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Gary Mix is a network specialist, Wedding Photography in Gainesville Florida
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Vignetting is a widespread photographic issue. It is found in most lenses and in all cheap compact cameras. It is a radial loss of brightness (due to exposure drop) visible at the corners. Vignetting is generally undesirable, but it can be used to draw attention to the central subject of the picture. There are in fact two aspect to vignetting: The common underexposed corners and the bright center, called the hot spot.

There are several causes of vignetting: 1. Mechanical. 2. Optical. 3. Natural. 4. Pixel.

Mechanical vignetting is typically caused by too thick a stack of filters, or a lens hood attached to the lens, or by secondary lenses. All three may reduce the light at the corners and darken them. The darkening will be abrupt or gradual depending on the aperture. The smaller the aperture, the more abrupt the vignetting.

Optical vignetting is inseparable from the lens makeup itself and is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. It is also called artificial vignetting. When lenses are used with a wide open aperture, the length of the lens becomes a problem: the edges of the lens will shadow the edges of the aperture and thus darken the edges of the photo. This kind of V is most pronounced in zoom lenses and wide angle lenses. The light in the lens is also reduced radially because the rear lens elements are shaded by the front lens elements, thus causing vignetting. One can often cure optical V by reducing the aperture two or three stops. Very large front lens elements tend to reduce this kind of vignetting and are typically used in wide angle lenses. Finally should be mentioned that the contrast of the sensor or film also plays a part: the stronger the contrast, the stronger the vignetting.

Natural vignetting (also called natural light falloff) is a, well, natural light falloff proportional to the angle the light reaches the sensor or film; it is not caused by the lens. Technically the falloff is proportional to the fourth power of the cosine of the angle of the in falling light on the film or sensor. Lenses in compact cameras are particularly prone to such falloff. So are wide angle lenses. Telephoto lenses show the least falloff. At large apertures both optical and natural vignetting will be present. The combined effect is often called illumination falloff or radial density.

Pixel vignetting is of course not relevant for film, but only for digital cameras. It is created because most sensors have an angle dependency of the in falling light. Light reaching the sensor at an oblique angle generates a weaker sensor reaction than light reaching it at a right angle, thus the corners become underexposed. Digital cameras often have a built-in compensation for this, used when converting the RAW image data to tiff or jpeg. If one works with RAW images, one will have to do post-processing to remove pixel V.

Understand vignetting in photos and how to avoid it. Based upon this article about vignetting in photos.<

Gary Mix is a network specialist, Wedding Photography in Gainesville Florida
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In the good old days, back in the darkrooms with photo paper, there was a black and white system called multigrade. Multigrade paper was available in six grades and each grade produced images with more or less contrast. Today software provides contrast adjustment, but not all contrast adjustments are equal or equally good.

What is contrast anyway? One (noun) definition of contrast could be: “The state of being strikingly different from something else, typically something in juxtaposition or close association.” It can also be defined as (verb): “To set in opposition in order to show or emphasize differences.” This means we in photos have three different kinds of contrast: contrast of luminance, contrast of color and contrast of saturation. Most software offers a single contrast slider that edits all three at once.

Software usually alters contrast by simply altering the difference between the middle value (128) and the individual red, green and blue values; like this: R= (R-128) * contrast + 128; and likewise for green and blue. But what if the image is generally dark or pale, then this algorithm will fail. In that case one will have to use the average values of the image’s R, G and B channels, like this: R=(R-RAverage)*contrast + RAverage. And so on for G and B. The problem with this method, in both cases, is that photos tend to become overly saturated and colorful with contrast enhancement. Similarly contrast reduction makes the picture look like you have placed a semi-transparent gray film over it.

When considering luminance contrast one can use the above algorithms after converting the RGB values to L values (luminance). Luminance is not just the average of RGB, since the channels are not equally bright. Green is brighter than blue and red and red is brighter than blue. Therefore one usually uses a weighted conversion like this: L = R*0.299 + G*0.587 + B*0.144. This algorithm is used in the YIQ colorspace. Photoshop has an option in their contrast editing panel called: “Use Legacy”. The problem with this method is that the image becomes pulled towards the primaries R, G and B if you raise contrast and pushed towards a uniform mid-gray if you lower contrast. It is not very useful.

However, there are several ways to enhance luminance contrast. First one can simply expand the range: if Lmax is 200 and Lmin 50, then one can for example expand the range so Lmax is 255 and Lmin is 0. But what if Lmax and Lmin already are at the extremes? Then one will have to pull the existing near mid tone L values towards the extremes. This leaves two situations: one will normally lose the mid tones that way and split the picture; alternatively one can use dedicated software that can preserve mid tones while pushing L values towards the extremes.

There is dedicated applications for luminance contrast adjustment. If you want to do it in Photoshop, you will have to convert the image to Lab mode, make the L channel alone active and edit contrast for that. Remember not to use Legacy mode or the picture will become either black and white or uniform gray. When editing contrast, don’t just look at the extremes and the contrast in the picture, but keep an eye on the mid tones.

Here you can read more about contrast. And here: contrast. This article, Problems of Contrast in Photo Retouching has free reprint rights.<