods generally require either molding if plastic or more advanced machining if metal (thus the preference for plastic). Newer Leica lenses utilize machined, rectangular metal hoods - precise, effective and expensive. Zeiss only makes one rectangular hood - the 21/25mm and Voigtländer uses exclusively round hoods. In the grand scheme of things, they're all pretty good. Some are more effective than others, especially if they're made for a particular lens - while others that are more generic or shared with other lenses less so.

Methods of attaching the hoods are also all over the place. Some clip on (with button releases), some screw into threads while others use a bayonet system. Leica even has another variant - the slide-out hood. Each system has its pros and cons and there is no "best" here. Some prefer detachable hoods so they can decide whether or not to use it, making for a smaller lens without the hood. Some prefer the solidity of a screw-in hood while others prefer the quick on/off of bayonet or clip on hoods. In some ways the slide-out hood is perfection - you can't lose it or forget it at home, you can either pull it out or leave it retracted. Though they do add a tiny bit of bulk to a lens and might not be as effective as a regular hood as a trade-off. Again, they're all pretty good.

Lens caps? They're either metal or plastic. Most grab hold of the filter threads and have either edge or center pinch releases and are made of plastic. Older lenses tend to use metal more often and slip on instead (which arguably makes them easier to lose). Can't go wrong with either really. They're one of the most often lost items of photography gear, so maybe plastic isn't such a bad thing - they're much cheaper to replace!

Unlike in the more consumer-oriented, mass market where plastic is everywhere - we enjoy a sweet spot. Small lenses that are built well and feel solid. If used properly and basic care exercised, there's no reason most lenses can't last a lifetime - if not several. Less expensive lenses might have issues crop up a little more frequently on average, such as the mechanics coming undone (aperture blades, ball bearing races, screws backing out, elements de-centering, etc.) or they might not have the same standards of quality control before leaving the factory - which leads to what's known as "sample variation" due to looser tolerances or plain sloppiness. As with anything mechanical - they're all subject to wear, accidents and plain old bad luck. This is one area where Leica lenses tend to fair better over lower cost alternatives; tighter tolerances and better quality control (though they're not immune either).

There's not a whole lot more to say about lenses as far as mechanics, really. They're generally all "pretty good." You could buy almost any lens, new or old, Leica or whomever - and have a well-built piece of glass in your hands. But how does it actually perform? That's the important thing, and what we'll look at next.

Optical Performance

Optical performance is something that varies much more widely. Not only is cost a factor, but also the technology available (and used) in a particular lens. Something to be aware of when you have lenses spanning nearly 60 years!

Technology has advanced in many ways over those years. While the laws of physics haven't changed, things such as coatings and advanced designs through the use of aspherical and floating elements (for example) have. The ease and cost of manufacturing these advanced lenses has come down as well. One could, as a broad generalization - say that newer lenses are better (on a technical level) than older lenses. That is to say, they're more highly-corrected. What does that mean? Optical flaws, or aberrations - are fewer and better controlled. The flatness of the field (frame) is better - which means that edges and borders are sharper. With the best lenses, performance approaches (or is equal to) the center of the frame. Distortion is less, images sharper and flare handling improved.

This is where we need to make an important distinction, and where things start to become more artistic, or personal in nature... More subjective. While modern lenses are technically better, it's the older, classic lenses - that many call "character" lenses or that have a unique "signature." It's exactly because of the particular optical performance of these lenses that gives them a unique look to the images they generate. Modern lenses, in their never-ending quest for perfection lack this character and have been called "clinical" by some.

While you could compare a handful of the latest, highest-end lenses - you might be left thinking, "they're all really, really excellent - from corner to corner." Some people prefer this. Others enjoy and seek out lenses that exhibit much more character... Be it slight softness, a "glow" or "swirly bokeh." Perhaps lower contrast which helps in scenes with a high dynamic range and/or shooting film (though it helps with digital as well). Some love the natural vignetting they get from a lens as a means to frame the subject and add a sense of depth to the image.

So let's take a quick look at some of the optical characteristics of lenses. We won't go into great detail and cover every single characteristic as it applies to optics but we'll go over the most common:

Sharpness is perhaps the most common optical characteristic that one thinks of, when discussing a lens. Is it sharp? Is it soft? Is it just sharp in the center with soft corners and borders, or does it extend out into same? Is the lens soft when shot at/near wide open and sharpen up as you stop down - or is it essentially sharp from wide open? Newer, more modern lenses tend towards the latter, while older or less expensive lenses towards the former. It's easy to get hung up on sharpness but often times, a strong image will be just that - whether or not every last detail in a scene is rendered in the image. You can sharpen your digital images to a degree in post-processing, but images shot on film are what they are. Sharpness is a "hard" characteristic of a lens; it can't be changed after the shot is taken. That is, it's either sharp - or it isn't.

Field curvature is related to sharpness and describes the plane of sharpness in your scene. Sometimes lenses are said to be "soft in the corners" yet it might actually be that the field curvature is more curved than flat - critical focus is not on the same plane throughout the scene. Newer, modern lenses and/or those that are more highly corrected tend to have a flatter field. One type of lens where this applies specifically is with macro lenses, or those used for scientific/documentary work. Accurate representation and even sharpness (especially when shooting documents) is very important in these applications. Though this isn't really the realm of the M system. A lens with a strong field curvature often improves as it is stopped down - if for no other reason than the depth of field is greater, covering up some of this error. So this characteristic isn't necessarily a "hard" one as you can exert some control over it.

