Photo Enlargement Glossary

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Photo Enlargement Glossary

Acid Free - ( sometimes referred to as "pH neutral") Refers to the fact that a material or substance is not acidic by nature. The level of acidity is measured using the pH scale, which ranges from 0 to 14. Note that this scale is non-discrete, which means it can take on fractional or decimal values such as 1.5, 4/5 or 6.75 rather than just whole numbers such as 6. A material or substance with a pH value of 7 or slightly more is generally considered to be "pH neutral" and can also be referred to as "acid-free" . A material or substance with a pH value clearly greater than 7 can be referred to as "acid-free" but not "pH neutral". In relation to printing, acidity ( pH less than 7 ) causes paper to decay over time. The first symptom of this is a yellowing of the paper, which will detrimentally change the colour balance of the image. More seriously, the yellowing is part of a chemical reaction that will eventually result in the paper becoming brittle. The speed of this decay will largely depend on the level of acidity initially present in the paper and exposure to other acidic chemicals or gasses over time (also see lignin).

Artefacts - In relation to a digital image, describes "unnatural" distortions that are introduced into an image due to some form of over processing. There are a number of different types of image artefacts. File compression artefacts (such as JPEG artefacts) are caused by too much compression being applied when saving a file. An image affected by JPEG compression artefacts will appear to consist of fuzzy squares. Halo artefacts can be caused by over-sharpening (see Print Sharpness), and usually manifest as a pale-coloured "glow" affecting edges or lines. Other forms of artefacts can be caused by the incorrect "handling" of light (see chromatic aberration)
[JPEG artefacts example] [oversharpening example]

Aspect Ratio - In relation to an image or print, refers to the ratio of the width divided by the height. Put another way, the aspect ratio provides a description of the proportions of an image or print. Most compact digital cameras record photos with an aspect ratio of 4:3. In contrast, most dSLR cameras record photos with an aspect ratio of 3:2.

Bokeh - (derived from Japanese word "boke", meaning 'blur') - In photography, refers to the creative use of lens blur as a composition technique. The aim of bokeh is to deliberately blur the background or foreground (or both) in order to draw the viewer's attention to a particular area of the photo. Bokeh is achieved by using a suitable lens with a large aperture setting, which in turn produces a narrow depth-of-field. It should be noted that some lenses tend to be more suited than others for achieving such an effect.
[bokeh example]

Buffering Agent - A substance added to paper during the manufacturing stage to increase the paper's pH value (refer to "acid free" definition) to 8 or higher. This is done to help prevent the paper from becoming acidic over time as a result of the absorption of acidic chemicals or gasses present in the environment (i.e. from the air or from contact with other material). The buffering agent has the effect of neutralising any acidic chemicals or gasses. The preferred buffering agent of archivists is calcium carbonate.

Canvas - In the arts/photographic industries, refers to a heavy, woven material used for printing or painting on. High-grade canvas is derived from 100% cotton, is acid-free and tends to be quite heavy (300 GSM or more). Lower-grade canvas is derived from a combination of cotton and synthetic material or entirely from synthetic material, and tends to be lighter in weight. It may or may not be acid-free. Virtually all valuable paintings employ 100% cotton canvas. Relative to other types of paper, canvas has quite a pronounced texture and noticeable weave. Certain types of photos or images are more suited to printing on canvas than others. In general, if it is important to show the very finest of detail in your print (and/or the broadest colour gamut/tonal range), it may be better to choose a smoother class of paper.

Canvas Stretching - Describes a popular method used to present canvas. Essentially, it involves wrapping a canvas print over a square wooden frame. Although there is often a small amount of ‘stretching’ involved, this is quite minimal in order to avoid distorting or damaging the print. A better, albeit awkward, description might be "tightly wrapping canvas over a wooden frame". Canvas stretching can be quite labor intensive, sometimes more lab our intensive than conventional framing. The techniques and, to a larger extent, the materials used to prepare a canvas frame can vary. For instance, different types of wood with differing strength, weight or shape can be utilised, and different types of fasteners can be used to attach the canvas to the frame. It is important to note that some inkjet canvas is not suited to stretching over a frame as it can be prone to cracking.

