Orquid "Phoenix"

Orquid "Phoenix"

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Everyone hears something about chromatic aberration but are there any other types? What causes them?

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3 Answers

up vote 9 down vote accepted

There are numerous kinds of optical abberations that you may encounter with a lens. Chromatic Aberration is only one of them. Some are more drastic, others are more subtle.

Lens Flare

Probably the most commonly known aberration is lens flare. Flare occurs when non-incident light enters the lens and reflects off of the various lens elements and/or diaphragm. The effect, when strong enough, can create bright spots and streaks, and may also have a detrimental effect on contrast where it occurs. Flare is usually caused by a bright near off-scene light source, such as the sun, or a bright light illuminating your scene.

You can mitigate or eliminate flare by using a lens hood. For telephoto lenses, a round hood will block all non-incident light. For wider lenses, a petal-shaped hood is best, as it takes into account the wide form factor of the sensor. Multi-coated lens elements help to reduce undesirable reflections, and when used for front and back lens elements, but particularly when used on all internal lens elements, can greatly reduce flaring. Filters, being an additional glass element with their own imperfections, will likely increase the chance of flaring.

Ghosting

Similar to flare, ghosting is the result of light bouncing off your sensor, reflecting off of the back lens element or elements, and returning to the sensor. Ghosting usually creates a soft, off-centered replica of your main image. It can look somewhat like what a person with astigmatism sees, a slightly blurred or streaked off-set copy of the scene.

Higher quality lenses use milti-coated lens elements to reduce reflection as much as possible, and they can limit the cases where ghosting is possible. It is impossible to completely eliminate reflection, however, and in the right scenarios, ghosting is always possible to a degree.

Distortion

Another type of aberrant lens behavior is distortion. It comes in two varieties: pincushion and barrel. In most zoom lenses, distortion occurs at the focal length extremes. Cheaper lenses often have more of a problem with distortion than higher quality lenses, however pretty much all lenses have some degree of distortion (including primes.) Many lenses have such a low degree of distortion that it is not a factor, and others are clearly noticable. Distortion may not be much of a problem if you are not photographing subjects that make the effects of distortion apparent, like brick walls or buildings.

In addition to pincushion and barrel distortion, many lenses will create distortion in perspective. Particularly with wide-angle lenses, distortions in perspective can be seen when using very wide focal lengths.

Certain types of lenses, often called TS or Tilt-Shift lenses, tend to generate very little barrel or pincushion distortion. Such lenses offer two additional controls over the normal focus and zoom: tilt and shift. Using these additional controls, a photographer can straiten out perspective distortion to one degree or another, and restore a proper degree of strait perspective to your images.

Spherical Aberration

Spherical aberration is another type of optical aberration that may occur in camera lenses. It results from the difference in refraction at the edges of a lens compared to the center, resulting in improper convergence of light rather than convergence into a focal point. Spherical aberration generally results in softer focus, rather than clear and sharp focus.

Spherical aberration may be corrected in a couple of ways. A combination of spherical convex and concave lenses may be used to correct the convergence of light. Modern high-end professional lenses often include an aspherical lens element. Aspherical lens elements cause less refraction at the edges and more in the center, resulting in proper convergence over a given focal length.

Some lenses, such as soft-focus portrait lenses, intentionally leave a certain amount of spherical aberration in place to produce more pleasing shots. In these cases, spherical aberration is a desirable effect, one which you may explicitly look for in a lens.

Coma

Related to spherical aberration, comatic aberration is a refractive problem that occurs in off-axis point light sources. Due to the difference in refraction near the edges of a spherical lens element, off-axis point sources may appear stretched and "haloed" at the focal plane. Coma is generally a combination of both spherical aberration of a point light source and chromatic aberration to produce an effect that looks like a comet.

Coma is generally controlled by using lenses of the appropriate curvature to minimize edge distortion. In camera lenses, a combination of lens elements is generally required to minimize such optical aberrations. Comatic aberration is a problem that largely affects those who do night photography or astrophotography, as point light sources are most common on these scenarios.

Diffraction

A final type of distortion is also possible, and prevalent on all cameras. Diffraction is an effect of light, given its waveform nature. When waves encounter an edge or opening, then have the tendency to bend around it. The diaphragm in a camera allows one to control the aperture, or the opening through which light passes on its way to the sensor. The aperture gives us control over how much light reaches the sensor...but as a result, it can also cause diffractive blurring via an effect called the airy disc.

At sufficiently wide apertures, diffraction is low enough that it does not cause any problems. However, all sensors have a diffraction limit, beyond which the effects of diffraction will begin to affect image quality. For most sensors, this is around f/8 to f/11. The larger the photosites and the more effective the microlensing around each photosite on a sensor, the higher the limiting aperture. When the aperture is stopped sufficiently far below the diffraction limit, the airy disc effect will allow light to bleed past the intended sensor pixel (photosite) and affect others. Apertures below f/22 or so will generally start causing enough loss in sharpness to counter the gains by having a tighter aperture.

