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I'm asking a more general question, starting from a particular case with which I'm dealing these days: I'm considering buying a Wide Conversion Lens for my Panasonic HC-X900M camcorder (see the item here) but I'm wondering what drawbacks it introduces and why the Wide Converter (also Leica glass) wasn't "integrated" in the lens' design.

Or more generally, what drawbacks/disadvantages a Wide Conversion Lens or a Teleconverter brings to an optical system (lens)?

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I'm not suggesting it's a duplicate as you specifically refer to wide converters too, but there is a similar question on teleconverters at photo.stackexchange.com/questions/889/… –  Nick Miners Nov 22 '12 at 11:38
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2 Answers

up vote 3 down vote accepted
  1. Reduction of light by 1 or more stops, depending on the specific design.
  2. An additional set of refracting elements in the light path, thus an additional distortion / aberration / Image Quality reduction factor.
  3. Observable decrease in sharpness at high contrast edges within image.
  4. In the case of add-on extenders specifically: Dust between the main lens front element and the extender back element, hence reduction of contrast and sharpness. Essentially, 2 additional dust / fingerprint prone surfaces in light path.
  5. Potential vignetting at corners, especially with telephoto extenders at maximum focal length.
  6. Potential introduction of pincushion / barrel distortion at various focal lengths if the extender is not perfectly matched with (and designed specifically for) the main lens.
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Why would a wide extender reduce light? –  Imre Nov 23 '12 at 6:50
    
The same way adding a UV filter reduces light by about 1 stop: Light transmission reduction while passing through layers of refractive media. –  Anindo Ghosh Nov 23 '12 at 6:56
1  
Wow, those must be really crappy UV filters, even the lowest-scoring UV-filter in Lenstip test had transmission rate close to 90% (but a loss of one stop would be 50% transmission) –  Imre Nov 23 '12 at 7:04
    
It's polarizing filters that reduce light by up to 1 stop by filtering out light with a certain polazrization. UV filters (or wide adaptors in general) do not lose a significant amount of light. –  Matt Grum Nov 23 '12 at 9:22
    
Fair enough, 1 stop is overstating it. –  Anindo Ghosh Nov 23 '12 at 9:29
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A front mounted wide converter will most likely reduce image quality by introducing new elements into the optical path, increasing the change of flare, as well as increasing the amount of refraction occurring which will result reduced corner sharpness and more distortion/aberrations (unless the adaptor is very well corrected).

A front mounted wide converter wont reduce the lens f-stop (a rear mounted wide converter would actually gain you stops, but reduce the size of the image circle).

A rear mounted teleconverter will reduce the maximum f-stop by the magnification factor (a 2x TC would turn an f/2.8 lens into f/5.6. A front mounted teleconverter wouldn't, but would be larger. Again using a teleconverter will introduce new elements, aberrations and reduce sharpness as you are enlarging the image. Rear teleconverters used with telephoto lenses can still yield very good images as the light reaching them has very little spread.

As to why the converter is not included in the design, that is so the reduction in image quality isn't forced on people who don't need the increase/decrease in field of view. Canon have announced a lens with a built in TC that slides out of the way, which is a potential solution to this problem, though it comes at a higher cost.

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