Chromatic Aberration - The term 'chroma' refers to colour. The term 'aberration' refers to some sort of unexpected variation. In relation to photography, chromatic aberration describes an undesirable side effect resulting when a camera lens cannot focus the different wavelengths making up light within the same focal plane. Chromatic aberration is evidenced when blurred (or "foggy") colour fringes appear along some of the edges of objects. It is a problem that more often occurs with the cheaper variety of telephoto lenses.
[chromatic aberration example]

Coating - Refers to a liquid substance that is applied to a print, typically by brushing or spraying, in order to provide additional protection against the fading effect of light and airborne contaminants, and against scratching or scuff marks. Note that the process of coating differs from laminating in that the latter involves encasing the print in a plastic-like wallet using a combination of heat and pressure. The main advantages of coating are: 1) it is more forgiving, and can be applied to a wider range of papers, and 2) It tends to be significantly less prone to yellowing over time. The main disadvantage of coating is that it tends to be more labour intensive to apply.

Colour Cast - (both subjective and objective measurement) In relation to a photo, describes a situation (not necessarily undesirable, especially in relation to warm colour casts) whereby the overall colour of the photo is affected by the tinge of another colour. Sometimes, a colour cast can be quite noticeable: for instance an old faded photo may show a clear yellow cast as a result of physical deterioration. On other occasions, the colour cast can be quite subtle and can only properly be identified using the features of certain image editors. One of the best ways to minimise the chance of an undesirable colour cast affecting your photo (or reduce the impact of the colour cast) is to manually select the most appropriate white balance setting prior to taking the photograph. More advanced techniques involve the use of tools such as external light meters and grey cards (or colour targets) to determine the most appropriate camera settings.
[colour cast example]

Colour Proof - It may refer to a reference print that a client supplies to a photo processing centre, along with the corresponding digital image, so that the photo processing centre can reproduce the digital image with colours that are representative of the reference print. Alternatively, it may refer to a reference print (usually smaller in size than the final "real" print) that the photo processing centre supplies to the client in order to assess whether the client would be satisfied with the colours of the final print.

Colour Quality Control - The processes employed by Giantprint Pty Ltd, both standards-based and proprietary-based, in order to control colour accuracy and colour consistency.

Crop - In relation to digital image editing, refers to the process of "cutting away" or discarding unwanted parts of an image. Typically, cropping is performed within an image editing program, whereby a square-shaped marquee (i.e. selection area) is placed over the part of the image that is to be retained. Upon clicking or pressing some button within the program, the unwanted parts of the image (i.e. those that fall outside the square marquee) are discarded. Cropping may be performed in an attempt to improve the composition of an image, for instance to remove a distracting object or person. Alternatively, some cropping may be required in order to print an image with an aspect ratio that differs from that of the final print dimensions. This is because, due to the different aspect ratios, the image may not fit within the allocated print dimensions. If, despite the different aspect ratios, the image must not be cropped prior to printing, some form of overall image resize will be required - for instance the image as a whole may need to be reduced in size by 15%.

Custom Colour Profile - A measurement taken to determine the colour reproduction characteristics of a specific printer, ink and paper combination (note that by "specific", we mean a specific physical instance in use somewhere). By using a profile that is custom-made rather than a generic (i.e. "canned") profile supplied by the manufacturer, the profile will more accurately represent the prevailing colour reproduction characteristics. This, in turn, will help to produce a print that is a closer match to a colour proof. Furthermore, the custom profile will typically allow access to a larger colour gamut than if the generic profile was used.

Delta-E - (in relation to printed output) An objective measurement used to determine the colour accuracy of a print when compared to some reference print (see colour proof). The lower the delta-E, the lower the difference in colour variation between two prints. Many professionals consider a delta-E result in the range of 3 to 6 to indicate a good colour match.

Digital Photo Enlargement - Commonly refers to the processes involved in making a digital photograph larger. It may refer to the process of increasing the size of the digital file (for example, a .jpg or .tiff file), increasing the size of the physical print which was derived from the digital file, or both. It is generally accepted that enlarging a photograph from a digital image is considerably more difficult than enlarging a photograph from an analogue (i.e. film/negative based) photograph. This is due to the fact that a digital image has only limited "information" available from which to derive a print. In order to produce a physical print with sufficient resolution and sharpness, special mathematical algorithms may need to be applied to the original digital file. The technique required to enlarge a digital photo will, to a large extent, depend on the size of the original file, the quality of the original photo, and the final print size desired. Poster size photos can be particularly difficult to achieve, and often require the expertise of a professional photo processing service in order to produce sufficient resolution and sharpness and thus achieve acceptable results. It should be noted that the techniques used to enlarge a digital photograph can vary quite widely from one processing centre to another.