While the diffraction of light is caused by the diaphragm in a lens, it should be noted that the resulting effect is dependent on the sensor in the camera. Large full-frame sensors in high-end DSLR camera bodies will exhibit problems due to diffraction less than the smaller sensors in entry-level DSLR camera bodies, which in turn will exhibit problems significantly less than the tiny, pixel-dense sensors in most point and shoot cameras.

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Great answer. Couple nit picks: On lens flare, loss of contrast does not have a prerequisite of a bright spot or streak, as your wording can imply. Coating helps mitigate flare (along with ghosting, as you mentioned). Also, petal-shaped hood is always the best on any lens, including telephotos. Most (consumer) telephotos have a rotating front element, which is why a round hood is used. Many pro telephotos use petal-shaped hoods. Some use round hoods w/o a rotating FE for cost and abillity to lay flat on the ground. Diffraction is an optical abberation of the pixel spacing, not the lens. –  Eruditass Jul 29 '10 at 17:00
    
@Eruditass: Technically speaking, diffraction is the bending of light around an obstacle, and has little to do with pixels. In the context of a camera, diffraction is caused when light passes through an aperture lower than its maximum setting. The distortion in the lights waveform creates something called an airy disc. When using small enough apertures, the airy disc effect can grow large enough to affect more than one pixel at a time, resulting in softened images. The larger your pixels, the smaller your aperture needs to be to become problematic, so the lens is involved, but not solely. –  jrista Jul 29 '10 at 17:46
    
@jrista: That is true, I just wanted to emphasise the point that the abberation is the same for all lenses and their apertures, but the appearance of the abberation is entirely dependent on pixel spacing, which you did not mention at all. This has nothing to do with a sensor being "better." –  Eruditass Jul 29 '10 at 17:51
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@jrista I think spherical aberration is worth a mention for a couple of reasons: first, that it has a bit of a special status, as it's a big part of the look of many classic portrait lenses that may not even be desirable to fully correct. Secondly that not everyone will be buying new asph lenses (I'm also wary of the idea that "most" current lenses include aspherical elements, but don't have evidence either way). In fact, I'd bet people buying older lenses will be much more interested in this answer than people buying new - it's what they'll have to deal with! –  ex-ms Jul 29 '10 at 18:23
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@jrista: Thanks for the retro hat-tip. I totally realise this is splitting hairs now, so feel free to disregard: it's not just soft-focus portrait lenses that have under-corrected spherical aberration. Slight under-correction tends to render a smoother out-of-focus area, and so it's used in a lot of "sharp but not clinical" lenses like the Nikkor 105/2.5. –  ex-ms Jul 29 '10 at 21:49
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I'm trying to write down a more photographer oriented answer. So the different lens-related issues are:

  • Vignetting - corners of the frame are darker than the center
    • noticeable in viewfinder/on screen
    • to avoid - stop down your aperture, filters only make it worse, so you might want to try without
    • post-processing - relatively easy to correct
    • special attention needed in panoramic shots
    • vignetting should not be confused with loss of detail nor loss of contrast in the edges
  • Lens flare - sun-spots or low contrast areas in frame
    • usually noticeable in viewfinder/on screen
    • to avoid - use lens hood, change your position to keep bright points out of the picture if possible, filters only make it worse, so you might want to try without
    • post-processing - hard to correct in areas with lots of detail
  • Diffraction - your image starts to be soft all over if you stop down too much (over f8-f16 depending on sensor element density)
    • noticeable on screen if zoomed
    • to avoid - know your diffraction limit and don't stop down over that, expensive alternative is to use tilt on tilt-shift lenses
    • post-processing - cannot be corrected
    • diffraction should not be confused with far more common focusing errors and camera shake (the latter is directional blur)
  • Chromatic aberration - green/magenta glow on the edges of highlights, usually called "purple fringing"
    • noticeable on screen if zoomed
    • to avoid - know your lenses' sweet spots, stop down
    • post-processing - easy to correct
  • Distortion - vertical and horizontal lines are curved in the edges of the frame (barrel distortion - outward curves, pincushion distortion - inward curves, complex distortion - wavy curves)
    • easily noticeable in viewfinder/on screen
    • to avoid - you can't unless you have better lens available
    • post-processing - relatively easy to correct (except complex distortion)
    • special attention needed in panoramic shots

Not really in the same category, but also very important aspect of lenses is resolution, or actually we usually care about resolution loss towards the edges due to various other aberrations such as coma, astigmatism, field curvature, etc. You can usually make things better when you stop down and you should try, because you can't correct it in post-processing.

One should keep in mind that there is no such thing as perfect lens and every lens has some degree of aberrations due to the compromises made during optical design.

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Strictly 'optical' (ie wavefront) aberations are:

  1. Defocus
  2. Spherical aberration
  3. Coma
  4. Astigmatism
  5. Field curvature
  6. Image distortion

But the ones that are mostly likely to affect your photographs are:

Chromatic aberation - different colors are focused to different positions in the image, this gives rainbows around bright objects, especially on the sky.

Flare/ghosting - light is scattered inside the lens, either from the glass or the metal body. This creates the row of colored circles going toward the sun that you sometimes see in movies and reduces the contrast of the image.

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