Lens Overview

General Information

The on-camera zoom lens covers focal length 8mm to 24mm as marked on the lens:

The 35mm equivalent of 8mm-24mm is 38mm-115mm, which covers not-so-wide-angle to moderate telephoto. This is a variable aperture zoom lens, which means the maximum aperture changes as the lens zooms. The lens marking shows that the wide side (i.e., 8mm) has maximum aperture F2.5, and the tele side (i.e., 24mm) has maximum aperture F4.0. The effect of zooming-in or -out can be viewed on both the viewfinder and LCD monitor. But, using the LCD monitor is preferable if the subjects are very close to the camera because of the parallax problem. See Autofocus Overview for the details. This lens also has very good quality for macro photography, and can focus down to about 2cm. See Close-Up for the details. In what follows, we shall provide a very general overview of a number of concepts and the meaning of some terms.

Focal Lengths

When you change focal length, you will notice two important effects: angle of view and magnification. When the focal length becomes smaller (resp., larger), the coverage in the image is wider (resp., narrower). It would be very helpful if you know the correspondence between the 35mm focal length and the focal length of 990's on-camera lens. The following gives this conversion:

For example, 12mm on a 990 is equivalent to 57.25mm on a 35mm camera. On the other hand, 105mm on a 35mm camera is equivalent to 21.92mm on a 990. The following images show the coverage of commonly used focal lengths. They are 35mm, 50mm, 75mm and 100mm (35mm equivalent). Note that 990 cannot achieve the 35mm focal length; however, you perhaps will not be able to distinguish the differences between 35mm and 38mm. The corresponding 990 focal lengths of 50mm, 75mm and 100mm are 10.49mm, 15.69mm and 20.88mm, respectively.

35mm	50mm
75mm	100mm
Click on the image to see a larger one

Focal length 50mm is the standard or normal focal length, because a 50mm lens (for 35mm cameras) covers approximately what human eyes can see. A smaller focal length than 50mm is called wide angle since its coverage is wider than that of a normal lens. See the images of 35mm and 50mm above. Focal lengths larger than 50mm are telephotos, because they can bring distant subjects closer like a telescope does. In fact, the on-camera lens only provide the so-called moderate telephoto, because its focal range is only 100mm strong.

Lens Attachments/Accessories

The on-camera lens provides the most commonly used focal range (i.e., 38mm - 115mm). The wide end is usually used for scenic shots and the tele end is good for portraiture. If the desired focal length is not in this range (i.e., wider than 38mm to cover a wider area or longer than 115mm to bring distant subjects closer), you have to use lens converters. Nikon manufactures a fisheye converter (FC-E8), two wide angle converters that can bring the focal length down to 24mm (WC-E24 and WC-E63), a 2X tele converter that extend the focal length to 230mm (TC-E2), and a 3X tele converter that extends the focal length to 345mm (TC-E3ED). Click here for an overview of these lens converters.

You might also consider to add a protection filter on the lens. The filter thread size is 28mm. The two most commonly used filters are UV filter and Polarizing filter (or polarizer for short). Click here for an overview of using filters.

Lens Flare and Ghost

When you point the lens to the sun or a very strong light source, lens flare and ghost may occur in your image. The left image below shows such an effect. The area near the bright spot is washed out. In fact, flare occurs even though the strong light source is not in the image. Flare occurs due to light bouncing off the glass surfaces of the lens (i.e., internal reflection) rather than being transmitted through. Because of this internal reflection, image contrast and tonality are reduced.

The right image below illustrates another effect, ghost, a string of color dots appearing in the image. The wash-out effect (i.e., flare) is still there near the upper-left corner, but is not as strong as the one shown in the left image. However, there is a string of dots, usually in green, purple or violet, appears in the image. These dots have the shape of the aperture of the lens and are not part of the actual scene. Therefore, they are called ghosts! Most low cost zoom and wide angle lenses, including the on-camera lens, suffer this problem. In a better lens, surfaces of glasses are multicoated with special anti-reflection chemicals to prevent flare and ghost. However, even though the on-camera lens is multicoated, flare and ghost are not eliminated completely. See Coated or Non-Coated for more details about lens coating.

Lens flare and ghost
Click on the picture to see a full size one

To overcome this problem, please do not point the lens directly toward or near a strong light source. If this cannot be avoided because it is your favorite scene, try to hold a paper or use your hand or a lens hood to block the incoming light. It usually partially solves this problem.

Also note that the right image has vignetting (i.e., shadows in the corners). This is because a polarizer is stacked on a UV filter and then zoomed out. To avoid this vignetting problem, do not stack filters on top of each other.

Field Curvature

There are two types of obvious field curvature: barrel and pincushion. Both types are slightly visible with on-camera zoom lens. Barrel distortion means straight lines in real world bow outward in images. The closer to the image edges, the worse the barrel distortion. Barrel distortion usually occurs in the wide angle side. On the other hand, pincushion distortion means straight lines in real world bow inward in images. Similar to barrel distortion, the closer to the image edge, the worse the pincushion distortion. Pincushion distortion usually occurs in the tele side.

Barrel distortion	Pincushion distortion
Click on the picture to see a larger one

The left image above shows an example of barrel distortion. The edge of the building and the pole bow outward. The right image above shows an example of pincushion distortion. A yellow line connecting the two endpoints of the roof is drawn. Comparing this line and the edge of the roof, you should see pincushion distortion.

Chromatic Aberration

Since the refractive index of all transparent materials varies with wavelength. It means a lens, in general, is not able to focus all three primary colors (i.e., red, green and blue) at the same point without applying some kind of optical correction. A lens in which two primary colors are corrected and united so that they focus at a common image point is said to be achromatic, while if all three primary colors are corrected and united is said to be apochromatic. A lens in which none of the primary colors are brought to the same image point suffers chromatic aberration. Therefore, the image produced by such a lens frequently contains color fringes that are not part of the actual scene. In digital cameras, the image capturing devices (e.g., CCDs) also contribute come degree of aberration. Because the sensors of an image capturing device are densely packed, it is possible that the color captured by one sensor may ``propagate'' to its neighbors, and, an effect that is similar to chromatic aberration occurs.

The areas marked by yellow rectangles in the following image shows the impact of chromatic aberration.

Click on the image to see a larger one

The lower-right corner and far right edge of the above image are cropped and shown in the following images. Click on the image to see the portions in original resolution. In the left image below, there is a purple fringe between the frame and garage door, and a orange fringe between the garage door and ground. In the right image blow, purple fringe also occurs along the window frame. These color fringes are the results of chromatic aberration and can reduce the quality of images. In general, boundaries between two high contrast areas are the places where color fringes may occur.

Click on the image to see a larger one

What Is Ahead?

Nikon Coolpix 990 supports two types of zooms, Optical Zoom and Digital Zoom. The latter provides a way to blow up the center part of a captured image using software algorithms. thereby offering a digital equivalent of telephoto lens with lower image quality. Also refer to Lens Converter Overview for more information.

Technical Data

The following technical information are taken from Nikon's manual. These information are for 950 and 990 only, and should be different for other Nikon cameras.