Dmax - (in relation to inkjet paper) Typically used as a measurement of the potential maximum optical density of black that can be achieved with a particular paper and ink combination. Optical density refers to the extent to which an ink and paper combination absorbs visible light. The greater the amount of light that can be absorbed, the darker the shade of black that is capable of being reproduced. Therefore, an 'informal' way of defining Dmax is that it represents the darkest potential shade of black that can be achieved. Dmax will fall somewhere within the Density Scale, which ranges from 0.0 to 5.0 (5.0 denotes highest optical density). It should be remembered that, in practice, a Dmax of 5.0 is unachievable. Dmax is commonly used as one of the important indicators of the colour gamut, or range of colours (or the range of brightness/tonal values, in the case of black and white printing) that the paper is capable of reproducing, assuming that the type of ink used remains constant. Another important indicator is the paper's "white point". Some examples: A typical high-end canvas print made with premium pigment ink recorded a Dmax of 1.71. A high-end glossy print, made with the same premium pigment ink, recorded a Dmax of 2.15.

dSLR Camera - An abbreviation of the term "digital single lens reflex camera". A camera that has a viewfinder through which the user views the actual image that is being projected through the lens. Essentially the same as a film-based SLR camera, except that a digital sensor replaces the film. dSLR cameras typically have more manual features and are priced above all-digital compact cameras. Prices vary from approximately $900 AUD to in excess of $60,000 AUD for a top-of-the range medium format Hasselblad dSLR.

Fine Art Paper - A class of paper that is popular amongst those producing artistic paintings, images or photos. In fact, there is a fine art paper to suit almost any application, including "regular" photographs. This class of paper is characterised by its matte (i.e. "unshiny") finish. It comes in a wide range of textures, from highly-textured to highly smooth. The smoother range of fine art papers are capable of reproducing the very finest of detail in an image or photo. This class of paper is synonymous with high-grade, acid-free paper.

Global Image Adjustment - In relation to the editing of a digital image, refers to any adjustment or correction that is made to an image as a whole, rather than to a specific area. An alternative definition is any adjustment or correction that does not require the application of selections or selection masks within an image editor. An example of a global image adjustment is an increase in the overall contrast.

Glossy Paper - A class of paper that has a gloss or "shiny" finish. Glossy paper comes in different levels of shininess. Typically, the shinier variants are referred to as "gloss" or "glossy" paper, whereas the less shiny variants are referred to as "semi-gloss" or "satin" paper. In general, this class of paper is well suited to reproducing the very finest of detail in an image or photo.

GSM - An abbreviation of the term "grams per square metre". Commonly used as an indicator of paper heaviness. Note that it is incorrect to use GSM as an indicator of thickness. Typical office printing paper has a weight of 80 GSM. A good-quality photo paper will tend to weigh 200 GSM or more.

Image Resolution - (objective measurement) In relation to an image on screen, refers to the average number of pixels per square inch (abbreviated "ppi") making up the image. The higher the ppi, the higher the amount of visual "information" in the image. As the average number of pixels per square inch increases, color gradations will appear smoother and the edges of objects will appear sharper. Therefore, increasing image resolution (by capturing, creating or post-processing the image at a higher resolution) results in finer detail becoming perceptible.

Image Noise - Describes the appearance of 'random' grains or particles in an image (usually distributed fairly evenly throughout the image). It may be helpful to think of image noise as being somewhat similar to the grainy looking interference you may have experienced when watching television. There are two types of noise that may affect an image: chroma noise and luminance noise. Chroma noise refers to 'random' grains that appear as a result of fluctuations in colour. Whereas, luminance noise refers to 'random' grains that appear as a result of fluctuations in brightness (i.e. luminance) levels. Of the two, it is chroma noise that has the greater effect in terms of making an image appear unnatural. There are different algorithms that can be applied to an image to lessen the effect of noise. However, it should be noted that, removing too much luminance noise using such algorithms can in itself result in an image appearing 'plastic-like' and unnatural. The level of chroma noise is largely dependant on the performance of the image sensor in a digital camera. However, the level of luminance noise is largely dependant on the ISO film speed setting (or sensitivity setting) used when taking a photo. For instance, a setting of ISO 1600 is likely to result in considerably more noise than a setting of ISO 100.
[image noise example]

Lens Barrel Distortion - An optical effect (not necessarily undesirable) which results in an image being distorted in a circular manner. The effect will tend to be more prominent towards the outer edges of the photo, with the centre often being unaffected. It is caused by a lens which has lower levels of magnification away from the centre. Lens barrel distortion is a trait of some wide-angled lenses.

Lens Flare - An undesirable optical effect which is characterised by faint-coloured, circular halos appearing in a photo. Lens flare results when light enters at an angle and then bounces around inside the lens (or between lens elements), prior to reaching the image sensor. The presence of bright light sources in front of the lens (but not necessarily in the field of view) is the most common cause. A lens hood can be used to reduce the possibility of lens flare affecting a photo.
[lens flare example]

Lignin - Refers to a naturally occurring, "glue-like" compound that is found in trees and certain plants. Lignin is responsible for binding wood fibres together. Over time, lignin breaks down and in the process produces acidic substances. Of course, the existence of acidic substances in paper is detrimental to print longevity, and so archivists prefer to use paper that is lignin-free where possible.

Local Image Adjustment - In relation to the editing of a digital image, refers to adjustments or corrections that are made to a specific area in isolation, without affecting (or with very little effect to) any other part of the image. An example of a local image adjustment is a correction made to a subject with red eye. Another example is the retouching of a subject's face in order to remove a blemish. In general, it is more difficult and time consuming to make local image adjustments than it is to make global image adjustments.

Matte Paper - A class of paper that has minimal or imperceptible shine. This is a feature typical of fine art paper as well. However, a paper described solely as "matte" will tend to be smooth and will generally not have any significant texture in its finish. Higher-grade matte papers are capable of reproducing the very finest of detail in an image or photo.

Metamerism - Although technically incorrect, the popular definition of metamerism is a situation whereby the colours in a print appear different when viewed under alternate light sources. As an example, the black colours in a print viewed under yellowish incandescent light might appear dark brown when viewed under whiter fluorescent light. Metamerism does not represent some flaw in a material or ink (although some pigment inks are known to be better than others at minimising the 'effect' of metamerism). Rather, it describes the fact that the colour of most (if not all) objects will display some variation depending on the light source under which they are viewed.

Optical Brightener - (also referred to as a "fluorescent brightener") A chemical agent added to paper during the manufacturing stage for the purpose of making the paper appear whiter (or "brighter") to our eyes. Optical brighteners work by absorbing invisible ultraviolet light and, in return, emitting bluish light that falls within the visible spectrum. It is essentially the same technique used to make "glow-in-the-dark" toys. Archivists argue that high-end archival papers (designed to last for decades or centuries) should not contain optical brighteners because these optical brighteners are likely to lose their effectiveness before the "raw" paper and ink combination show any signs of colour fade. In such situations, as the optical brightener deteriorates, the image will develop a detrimental colour shift as the paper's white point returns to its natural state, which will typically be a shift to a yellowish colour.

Overexposed - An undesirable situation whereby a camera's digital image sensor or film was exposed to too much light under the prevailing conditions. An overexposed photo will appear too light (i.e. white) overall, with an absence of the darker tones required to provide sufficient contrast. Sometimes, the level of overexposure is so great that the photo cannot be corrected by an image editing program.
[overexposure example]

Panorama - (in relation to the dimensions of a print) There is no agreed standard as to what print dimensions (or aspect ratio) constitute a panorama. Informally, it refers to a print that has one dimension that is 'considerably' longer than the other dimension. Indeed, this description is vague. We would argue that a panorama is any print that has one dimension that is at least two times longer than the other dimension (i.e. with an aspect ratio of at least 2:1, such as a print measuring 24 inches by 12 inches). Notice that we don't refer to the width being two times as long as the length, or vice versa. This is by virtue of the fact that a panorama can extend either horizontally or vertically.
[panorama example]

Paper Curl - Describes the tendency of a paper print to remain curled after the printing process has completed. The extent of this undesirable curling will largely depend on (i) the roller mechanism employed by the printer, and (ii) the type of paper used, and (iii) the method used to store the print during transit. Ideally, a print should be stored in a flat position, although in reality this may be impractical during transportation. If a print must be curled up, for instance when it must be placed inside a document tube, it is important to select a tube with a diameter that is as large as practical (typically a minimum of four or five inches) in order to minimise the curling effect.

