Lens Primer - Advanced Topics
- Optical Aberrations
- Lens Design Elements
- DoF Scales and CoC
- Focus Shift
In previous articles of this series, we discussed both the basics and the creative possibilities of lenses. In this next installment, we'll cover more advanced topics such as optical aberrations, ergonomics, as well as mechanical/optical characteristics and materials.
As alluded to in the first article, there's a lot more to lenses than meets the eye. When it comes to image quality - a lot more. Pick up any modern lens and shoot it stopped down a ways and they'd mostly all appear pretty similar. The aforementioned, "at f/8 every lens is good" notion. But depending on the speed of the lens, the quality of its design and manufacture and the shooting aperture - you might have to contend with optical aberrations, or "defects." Old, fast and/or cheap lenses generally have more than new, slow and/or expensive lenses though this isn't a hard and fast rule. The most common types of aberrations include:
|Type of aberration||Description and effect on image|
|Coma||Off-axis light rays focused as comet-shaped flare|
|Field curvature||Inability to bring into focus an object on a flat image plane|
|Flare||Reflections of lens elements and stray light contamination of the image|
|Diffraction||Spreading of parallel rays of light causing blurring of details|
|Distortion||"Pincushion" or "barrel" shaped curving and distortion of image borders|
|Vignetting||A darkening of the image corners (can be optical and/or mechanical)|
|Spherical aberration||Light rays focused at different points causing blurring of details|
|Chromatic aberration*||Different wavelengths of light focusing at different points resulting in color fringing * Includes lateral, axial/longitudinal, primary and secondary|
Lens flare is both a boon and a bane to photographers. It usually occurs at inopportune times, and can ruin a photograph. Other times it can be exploited purposely to produce flare and "sunstars" as a creative element in a photo. A form of it, called veiling flare - is in fact responsible for the infamous "Leica glow." The more modern a lens typically, the better it handles flare. This is due to improved coatings, lens designs including baffles and careful element layout and grouping as well as effective hoods. Flare can occur when there are bright specular highlights in your scene, that can bounce around inside a lens. An extreme example of lens flare:
Some common defects seen in images include coma - where objects in the borders and corners of an image are smeared into a shape reminiscent of a comet. Another is spherical aberration, where focus is not quite achieved, softening details (diffraction appears similar). Faster lenses often suffer from chromatic aberration when used wider open and is seen especially in extreme contrast areas - a phenomenon that tends to happen with digital sensors in part because they're completely flat. Though not all that appears to be chromatic aberration actually is. Sometimes it's due to the behavior of the sensor itself (receptors, microlenses, detail frequency vs. resolution, bayer pattern, etc.). One example of such a distinction is "purple fringing." Film on the other hand, records light more organically (literally) and has different color layers making up the overall thickness of the emulsion and exhibits it to a far lesser degree, if at all. Black and white film naturally, does not show it in color - but rather as a slight softening of detail. Some extreme examples of coma, spherical and chromatic aberrations, respectively:
When discussing wide angles especially - you'll often hear of distortion. Ideally, straight lines in your scene should appear as straight lines in your image - especially in the border areas. Wide angle lenses that are under-corrected tend to exhibit what's called barrel distortion as other lenses can go the opposite direction and exhibit pincushion distortion. Another common defect and visible in almost all lenses to varying degrees is called vignetting, or a darkening of the corners. Wide angles and fast lenses tend to do this to a greater extent than longer, slower lenses. Extreme examples of vignetting, barrel and pincushion distortion, respectively:
Lenses can exhibit one or more of these aberrations and it is up to the lens designer to choose an optical formula that will correct one or more of these. The designer is often limited by constraints such as lens cost, size and weight and even the desired look or performance of a lens. Often new lenses are based on existing, perhaps even old formulas though will usually include modern technology such as lens coatings. Different aberrations typically require different approaches to correcting them.
Lens Design Elements
In order to correct for a variety of aberrations and to increase overall lens performance and image quality, manufacturers incorporate a variety of design choices. Let's take a look at some common corrective measures:
Aspherical Elements (e.g. Leica ASPH or Aspherical)
Aspherical elements by definition are elements that do not have a truly spherical shape, as is most common. One or more such elements are typically utilized to combat spherical aberration. As a downside, the bokeh is often affected negatively - which of course, is subjective. In the early days, Leica hand-ground and polished these elements, which lead to them being difficult and slow to manufacturer as well as relatively expensive. Modern advances allow for computer and machine-assisted grinding or even molding of such elements which has brought the cost down considerably. Not all lenses, and in fact most - do not feature aspherical elements - and those that do usually mention it in their name. This is most obvious with Leica lenses, which usually have "Aspherical" on earlier lenses or "ASPH" on later lenses as part of the name. Cosina/Voigtländer usually uses "Aspherical" as seen in their 35mm f/1.2 Nokton for example. It is the increased difficulty in manufacturing, cost and availability of these elements that has forced Cosina/Voigtländer to create a second version of this lens even.
