Camcorder Lens Defects Explained

Jeff is one of our primary moderators at the DV Info Net Community.

A short bio: I started in video production and photography in 1979 when I moved to Cincinnati. Over the course of the next 17 years the company grew to 3 partners and 17 employees. I became an Avid Certified Editor in 1993 and we were the first company in Cincinnati to on-line with Avid Media Composers. I sold my interest in the company and worked on an independent production about free agents in the National Football League. I was the Director of Photography and we went to over 20 NFL cities shooting spring tryouts. The film was off lined by me, on an Avid. The film went to Sundance, but was never picked up by a distributor. Local network TV was my next calling and I worked shooting and editing, on Avids. In January 2001 I bought a Mac and started using FCP to edit productions for small non-profit groups. Today I operate a small production company doing work for Fortune 500 clients and non-profits. My work includes DVD authoring, web site design, video production and post production. In my spare time I teach photography and digital photography at a local fine arts school.

Here are the ten optical defects and what the camera operator can and cannot do to correct the defect.

1. Astigmatism – The inability of the lens to bring to focus both vertical and horizontal lines on the same plane. Astigmatism is caused by axial rays (not parallel to the lens axis). It will appear that lines of equal density (darkness) are less dense horizontally or vertically. Astigmatism is improved by stopping down the lens (smaller lens opening, larger F number).

2. Coma – Coma causes parallel oblique rays passing through a lens to be imaged (focused) not as a point, but as a comet shaped (oval) image. Coma can be improved by stopping down the lens.

3. Curvature of Field – The plane of sharpest focus becomes curved, not flat. It is caused by rays from the outer limits of the subject plane coming to focus nearer to the lens than the axial rays (image comes to focus in curved shape, away from the CCD). This defect is not improved by stopping down the lens.

4. Longitudinal Chromatic Aberration – The inability of a lens to focus all colors (wavelengths) at the same plane on the lens axis (shorter wavelengths come to focus in front of the CCD, longer behind). This defect is not improved by stopping down the lens. This is normally only noticeable in long telephoto lenses. It is reduced by the use of special glass elements, referred to as ED, ID, LD or Fluorite.

5. Lateral Chromatic Aberration – lateral displacement of color images at the focal plane (CCD). Caused by different sizes of images by produced by different colors even though the image is all on the same plane (CCD). Produces color fringing of red or blue. Not improved by stopping down.

6. Spherical Aberration – Inability of all rays to focus at the same point. Marginal rays (at the edge) through the lens come to focus closer to the lens than do paraxial rays (rays parallel to the axis or center). This causes the focus to drift as you stop down the lens. This may be part of the XL1 focusing problems. However, it’s just my opinion.

7. Distortion – distortion causes the image of a straight line, at the edges of the field (CCD) to bow in or out. Bowed in, pincushion, bowed out barrel distortions. Produced by variations of magnifications over the field of the lens, particularly at the ends of its range (max wide angle, max telephoto). It does not effect sharpness, only shape of the image. It is not improved by stopping down. Very common in extreme wide angles and lenses with WA adapters.

8. Flare – Flare is non-image forming light. Reduces contrast and color saturation. Flare is caused by very bright subject areas and produces internal reflections in the lens. Stacking of filters can increase flare. Lens coatings by the Mfg. keeps flare to a minimum. The use of a suitable hood (not the stock hood) can also aid in reducing flare.

9. Ghost Images – Not a true defect but is seen quit often and sometimes confused as one. Distinct images, usually in the shape of the diaphragm or a very bright light source included in the scene. Caused by high intensity light producing rays which bounce around in the lens and form an image. Often multiple images appear in a row across the scene, starting at or near the source. Ghost images can be reduced by the use of a suitable lens hood.

10. Diffraction – Again, not a true defect, but a property of light. Diffraction is the bending of light rays as they pass a small opening or past a sharp edge. Common when a very small diaphragm (F16, F22, etc.) is used. Diffraction causes a point to be less sharp.

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