Paper White Point - The colour (or colour temperature) of paper before any ink/image is rendered on the paper. The white point will represent some shade of white. When a paper is relatively closer to 'true' white, it is described as "brighter", "cooler" or "bright white" . When a paper has some sort of 'yellow' tinge, it is described as "warm" or "warmer". Artists and photographers may deliberately seek out "warmer" coloured papers for the effect they have on a particular photo or image. It should be noted that, in many cases, the difference in white point between various papers is very subtle, even when they are placed side-by-side (of course, this definition ignores 'fancy' coloured papers with highly saturated hues of purple, red, etc).

Paper Texture - The degree of roughness or coarseness of a paper, both in terms of feel and in terms of its appearance. A paper with a high degree of roughness or coarseness is described as "highly textured". Sometimes also referred to as the paper's "tooth", although this is more often associated with the tactile feel of the paper alone.

Perspective distortion - An optical distortion (not necessarily undesirable) that results in part of an object appearing to be 'unnaturally' large in relation to another part of the object. Photographers often encounter perspective distortion when photographing the entire facade of a tall building at a close distance. In this situation, the sides of the building will appear to be converging towards the peak. The effect can be reduced by standing further away from the object when shooting. Otherwise, tilt-and-shift lenses can be used to correct for the distortion, as can certain image editing programs.
[perspective distortion example]

Pigment Ink - A class of ink created by adding very finely ground colourants (derived from solid coloured material) to a liquid solution. The colourants are insoluble - i.e. they don't dissolve in some solution to produce colour the way dye ink does. The colour of pigment ink is produced by the many tiny coloured particles adhering to the top of the paper. The molecules of these pigments are relatively large, making it more difficult for them to break down. The result of this is that pigment ink tends to have superior fade and water resistance compared to dye ink. The main disadvantage of pigment ink is that it tends to have a smaller colour gamut. However, with recent advances, this difference now tends to be quite small, particularly in the high-end printing market.

Pincushion Distortion - An undesirable optical distortion which results in an image appearing to be stretched (or pinched) inwards towards the centre (think of the inwards distortion you get when you push your thumb down on a cushion). It is caused by a lens which has lower levels of magnification towards the centre. Pincushion distortion is a trait of some telephoto lenses.
[pincushion distortion example]

Pixellation - (subjective) Describes an undesirable situation, either in print or on screen, whereby at a typical (or reasonable) viewing distance you notice that your print or image is made up of individual little squares rather than fine gradations of colour. Pixellation results when there is insufficient resolution in a print or image.
[pixellation example]

Poster Hanger - As an alternative to conventional framing, describes a device used to display a poster. These devices usually consist of a horizontal rod or strip to hold the top part of the poster, and another rod or strip attached to the bottom of the poster which acts as a weight preventing the poster from curling. Poster hangers vary in terms of their appearance, the mechanism used to load the poster and the mechanism used to mount the device to the wall or ceiling. The main advantages of a poster hanger are: (i) it is considerably easier to attach a poster to these devices than it is to frame a poster, and (ii) it is considerably easier to swap prints between these devices than it is to swap prints between conventional frames, and (iii) it offers outstanding "minimalism". Without the bulk of a conventional frame and without the possibility that light will reflect off glass, there is very little to distract from the actual print. The main disadvantage of a poster hanger is that is doesn't offer the same level of protection offered by a frame with a glass cover, although laminating or coating a print can offset this disadvantage to some degree.

Posterisation - (also referred to as "banding") Describes a situation whereby the colours or tones making up a photo are restricted in a manner which produces sharp (or less gradual) changes in colour or tone throughout the photo. The more extreme cases of posterisation are typically the result of deliberate image editing, with the intention of making a photo appear cartoon-like. The less extreme cases of posterisation are often caused by: (i) unintentional mistakes made with colour space conversions, or (ii) excessive editing of photos that have a "low" bit depth (e.g. 8 bits of colour or tone), or (iii) the conversion of a photo to a higher bit depth, for instance from 8 bits to 16 bits of colour or tone.
[posterisation example]

Prime Lens - A lens with a fixed focal length (e.g. only 50mm) , as distinct from a zoom lens which has a variable focal length (e.g. 35-70mm).

