BetterPhoto Q&A
Category: Digital Cameras and Accessories

Combo Lens - are they too good to be trueÉ

Do you have any photos taken with such a criter you could share, would you recommend it...

Thanks
G Smith

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September 18, 2008

On converter lenses:

The camera lens is a converging lens. A side view of the typical lens is shaped to resembles the Latin name for the lintel seed ‘lens’(). The camera lens is a converging lens. Rays of light are caused to bend inward as they transverse the optical system. We label lenses according to their strength. When the camera is focused on a distant object, the distance lens-to-chip (lens-to-focal plane) is measured. This distance measurement is the focal length. Typically the focal length is fixed during manufacture. Long lenses magnify and give narrow fields of view. Short lenses yield a wide-angle view. Many camera models sport non-interchangeable optics. On these models one must resort to unusual measures if one desires to alter the focal length. As an example if one desires a narrower field of view (higher magnification) your only option will be to mount a converter lens ahead of the built-in-optics. The idea is to alter the focal length. If a 1.5X converter lens is mounted in front of a camera fitted with a 75mm lens, the combination total will be 112 ½ mm. If a wider field of view is required, one mounts, say a 0.75X converter lens. Now the combination will be 56 ¼mm.

Now you need to know that every lens design has weaknesses. The lens maker strives to correct each and every one. This requires a complex stacking of lens elements each with different powers plus the use of different types of glass. What do you think happens when you mount a converter lens affront? The answer is, the amended array will always be degraded as compared to the original design.

How good will the new array be? Everything depends on the definition the original lens provided and the quality of the converter. Often the combination will yield reasonable results especially if the camera lens is stopped down around three or more f/stops from maximum aperture.

Now on a wide-angle converter, the front element will be a strong positive. Likely it will consist of two, cemented together lenses. Such a sandwich is called an achromatic doublet. Such a lens serves well as a supplemental close-up lens. That’s the so-called convertible ‘macro’. This application is a brazen maltreatment of the word ‘macro’. A ‘macro is a stand alone lens designed to image at unity i.e. life size. However, the achromatic doublet likely will function as a first class close-up lens.

Alan Marcus (marginal technical gobbledygook)
alanmaxinemarcus@att.net

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September 18, 2008