Spherical aberration is a more complex performance characteristic of a lens and it too affects sharpness. Essentially, it's seen as not all light rays focusing on the same plane. In an effort to control it better, manufacturers use aspherical elements primarily and Leica has been producing newer lenses with Floating Lens Elements (FLE) as well. Spherical aberration is a bit of a double-edged sword however as it also affects "bokeh" - or the characteristics of the blurred fore and background aspects of a scene. It's good for bokeh, but bad for sharpness more or less.

Distortion is another commonly seen characteristic of lenses. Are straight lines captured as straight by the lens? Or do the lines change, especially towards the corners and edges? More prevalent with wide angles than standard or telephoto lenses, distortion can take generally one of three forms. The first is barrel distortion, where the edges of an image tend to bow outwards (like a barrel). The opposite is called pincushion distortion, and is where the edges of an image bow inwards. With film you're kind of stuck with the distortion as-is but with digital post-processing, either barrel or pincushion distortion is easily corrected. A third type of distortion is more difficult to deal with, and is often called "mustache" or "wave" distortion. It doesn't just simply bow out or in, but varies along the axis. While not impossible to correct, it's often more trouble than it's worth.

Vignetting is typically a darkening of the corners of the image. The cause can be a combination of optical and/or mechanical influences. Optical vignetting has to do with the path of the light rays through the various elements that form the image and how well corrected the optical formula is. Mechanical vignetting is a little more straightforward - basically something intruding into the path of light rays, such as a hood or filter(s). Just how much the darkening intrudes into the frame and how dark it is varies. Your camera, or more accurately - the medium plays a part as well. Film or full-frame sensors show the entire image formed by the lens (and thus all of the vignetting) while a camera such as the M8/M8.2 have a "crop" which chops off the corners and lessens the vignetting visible. While film will show vignetting unaltered, digital bodies can apply in-camera correction to coded lenses. Since vignetting can be measured, you can create a mask to reduce if not eliminate it in the final image. Furthermore, in post-processing digital images you can further tweak correction for vignetting (or go the opposite direction and add more). Therefore vignetting is a "soft" error - easily corrected in most cases.

Chromatic aberration is the last major characteristic we want to mention. It wasn't particularly a problem with film but digital photography has made it more obvious than ever since we can now view our images at 200% or even 400% magnification. Essentially, it's the focusing of different wavelengths (colors) of light at different planes. This is usually observed as red or green fringing around contrast borders (details) in an image. Through careful selection of glass and other elements (e.g. flourite) it can be controlled. Leica has been updating their newest lenses to feature apochromatic correction (denoted by "APO-" in the lens name) to reduce if not eliminate this phenomenon. Related to chromatic aberration is what is often called "purple fringing" - but care should be taken when describing it as it can be caused by other issues; typically the overexposure of highlight areas causing "sensor blooming." That is, not lens related at all.

So, taking into consideration all of these aberrations, what makes a lens better than another? Essentially and in the strictest sense - how well they're controlled/corrected. This is why modern lenses are generally considered "better" than older or less expensive lenses. Corrections complicate the optical design, often much more so, though the use of additional elements, those made from exotic (or at least expensive) materials, additional coatings and other tricks.

But does a highly-corrected, technically excellent lens necessarily make it "better?" Not necessarily.

So What's Best?

As we mentioned earlier, these modern, "perfect" lenses are sometimes considered boring. Lacking uniqueness or character... "Clinical." Some folks enjoy and seek out lenses with vignetting, or high levels of spherical aberration. If you're shooting portraits, the ultimate in sharpness is often undesirable, especially when shooting women. Seeing every pore and blemish detracts, rather than adds to the photo. A gentle rolling off of sharpness into the corners and edges can add to the feel of a portrait.

Bokeh, which we've also mentioned - is a highly subjective and debatable aspect of a lens. While not a new concept, in recent years it has gained popularity among "bokeh aficionados" that love to shoot wide open, all the time - in an effort to maximize the area of the photos that's out of focus (and thus, exhibits the most bokeh). Some go so far as to shoot holiday and other lights at night, even completely out of focus - leaving just nebulous blobs of color.

In the M system, most lenses are built well mechanically and perform well optically - some better than others. But each photographer is unique. What they like to shoot, how they like to present their subject matter, the medium, etc. Some might be restricted by budget, some might prefer only the absolute best. One of the nicest things about the M system is precisely that there are nearly 60 years worth of lenses, running the gamut - from which to choose. Covering every focal length and speed, optically perfect or with a signature. You really can't go wrong with most all of them and experimenting is half the fun as they say.

Only you - the photographer - can really say what is "best" for you. Don't be afraid to buy an old lens (in decent condition) to try it out. You might just surprise yourself and find a new style.

So in a nutshell, then, what does more money get you these days? As with most things, better materials, tighter tolerances (fit and finish) and optically - better correction. That is, more even performance from corner to corner and from wide open to stopped down. Sharper lenses with less distortion, vignetting and other aberrations. We left out lens speed as it's a separate animal. Sure, faster lenses tend to cost more than slower ones - at least well-corrected ones. But in general, what we've discussed here applies to all lenses regardless of speed or even focal length for the most part.

Further Reading

If you're just getting started with photography or new to the M system, we have a variety of other articles that may be of interest: