I've never used a wide angle converter (aka wide angle adapter) before, simply because I assumed they were cheaply constructed devices that would not provide acceptable image quality. I ended up trying one though and here are the results.
I only have one lens, the Tamron 28-75 f/2.8, which has a 35mm equivalent focal length of 42-112 on the DX (APS-C size) sensor of the Nikon D80. The 28-75 was fine for close-up portrait shots of our baby but as he grew older, I started taking more environmental portraits, which made the need for a wider angle lens more apparent. I would have liked to get a Tamron 17-50 VC (or perhaps the recently announced Sigma 17-50 OS) or even a non-VC Tamron 17-50 but there's no room on our budget for that, particularly because I was supposing that I would still use the 28-75 more often. I was contemplating getting the 18-55 Nikon kit lens as a cheaper substitute but I've been spoiled by the speed and constant aperture of the 28-75. Just as I was about to apply for a spousal permit to buy the 18-55, I thought about a wide angle converter.
(For new photographers, a wide angle converter or adapter is a wide angle lens that is screwed on to your existing camera or camcorder lens like a filter or hood.)
I never considered them before but now wondered if a wide angle converter would be a viable option. My first concern was whether my aperture would decrease. Tele-converters (lenses which are attached between your lens mount and camera mount to increase the effective focal length) have an aperture penalty proportionate to the tele-converter's magnification (1.4x = loss of 1 stop; 2x = loss of 2 stops). I have since found that there is no such penalty with wide angle converters. I drooled at the tantalizing possibility of using a 0.7x wide angle converter to get a 20-53 f/2.8 at negligible cost while still keeping my 28-75.
My second concern was the effect on image quality. I've used UV filters before (for lens protection purposes) and have avoided using them because of increased glare and significantly decreased contrast. If a relatively simple device such as a UV filter could degrade image quality so significantly, I wondered how much worse a wide angle converter would be. I was warned about the severe image degradation by a more knowledgeable friend who had tried one a long time ago. But I wondered whether or not those problems could be fixed in postprocessing. I also thought that an APS-C size sensor would only get the image from the middle part of the lens, therefore image quality problems at the edges would be mitigated. Plus, I was hoping the problems wouldn't be apparent with the webpage-only resolutions I use.
Well, I found a very inexpensive (less than $12 + shipping) wide angle converter for my lens (67mm filter size) that seemed to be a rebranded version of another wide angle converter that got a decent review on Amazon. The item I bought (67mm 0.43x by "Neewer") doesn't seem to be available anymore but here's a link to a very similar one from the same manufacturer and same seller. The magnification factor was 0.43x, wider than I was looking for, but I had read about some wide angle converters having an effective magnification that was much narrower than advertised. So I thought it was worth a shot.
This particular wide angle converter has two components. The first component is a close-up lens that can be used without the other component to decrease the minimum focal distance for your lens, thus getting macro-like capability. The second component is stacked over the first component and results in the wide angle.
I was originally going to run it through a number of tests for image quality and field of view but it turned out unnecessary to do so because the deterioration in image quality is noticeable on a casual glance. Specifically, the image suffers from severe spherical aberration at the edges, chromatic aberration (purple fringing), uneven distortion, diminished contrast, and the colors become more bluish compared to the normally warm colors of the 28-75.
WITHOUT wide angle converter (click to follow the link then click on the magnifying glass for 100% view):
When the lens is zoomed, the image quality deteriorates even more, so much so that it begins to look like a lensbaby picture. Sample:
As other people have commented on cheap wide angle converters in general, the advertised magnification factor doesn't seem to match the actual field of view. In the comparison shots above taken from almost the same position, using this focal length simulator, it seems that the shot with the converter is about 20mm (30mm equivalent), i.e., a factor of 0.7x rather than 0.43x.
You get what you pay for with this particular wide angle converter. If you really want to get a wider angle but for whatever reason cannot or don't want to use a real wide angle lens, then perhaps a wide angle converter might be ok for snapshots as long as the image won't be looked at closely and you don't plan on zooming your lens with the converter. When buying one, you might also expect that the actual field of view might differ significantly from the advertised magnification factor.
When buying one, I've seen some being sold for $50 or more. At that point, the price gap between that and a real wide angle lens (even a cheap one) would be low enough to make the real wide angle lens a better value in my opinion.
Needless to say, my search for a wide angle solution continues. I'm now deciding between several potential wide angle lenses and will be reviewing the lens I get in part 2 of this entry.