The lack of aspherical elements does not mean a lens exhibits any more or less spherical aberration necessarily. The particular optical design might well be acceptable without.
Floating Lens Elements (a.k.a. "FLE")
Floating lens elements (often abbreviated as "FLE") are a group of one or more elements that move independently of the the remaining, fixed elements. The primary reason is to improve lens performance at closer focusing distances and to combat focus shift. One example of a lens using such a system is the newest Leica Summilux 35mm f/1.4 ASPH (often referred to as the "FLE" version). The previous generation suffered from focus shift (more so when used on digital bodies) and the addition of floating elements improves this dramatically - though not completely.
The lack of floating elements can imply that a lens potentially has focus shift, but that's not a given and in fact relatively rare.
Lens Coatings (e.g. Zeiss T*, B+W MRC and Heliopan SH-PMC)
Lens elements and filters can be non-coated, single-coated or typically multi-coated; the latter being generally the case for modern optics. Older lenses and filters may only be single-coated or lack any coatings at all, as is often the case with "pre-war" vintage lenses. Coatings are utilized to combat flare and reflections (a.k.a. "ghosts"). In the case of filters, it is generally advised to use only multi-coated. Lenses on the other hand are what they are; you don't usually have a choice per se. There are exceptions, naturally - you can buy a vintage version of a lens - or its modern equivalent. Voigtländer offers their currently-available 35mm f/1.4 and 40mm f/1.4 Nokton lenses in either single- or multi-coated versions.
Non- or single-coated lenses are often sought after due to their low-contrast characteristics which make them ideal candidates for black and white and digital photography. The latter because of the tendency for digital sensors to "crush blacks" (lose shadow detail) and "blow highlights" (lose highlight detail) especially with high-contrast lenses. Lower contrast lenses minimize this to a degree and can be compensated for during post-processing.
Almost all modern optical lenses and filters are multi-coated; not just on the front and rear elements but also internally - with the exceptions noted above.
Apochromatic Lenses (e.g. Leica APO)
Apochromatic, or "APO" lenses are designed to focus the different wavelengths that make up "white" light - at the same point on a flat image plane. This is done to combat chromatic aberrations, typically with telephoto lenses. Usually such a design is noted in the name, for example the Leica APO-Telyt-M 135mm f/3,4 ASPH lens. Naturally, there are exceptions - the Leica Summilux 50mm f/1,4 ASPH is at least partially an apochromatic design.
There's not a lot to cover when it comes to ergonomics of lenses, but there are a few things worth pointing out. In a general sense, lenses are cylindrical and have both a focus and an aperture ring. Not too much mystery there. How consistent are the look, feel and location of them between a manufacturer's lenses - let alone among different manufacturers though? There's a lot to be said for lenses that you can pick up and use essentially "blindfolded" knowing that the controls will generally be in the same location, function and feel the same way. They should also be comfortable to use and easily controlled without sloppiness in their action. This is the notion behind good ergonomics.
