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What is Vignetting?
Vignetting refers to the effect by which the edges of the image are either darkened or desaturated compared to the center of the image. It can be either a sudden change or a gradual gradient, an intense reduction or a subtle one, depending on the type of the vignetting.
As you might imagine, this can be both a powerful tool or a pervasive nuisance to the photographer! While a vignette can draw attention to the center of the photograph, making it well suited for a variety of subjects such as portraits, it can also make a photograph feel dated or overdone, or simply sacrifice image quality around the edges.
So, to either intentionally add - or consciously avoid - the vignetting effect, it's important to understand the different types and causes of the vignette:
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Mechanical vignetting refers to vignetting that occurs as a result of something attached to the lens barrel physically blocking out the light along the periphery of the incoming light. Thus, it is also sometimes known as physical vignetting. This can be the result of poor camera manufacturing and design, but not always. Mechanical vignetting can also be the fault of the camera user for using an badly fitting and misaligned lens hood, stacking too many lens filters at the end of the lens, using poorly made DIY lens attachments, or the like. There are also lens filters designed to deliberately add the vignetting effect for artistic purposes.
Mechanical vignetting can cause total blackening of the edges, and the darkening effect is usually quite abrupt. A quick fix for mechanical vignetting is to lower the f-stop, which can either eliminate or dampen the vignetting effect, though this is obviously not ideal.
It's easy enough to check whether mechanical vignetting is an issue or not: take off all your lens attachments and see whether there is still a vignetting effect around the edges. If the vignetting problem persists, your problem lies elsewhere. If not—well, go through all your equipment and see which attachments (or combinations of attachments) are the problem! You may end up rethinking your accessory usage a bit.
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Optical vignetting is caused by the addition of multiple lens elements, such as a telephoto lens, a macro lens, a wide angle lens, etc. Each additional lens piece, by continuing to refract the incoming light, also works to narrow the effective lens opening by which light enters the image sensor. This is an inherent element of lens design, made conspicuous by increasing the amount of refraction and thus vignetting that occurs. The vignetting that occurs as a result of this tends to be subtle and gradual.
The solution? Well, be careful with what lens elements you buy! Optical vignetting is more likely to occur with cheaper, less well-made lenses, but this is only a generalization. Research different lens elements carefully before purchase lest you make an unpleasant discovery regarding excessive optical vignetting later.
Also, just be mindful of how many lenses you have stacked on your camera! Try not to overdo it, as no matter how good your lenses are, if you have enough of them stacked together, you can and will encounter conspicuous optical vignetting.
A more temporary solution is to decrease your f-stop (aperture), the same as with mechanical vignetting.
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Natural vignetting differs from the previous two types of vignetting, as it is not caused by a blocking of the light. Rather, it has to do with the image sensor.
Natural vignetting occurs according to what is known as the cos^4 (b) law, also known as the illumination falloff law. The amount of light reaching the edges of the image sensor decreases as compared to the center for a number of reasons, all relating to the angle (here represented by b) at which the light hits it. First, as according to the inverse square law (cos^2 (b)), it will take light ever so slightly longer to reach the corners of the image sensor, and thus slightly less is received from when the aperture opens to when the picture is taken. Second, still less light will reach the corners of the image sensor because the same amount of light that is reaching a certain area at the center of the image sensor is spread out over a larger area at the edges, which creates another cos (b) amount. Thirdly and lastly, the effective lens opening relative to the corners of the image sensor appear to be elliptical, not circular, which transmits still less light, producing yet another cos(b) term. (This is only approximately related to cos(theta), but is close enough for purposes of photography theory.)
This, and optical vignetting, both contribute to an overall vignetting effect that is pervasive and difficult to do anything about. There's really not much you can do to avoid this. Some lenses are more prone to it than others, particularly modern ones that are designed to deliberately avoid this problem by attempting to have the light strike the image sensor as parallel to each other as possible. This tends to be only a very subtle effect, however, and may be difficult to notice with anything less than the professional eye, so don't worry overmuch about it.
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This is the only type of vignetting that is unique to digital cameras, as by definition it cannot be found in film cameras.
Pixel vignetting has a similar cause as natural vignetting, as is the result of the slightly oblique angle at which light hits the edges of the image sensor relative to the center. However, while all the effects of natural vignetting also apply to film, pixel vignetting refers to the specific vignetting that occurs because of the fact that the image sensor picks up more light when it is head-on as opposed to slightly at an angle.
This is a subtle effect, but when combined with both optical and natural vignetting, can be quite conspicuous. Also as with optical and natural vignetting, there isn't much that you as the camera operator can do about this. Your best solution is simply to attempt to research cameras that have higher grade optics, or careful application of digital editing software to remove any pixel vignetting that occurs.
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This refers to, obviously enough, vignetting that is digitally added via image editing software. This can be with any color, though black is typical, with any degree of subtlety or abruptness that is desired. Anything is possible! Software may be used either to emphasize (or deemphasize) an existing vignetting effect.
Software vignetting is done on the part of the photographer for artistic purposes, primarily to emphasize a subject in the center of the image. You can learn more about artistic uses of the vignette by reading Beginner's Guide to Vignetting, and a comparison of black and white vignettes by reading An Introduction to Black and White Vignettes.