Printer Calibration - Refers to the processes used to ensure that a printer produces colours that are consistent in relation to a pre-determined baseline. Over time, printing devices wear and ink and paper formulations vary (even within the same batch of such items). The result of this change in state is that the same digital signals sent to the printer to achieve a specific colour will no longer produce the same colour. Some level of compensation (i.e. correction) is then required to allow the printer to once again print colours that are consistent with the baseline. As part of the processes required to achieve predictable colour, a printer should be calibrated regularly, and at least each time an ink cartridge is changed or paper is loaded.

Printer Colour Profile - An objective measurement used to describe the colour reproduction characteristics of a printer, ink and paper combination. Colour profiles are typically specified by using an ICC (International Color Consortium) colour profile.

Print Colour Accuracy - Refers to how accurate the colours of a print are in relation to the colours of another print (i.e. relative to a printed proof) or in relation to the colours of the image on a screen (i.e. relative to a screen proof).

Print Colour Fade - An undesirable situation whereby the colours in a print fade over time. One of the main causes of colour fade is exposure to light; in particular, ultraviolet light (uv), either through exposure to sunlight or exposure to certain other light sources. Acidic gasses or substance that come into contact with a print are another common cause of print fade (also see acid-free). A third cause is exposure to ozone gas, which occurs naturally but is also produced by certain electrical equipment, such as laser printers or certain air ionisers. Some common methods used to combat print colour fade are: (i) minimise exposure to uv by physically keeping the print away from uv sources, or (ii) placing the print in a frame with uv-filtering glass or acrylic, or (iii) directly laminating the print with a uv-resistant laminate or coating, or (iii) using fade-resistant inks (i.e. archival inks). Note that framing, laminating or coating can result in significantly lower exposure to ozone gas or acidic substances.

Print Ghosting - (also known as "out-gassing" or just "gassing") An undesirable situation whereby a picture frame's glass (or acrylic) becomes fogged from the inside. This is usually caused by inkjet prints mounted within a frame that have not been allowed to cure (i.e. dry) sufficiently. In this situation, liquids from within the ink evaporate and then condense on the inside of the frame's glass.

Print Resolution - (objective measurement) In relation to a printed photo or image, refers to the average number of dots per square inch (abbreviated "dpi") making up the print. The higher the dpi, the higher the amount of visual "information" in the print. As the average number of dots per square inch increases, color gradations will appear smoother and the edges of objects will appear sharper. Therefore, increasing print resolution results in finer detail becoming perceptible. It is important to note that, due to the different physical rendering methods, an image's ppi on screen will rarely equal the image's dpi when it is finally printed. As an example, in order for a dot-matrix printer to render the equivalent of a single pixel shown on screen, it will have to produce a number of dots (i.e. one screen pixel = multiple printed dots). In general, a print with a resolution of 1200 dpi or greater is considered a high-resolution print.

Print Sharpness - (subjective) The amount of fine detail or visual "information" perceived to be present in a print. The definition (or lack of definition) in edges and lines is often a large factor in our perception of print sharpness. An alternative definition of print sharpness is the degree to which a photo or image rendered on paper appears "unblurry".

Red Eye - In relation to a photo, an undesirable situation whereby the colour of a subject's eyes appear reddish. This is caused by the light from a flash reflecting off the retina of the eye. The same effect can occur with animals, although, the reflection from the retina will tend to produce a different colour to red. In order to avoid red eye when taking a photo, it is recommended that an external flash is triggered whilst at some distance (within the same horizontal plane, i.e. to the left or right) away from the camera. If the subject in a digital photo appears with red eye, mathematical algorithms within software can be used to make a correction.
[red eye example]

Underexposed - An undesirable situation whereby a camera's digital image sensor or film was not exposed to a sufficient amount of light under the prevailing conditions. An underexposed photo will appear too dark overall, with an absence of the lighter tones required to provide sufficient contrast. Sometimes, the level of underexposure is so great that the photo cannot be corrected by an image editing program.
[underexposure example]

Vignetting - (also called "falloff") In relation to a camera's optical system, describes an undesirable situation whereby the corners of an image show reduced levels of brightness, sometimes approaching black. Vignetting can be caused by such things as the design of the lens itself, or by a poorly fitted lens hood.
[vignetting examples]

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