There are a variety of focus aids available among lenses. The most common is a simple ring, usually with knurling to improve grip. Leica took this to the next level, offering a "focus tab" on some of their lenses (though other manufacturers offer them as well). Essentially it's a protrusion of the focus ring that a finger can slip into - affording two primary benefits. The first is being able to tell, sight-unseen - at what range the lens is focused. This is a great boon for street shooting, for example - where zone focusing (focusing by distance rather than through the viewfinder, a.k.a. scale focusing) is used a great deal. The second benefit is the speed at which such a lens can be focused. The focus "throw" (or amount of rotation required) is usually short and can be quickly adjusted in such a manner. One downside is that some lenses that feature such a tab - do away with the normal focus ring and offer just the tab. Here's a focus tab as seen on a chrome Leica Summilux 50mm f/1.4 ASPH lens:
Zeiss, with their ZM line - offers essentially a "nub." Their usefulness is questionable however. While they provide an indication of where a lens is focused, they do not offer the benefit of single-finger control as with a full-on focus tab. In the sense of ergonomics however, Zeiss ZM lenses are very good in that they're all exactly alike (for better or for worse). You can expect the same behavior no matter which lens you pick up and mount to your camera. One reason for this is that the ZM line essentially came out over a short period of time, so they're all similar in design. The ZM focus nub is so small, only a tiny picture of it is required:
The real wildcard among manufacturers is Cosina/Voigtländer, which offers lenses not only with traditional focus rings and focus tabs - but a third type, or focus levers. Somewhere in between the Zeiss nub and the focus tab is the lever. A small protrusion that behaves much like the nub but protrudes roughly two to three times as much. This allows for limited fingertip control, however only in one direction - without having to move your finger to the other side of the lever (or using two fingers to craddle it). From an ergonomic standpoint, and if you have a bag of mixed lenses - this is a nightmare. If you pick and choose your lenses for specific purposes though, it's not a huge issue. Cosina/Voigtländer lenses have been around for some time, which explains some of the variation. But even on new or updated lenses, there's no particular design direction. Here's a typical Voigtländer focusing lever, as seen on the 28mm f/3.5 Color Skopar:
The word "bokeh" is a Japanese word that basically refers to the blur (and aesthetic quality thereof) of the out of focus areas in an image. Being that a large component of it is the aesthetic value, it is therefore largely subjective. Everyone has a different sense of what appeals to them. Some people prefer completely blurred out washes of color to their backgrounds while others prefer to have more definition.
Characteristics of "good bokeh" are generally considered to include a smooth blurring of background details, with highlights that appear round in shape. This is opposed to "bad bokeh" which is often described as "nervous" or "caffeinated." What this means is that out of focus objects might have a sort of double image, or seem to "vibrate." That is, they're not calming or pleasing as smoother, simpler blurring imparts. The bokeh starts to draw more attention than your subject. Another example of "bad bokeh" would be out of focus highlights that appear pentagonal, hexagonal, octagonal, etc. or clearly show the number of aperture blades, especially if they're not rounded. Or stretch into ellipses around the image borders. Another example of bad bokeh is called "swirly bokeh." Certain lenses produce a background blur that seems to encircle the subject, or swirl around.
What you'll notice about the above paragraph are all the quotation marks... As mentioned, bokeh is very subjective. What matters to one person might not matter to the next, if at all. It's important to realize that "most people" (suggesting the average layperson) that looks at your photographs will likely never even notice such details. A photo, to them - is simply, "nice." So don't get too hung up on the concept of bokeh - while it's an artistic concept, a lot of it is rooted in technicalities. There are some people that enjoy railing against the norm and seek out "bad bokeh" while others refuse to shoot at apertures less than wide open if at all possible to create the most pronounced effect possible.
So what contributes to good or bad bokeh? Bokeh is entirely the product of an optical design and the choices the lens designer made. Lenses normally criticized for having spherical aberration are usually the ones with the most pleasing bokeh oddly enough. it's the correction (or more accurately over-correction) of spherical aberration through the use of aspherical elements that tends to make background details more defined; often at the risk of becoming "nervous and caffeinated." This is one reason older Leica Summilux lenses, so-called "pre-ASPH" are so sought after. Not that the latest versions have bad bokeh per se, just that it's not as pleasing (to some) as the previous versions.
Another important factor in bokeh quality is the rendering of out of focus highlights. As mentioned, they should ideally be round and smooth. The more aperture blades a lens has or if they're curved - the rounder the opening of the aperture is. Wide open of course, every "aperture" is round (technically, the aperture blades are fully retracted and not in the optical path). As you stop down however, the shape is clearly visible - though the importance diminishes as the depth of field increases (resulting in less out of focus areas).
DoF Scales and CoC
Say what? First a little pre-requisite background. The circle of confusion (CoC) is an optical spot caused by a cone of light rays from a lens not coming to perfect focus of a point source and is used to determine the depth of field. The standard value for the circle of confusion is related to a particular format (e.g. full frame or 35mm film) though it has more to do with the viewing conditions for your images. That is, will they be seen as tiny web images seen up close, or large gallery prints hung on the wall and viewed at a three foot distance? The point being, that what's considered sharp on small images at close distances - is distinctly different from larger prints viewed at greater distances. As you enlarge your image, the circle of confusion needs to be taken into account. Anyway...
