Understanding lens-related descriptive terms

I realize that in almost all of my past lens reviews, I use terms whose meanings may not be immediately obvious to everybody – allow me firstly to apologize for that, and remedy the situation by explaining some of the more comment terms I use. Note: This is not a technical lexicon (that’s in the works) but rather an attempt to help readers get an understanding of what I mean by ‘luminous transparency’, for instance. I know a lot of you are going to ask for images; I’ll add them in the future once I can find suitable examples. In the meantime, read the text and see if you can spot what I’m seeing in review images, or alternatively, make a small donation via Paypal to contribute towards a straightjacket if you think I’ve gone completely insane. MT

Aberration
Any sort of limitation in the optics that causes distortions in the final image, i.e. straight lines not rendered straight, curved planes of focus, incomplete spectra resulting in inaccurate colors etc – it’s a catchall term for imperfections.

Astigmatism
An effect where the left and right sides of the lens do not render symmetrically, e.g. left edge softer than right edge. Usually the product of misaligned elements. Note that most lenses are capable of theoretically much better results than in reality; the limitation is due to manufacturing tolerances and the impossibility of aligning everything perfectly. Such optical limitations are far more obvious on high resolution sensors, which means that you should really test the specific lens sample you’re going to buy before you actually pay for it…
Note that astigmatism can also be a design flaw where sagittal and tangential rays come into focus at different planes; it looks quite similar to coma. (Thank you Oskar O for pointing this out.)

Bokeh
A general reference to the out of focus areas in an image. Smooth bokeh has no uneven luminance values across the out of focus areas, or hard/ sharp edges, and renders even complex out-of-focus subjects in a uniformly blurred manner. Nervous bokeh is mostly smooth, but can be provoked into double images with the wrong kind of out of focus subject. Bad bokeh is full of oddities like double images, hard edges, artificial highlights, texture in blur areas etc – in other words, it distracts the eye from the primary subject and outweighs it visually. Swirly bokeh is where the background areas are rendered in a spiral pattern as though rotated; this is a trait of many fast lenses – especially the Leica 50/1 Noctilux.

Chromatic aberration
Light is made up of different wavelengths; uncorrected, these focus at different distances away from the lens, resulting in a lateral spread of color across the focal plane. Well corrected lenses use pairs of chromatic doublet elements that bend all of the wavelengths of light by the same amount at every doublet; this ensures that all wavelengths of light focuses in the same plane. Chromatic aberration, or CA, manifests as red-blue fringing on high contrast areas – this is the image separating out into its constituent spectra. Red and blue are usually the most obvious because they have the greatest difference in wavelengths – between 450 and 750nm or thereabouts.

Cinematic
This is one of the tougher properties to describe, and very personal. For me, a lens is cinematic if it has a few properties – smooth background bokeh, and transparency in foreground bokeh; slightly biased but accurate color transmission – usually warm; minimal to zero chromatic aberration; gentle flare; a well-defined plane of focus. These properties usually require at least a short telephoto to achieve, but there are some shorter lenses that do admirably well – the Zeiss ZF 21/2.8, ZF 28/2, Nikon AFS 24/1.4, Leica 35/1.4 ASPH FLE, 50/1.4 ASPH and 50/0.95 ASPH come to mind.

Cold
A lens whose color transmission is biased in favor of the shorter wavelengths, i.e. blues; it attenuates reds.

Coma
The spreading of point light sources; usually at the edge of the frame. Points will not be rendered as points, but instead somewhat egg shaped – a corona of white with a bright ‘yolk’ at the center.

Contrast
The way a lens renders luminance differences in a scene – high contrast means that these are exaggerated; low contrast means that they are minimized. Low contrast lenses are good for high-contrast scenes, because they help to limit the dynamic range of the transmitted image – and thus leave it within the sensor’s capabilities.

Dark
A lens with high saturation and very low flare or internal reflection; ‘darkness’ in a lens mean that colors are rendered in a rich, saturated manner; low flare and internal reflection reduces the overall key of the image and eliminates bright but low-contrast areas.

Distortion
A flaw in the image where straight lines in the subject are not rendered as such. Perspective distortion is when verticals or horizontals that are not perfectly parallel to the imaging plane converge or diverge; lens-induced distortion (barrel or pincushion) is when straight lines are rendered as curving.

Field curvature
An ideal lens projects a flat subject plane onto a flat image plane; most lenses suffer from a mild degree of curvature in both, which is to say that the projected surfaces of subject and image are the opposite sides of a sphere – curving away from the sensor plane and towards the optical center of the lens at the edges. In real terms, this means that the plane of focus in the subject isn’t always the same distance away from the camera.

Flare
A bright spot in an image caused by internal reflections off surfaces within a lens; usually caused by stray non-image forming light coming in at an oblique angle and reflecting off elements; this kind of flare is well-defined and usually lens-shaped. Flare can also take the form of a general lowering of contrast that appears as a white haziness across the image; this is caused by light scattered off the internal surfaces within a lens assembly – including but not limited to edges of individual elements, helicoids, etc.

Flat field
A lens that does not have perceptible field curvature. Most macro lenses and super telephotos are flat field.

Gentle
A rather abstract concept that is used to describe tonal gradations and contrast; shallow output gradation for a given input gradation would be shallow; the opposite would be strong.

