Tuesday, 30 July 2013

Camera Lens Corrections - Vignetting

Camera Lens Vignette
Camera Lens Vignette

Like all other things; lenses are also not 100% perfect; Almost all of the lenses are prone to one or the other imperfections; photographers have no way but to live with them; however they can adopt many techniques to correct these faults either during shooting or post production to even these aberrations.

It is a fact that all types of lens- imperfections cannot be corrected, some can be and some cannot be. In certain situations these imperfections could actually be used to add to the aesthetic appeal of an image. In this article we will discuss three of the most commonly found lens- faults, why they occur, how they can affect the final image etc. Here are also some tips and methods to minimize or eliminate these aberrations altogether; both while shooting as well as in post- production.

The most common imperfections found in lenses are:

  1. Vignetting: vignette is a progressive darkening that occurs in the edges of the image. This phenomenon is known as vignetting; it occurs due to slight reduction of exposure on the corners of an image.
  2. Distortion: it is yet another fault where otherwise straight lines bend inwards or outwards in the photographs.
  3. Chromatic Aberration: is color fringing along high contrast edges.

Vignetting - What is Vignette?

Lens projects a circular image(called the “Circle of Illumination”) on to the sensor; since digital camera’s sensor is rectangle in shape, they only actually record the middle portion of the projected image circle; the rest is cropped out.

Circle of Illumination
Circle of Illumination

The‘Circle of Illumination’ is not the same for all lenses. Some lenses project a very wide circle while some others project a circle which completely fits into a 24x36 capture area. Vignette is rare in lenses which project a wide circle of illumination (typically tilt shift lenses); while in lenses which produce a smaller circle of illumination (e.g. Circular fish eye lenses) vignette may be very prominent.

When light passes through the lens, it directly hits at the center then spreads from there to the fringe areas traveling more distance. This causes in slight reduction of exposure on the corners of the image causing darkening.

There are three types of vignettes namely mechanical, optical, and pixel.

Mechanical Vignetting

Mechanical Vignetting caused due to stacking ND filters
Mechanical Vignetting caused due to stacking ND filters

Mechanical Vignetting (or Physical Vignetting); when light coming on to the lens is blocked by some external objects such as a wrong lens hood for the focal length, filters that are too thick, stacked filters, secondary lenses or elements etc. It appears as a strong, abrupt darkening usually only in the very corners of an image.

How to Eliminate Mechanical vignetting

Mechanical vignetting cannot be avoided by adjusting any of the variables like aperture, focal length etc. Stopping down the lens to a narrower aperture will only make it more pronounced. The only way left is to find the obstruction and removing it.

Optical Vignetting

Photo by: Niels Linneberg

Optical vignetting is caused due to light fall off towards the edges; which is inherent in the lens design. Most lenses transmit more light at the center than the edges; this too causes vignette. Optical vignetting is most commonly found in some fast lenses and wide range zoom lenses; it is more pronounced when focusing on distant objects and when shooting wide open.

How to Eliminate Optical Vignetting

Optical vignetting could be easily eliminated by stopping down the lens to a narrower aperture (higher f number).

Pixel Vignetting

Pixel vignetting occurs as a result of the physical depth of photon wells that capture light at each pixel.  It occurs when light entering the well becomes more oblique. Even though this is a digital imaging phenomenon, it can be influenced by the lens in use. It mostly occurs when one uses wide-angle and/or wide-aperture lenses.

How to Eliminate Pixel Vignetting

Pixel vignetting can be minimized by using a longer focal length lens and also by changing the direction of light.

Some General Tips for Reducing Vignetting

Photo by: Rawhead Rex

  • When lenses designed for full-frame format is used in crop sensor camera; vignette becomes less noticeable as they crop out more from the circle of illumination. A full frame camera however uses more of the circle and there will be more difference in exposure between the center of the frame and the edges.
  • Vignette is most pronounced when lenses are focused at infinity.
  • Shooting with proper exposure will help reduce vignette.
  • Or shoot with the lens stopped down to a narrower aperture.
  • Changing the lens or shooting with a longer focal length lens also could be considered for the job.
  • Artificial Vignetting
  • When one is forced to live along with something; it is natural to think whether any advantage can be made out of that. But that is not the case here, we have already mentioned that certain subjects and situations actually benefit by having a vignette effect so sometimes photographers add vignette to their images while post processing is done; this also enables them to draw attention to a central subject, and to make the frame's edges appear less abrupt. The proper way is to apply Vignetting after image cropping is done (known as ‘post crop vignetting’). Here’s an article that explains how to apply dark vignette effect using Adobe Photoshop.

Let us deal with ‘distortion’ in the next article; have a good day!