The main point of this section relates to the depth of field scales as seen printed on lenses. They indicate what range will be in focus for a given aperture on a lens. The caveat here is that they're generally considered "optimistic." Without getting too detailed, they're calibrated for a particular assumption of image size and viewing distance. In our case, roughly for 35mm film prints at a size of 8x10" viewed at about 15" or so. There is a lot more to all of this. It's confusing, and there are better articles out there on the web to explain this in much greater detail. It's not important to the point we're trying to make in this section.
Why does any of this matter? Besides getting less in focus than the scale indicates, there are two other cases that can cause you grief. First off, if you plan to shoot the lens at its "hyperfocal distance." This is the distance that a lens is focused for a given aperture such that from the point of focus to infinity is in focus, thereby maximizing the depth of field available. When shooting film, you'll generally be okay in using the scales as marked. Though when shooting the M9 with its full frame sensor, the resolution is higher. Therefore, it is advised to use the next widest aperture when setting your lens for the hyperfocal distance. That is, if you're shooting at f/11, you should stay within the f/8 limits instead. Consider the scale below:
The second situation is when you're shooting a digital camera with a less than full frame sensor, such as the M8. Here the margin is a bit more critical and you should use the scale marks that are up to two stops open from your shooting aperture. For example, you're shooting at f/8 - but should use the marks for f/4 (or in between f/5.6 and f/4). What happens if you don't take all this into consideration? Simply, you get less in focus than you intended, and in the case of setting a hyperfocal distance - infinity will not be sharp!
There's a lot of misinformation out there when it comes to focus shift. Basically, focus shift is a displacement of the sharp plane of focus as the aperture changes. In other words, when properly focused at an arbitrary aperture, the sharpest part of the image will shift forwards (front focus) or backwards (back focus) to a certain degree as the aperture is changed. It is a function of lens design and caused by under- or un-corrected spherical aberration.
That last sentence bears repeating, as this is the most misunderstood aspect of focus shift. It is a function of the lens design - there's no such thing as "my copy does not focus shift" as seen on the forums. If the Leica Summilux 35mm f/1,4 ASPH lens has focus shift, then all Leica Summilux 35mm f/1,4 ASPH lenses have it. Not just this one, or that one. The confusion comes from two principles. Namely, a) the lens is adjusted from specification in such a manner where increasing depth of field masks the shift, or b) the personal style of the photographer falls outside of the parameters where focus shift occurs - or occurs very slightly.
Lenses that have a focus shift do so within a narrow set of circumstances. Typically it occurs anywhere from wide open to about mid way, roughly f/5.6 - and occurs at closer distances. Stopping down or shooting at greater distances both increase the depth of field, which masks the shift. To further complicate things, the amount and direction of shift is subject to the aperture and distance shot at. As complicated as it sounds, in practice it's not that bad - depending on the individual. Some prefer a lens that works as expected, period. Others can overlook a little bit of mental work, a moment of thought - required with such lenses. Why would anyone put up with this?
Focus shift at face value sounds like a reason to avoid a lens altogether. It's certainly not everyone's cup of tea. But the reward of using such a lens usually makes it worthwhile and in the grand scheme of things the focus shift is often very small and only within certain distances - which may very well not be the way you shoot. Some lenses that are known for having focus shift include the Leica Summilux 35mm f/1,4 ASPH ("pre-FLE"), the Zeiss C Sonnar T* 1,5/50 ZM and Cosina/Voigtländer 50mm f/1.1 Nokton. As you might notice, no manufacturer is immune - and they're all fast lenses! Having the speed of such lenses comes at a cost, and sometimes that's focus shift.
Focus shift is often typical of fast, old or cost-conscious lens designs though not necessarily and can sometimes just be a design decision, even on the newest lenses. Slower lenses are easier to design (sound familiar?). They require less corrections, which in this case is spherical aberration. Manufacturers correct focus shift by the use of aspherical elements, elements made of different refractive indices (e.g. flourite) and floating lens elements (which affect mostly close-range focus). The downside is increased cost, complexity, size and weight.
Other than speed, why put up with focus shift? In some cases, the bokeh! Remember, focus shift is due to under- or un-correct spherical aberrations. It's these same aberrations that contribute to the bokeh in the eye of the photographer. A classic example is the Zeiss C Sonnar T* 1,5/50 ZM. Since it's based on an old design entirely because of its unique signature, the focus shift comes along with it. Correcting for it would ruin the signature.