Latitudinal chromatic aberration
Colored fringes (multiple images) on a solid edge caused by different wavelengths of light not focusing in the same place.

Longitudinal chromatic aberration
Colored fringing in the foreground and background areas outside the plane of focus, caused by prismatic separation of different wavelengths of light by the lens elements.

Luminosity
Another useful but slightly vague term; my idea of a luminous lens is one which has some minor degree of flare and moderate to low contrast, resulting in a bright or high-key image. Blacks are not always rendered as true black, and images shot with the lens at the correct exposure turn out somewhat brighter than expected. Not necessarily representative of low microcontrast.

Macrocontrast
The projected difference in tonality in the image, relative to the perceived difference between bright and dark areas of substantial actual luminosity differences in a subject. In short: how black are the blacks, and how white are the whites?

Microcontrast
Not quite the opposite of macro contrast – microcontrast is the lens’ ability to maintain small luminance differences in a subject and transfer them to the imaging plane. Note that lenses which have low CA are typically high microcontrast, as there is little ‘averaging out’ of fine detail structures when all of the light from the subject lands up at the same spot.

Neutral
A neutral lens is one without tonal or hue biases in any direction (warm or cold, light or dark). It reproduces exactly what it sees.

Purple fringing
Any portion of the image, usually high contrast, that has a purple glow around it; this is often confused with a lens effect but is actually a sensor effect; it’s due to minor internal reflections off the coatings of the microlens array covering the individual photosites. The color is uniform around the high contrast area, and does not shift left-right as chromatic aberrations would.

Resolution/ resolving power
The ability of a lens to reproduce fine detail structures. Related very intimately to microcontrast – a lens that can resolve high amounts of detail must necessarily be able to resolve small luminosity differences in adjacent areas.

Sharpness
A perceptual effect rather than an actual, measurable property; not to be confused with resolution. Something can be sharp with little resolution – i.e. able to project large differences in macro contrast with little chromatic aberration, creating a sharp edge; but have poor microcontrast resulting in low resolution, but high perceived sharpness. Rather, sharpness is a subjective measure for how much difference we perceive between adjacent areas of different tonal values.

Spherochromatism
The presence of chromatic aberration in bokeh – out of focus highlights separating into distinct red-green portions.

Tonality
The lens’ ability to reproduce luminance and color differences in a scene, and the nature of how it renders these differences in the image.

Transition zone
The plane represented by the distance from the camera where things just start to lose critical focus; a bit in front of and a bit behind the actual focal plane.

Transmission
The lens’ ability to capture light and transfer it to the image plane, limited only by the physical aperture/ iris of the lens. Applies to both the overall amount of light – T stop vs f stop – and the spectral transmission of the lens,i.e. which light spectra the actual types of glass allow through. See also this article on T stops and F stops

Transparency
The ability of a lens not to impart any of its own distinguishing characteristics on the image; it renders the scene as-seen, without any distracting properties or aberrations. My idea of a transparent lens is one with high microcontrast, moderate macrocontrast, natural spectral transmission with good saturation, and a T stop that’s close to its F stop. Bokeh is simply blurred and does not exhibit nervousness or swirliness; foregrounds and backgrounds are equally smooth. Such a beast is a rare piece of optics indeed.

Warm
A lens whose color transmission is biased in favor of the longer wavelengths, i.e. reds; it attenuates blues.

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Comments

  1. Great site!!
    Suggestion:
    Would be great if you attached some pics to illustrate the definitions for the less tech inclined like me..

    • Thanks for the suggestion. I’ll have to look for them – I generally try to avoid lenses with these issues (and they’re not always separable, so you might see two or three properties at the same time).

  2. Matthew D says:

    Helpful for sure–thanks as always.

  3. Good article, it’s important to have a proper terminology of the topic.

    I don’t wish to sound nitpicking, but I think there is a slight possibility for confusion in the part about astigmatism. You discuss the possibility of astigmatism mainly through decentered lens elements, but there is also astigmatism that is part of the design and present in correctly manufactured lenses, where sagittal and tangential rays come into focus in different planes (illustration: http://en.wikipedia.org/wiki/Astigmatism). I felt from reading that one may get the impression that astigmatism would only be due to a lack of symmetry.

    About sharpness, I nowadays find that a good MTF measurement is very descriptive of sharpness as it takes into account contrast as well as size of detail. The only problem is to find a good MTF measurement…

    • Thanks for the correction – you are of course right, I have amended the article to reflect that. It looks quite similar to coma, though.

      Agreed re. MTF, but the caveat is always a) whether it’s a measured or theoretical value and b) sample variation/ manufacturing tolerances. There are also sensor effects – e.g. edge purple fringing or CA on close back focus lenses – which are almost certainly not taken into account. A well-dcoumented example are the fast Leica M wides – they’re fantastic on the M9, and M Monochrom, but lousy in the corners with the Sony NEX – the sensor microlenses simply aren’t designed to correct for non-telecentric optics.

      And the whole MTF thing is another article unto itself…

  4. Thanks you! Never too old to learn something.

  5. I love the super geeky part of you Ming. I can definitely learn from this. Thank you for posting.

  6. Michael Sin says:

    Excellent work! Ming, You are very thoughtful…and hard working.

  7. Can i ask another newbie question?

    Why is it that lens that are capable of bigger apertures called “fast” lenses?

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