A Viewfinder Darkly

Photography tips and tutorials

Pentax’s New 18-250mm Digital Zoom Lens

October 5 2007

by Phil Northeast

Equivalent to a 28-380mm focal length zoom in 35mm terms, the DA 18-250mm lens handles landscapes and portraits as well as zooming in on sports and wildlife.

The new SMC PENTAX DA 18-250mm F3.5-6.3ED AL [IF] zoom lens promises extra versatility with its wide range of focal length while still producing quality images.

Pentax DA18-250 mm

Pentax DA18-250 mm zoom lens

Zoom lenses with such a large range of focal lengths do suffer a performance disadvantage compared to prime lenses and other zoom lenses that cover a smaller range of focal lengths. Modern Computer Aided Design and manufacturing techniques are overcoming the performance drawbacks zoom lenses with wide zoom ranges.

The advantage for the photographer is the flexibility offered by lenses such as this new zoom from Pentax. This places the lens in the category of an excellent ‘walking around lens.’ Many photographers have favourite long lenses for sport and wildlife, and short lenses for landscapes and portrait work. The flexibility allows the capture of those unexpected moments, without carrying around a collection of more specialist lenses.

Another benefit is in dusty conditions the reduction in the need to change lenses means less chance of getting dust on the sensor.

Pentax claim the optical design is optimised for the sensor of Pentax DSLR cameras to effectively minimize flare and ghosting in images that may be more prevalent in digital cameras.

This makes the new lens unsuitable for Pentax film cameras, although it will physically fit as the basic physical design of the Pentax K mount has not changed since its introduction on the 1970s.

However, quality lenses with good lenses coatings, such as Pentax’s own Super Multi Coating (SMC), produce excellent result in older lenses suitable for both film and digital SLR cameras.

Extra low Dispersion (ED) glass in the lens reduces chromic aberration. This is where the different wavelengths of light refract slightly differently as they pass through a lens element. Chromic aberration is noticeable as producing a colored fringe around the edges of objects in an image. If optical design heads in the opposite direction, you end up with a prism.

Aspherical elements (AL), refers to the shape of some of the individual lenses within the zoom lens. Instead of the more common, and cheaper to make, simple spherical shape, these elements have a complex shape allowing the one element to do the job of multiple elements.  Their inclusion in a lens design produces high optical performance with less weight and a compact size.

The Inner Focus (IF) design keeps the front element of the lens in the same position during focusing. A common characteristic of zoom lenses is to rotate the front element as part of the focusing operation. This makes very difficult when using a polarizing filter. These mount on the front of the lens and rotating the filter element adjusts the filter producing the optimum effect.

To overcome this annoyance designers place the moveable focusing element well inside the lens. This also keeps the size of the lens down. While some zooms may look reasonably compact, they can increase their length noticeably during focusing.

One indication of the design quality of this lens is its close focusing performance with minimum distance of 17 inches (0.45 meters) right across the long zoom range.


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