First party manufacturers offer wide angle converters today for their current lenses. Sony offers 4 such converters for E-mount. The Sony FE 2/28 has a sensor to detect the converters allowing it to send the proper lens profile to the camera. The Sony converters also work on lenses there are not designated for.
There may be many reasons why third party converters don't work well, but photography era is not one.
Yes - this is a good point: There are camera makers that produce "matched" multipliers for telephoto and wide adapters - usually for specific lenses. This is often to make a very expensive camera or lens more "valuable" to a user. Fuji springs to mind as a current example with several options while Panasonic have a moderate wide-angle adapter for the 14mm M4/3 lens.
Olympus had a series of well made but heavy conversion lenses made for the era when they abandoned film SLRs with interchangeable lenses and made the first "bridge" cameras for 35mm film - in cooperation with Ricoh (now Pentax). Those often have 55 or 58mm filter threads, and are often found on eBay.
This thread is on wide angles - but I will just mention that some of those moderate telephoto converters in the Olympus TCON 1.3-1.5x range are excellent; for example turning the Lumix 25mm F1.7 M4/3 "standard " lens into a moderate fast telephoto for portrait work, as it still keeps its wide F1.7 aperture. In that setting, soft edges is part of the appeal for shallow depth of field portraits. At the 70mm equiv. kit zoom setting most lenses will be at F5.6. However, the inexpensive olympus 42.5mm for M4/3 is just so wonderful, and not too expensive second hand - that my advise is to save u and buy that (or the very good hard to find Chinese Yi 42.5mm F1.8 AF lens for M4/3, at 1/3 the price)
Panasonic and Sony made adapter lenses for their digital bridge cameras that are good - but are often heavy and cost £/$100s. The uniform factor is that these were matched to cameras with zooms that went no wider than 35mm equivalent - and the conversion lenses took them to 28mm equiv. Using them on cameras already reaching 28mm wide takes them beyond their design specification - with loss of image quality. Many of these lenses are huge and heavy - so beware !
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have read widely on the topic, and looked at a lot of websites and videos as well as tried conversion lenses out for myself. The conclusion looks to me to be:
[1] matched multipliers and wide conversion lenses are near uniformly excellent - when paired with the lens and camera for which they were designed for - but they are often expensive and heavy and typically are only matched to moderate wide-angle parent lenses
[2] "generic" wide converters are near uniformly poor when used on wide angles at 28mm full frame equivalent or wider - they distort and have soft edges with chromatic aberration - a combination of faults that is very hard to fix in software.
[3] moderate conversion lenses (x0.7 or lower strength) with multiple lenses and a big difference in front filter to back filter sizes are good for video work where the soft corners are not the key subject matter. This is even more so when you switch to a "letterbox" format such as 16x9 or wider. I have an "opteka" brand example that is 52mm rear thread and 72mm front, also sold under lots of different brand names today on the web. For still photos taken stopped down to f8, with barrel distortion corrected they can be just OK - but I have never good a "good enough" image for a 10x12' print from them. But - for "vloggers" shooting web-standard video they can be a "Best Buy".
[4] stronger wide converters such as the widely advertised x0.45 models (with 10x macro) are a uniform disaster when applied to an already wide-angle lens. They distort and vignette such that they are unusable. However, as an experiment, I found them to be "just OK" on standard 50mm equiv. length lenses. However, every camera system sells a cheap "kit zoom" that covers the 28-70mm equivalent range - and these are optically way ahead.
I would love to "find" a perfect conversion lens - one that took a 28mm wide kit zoom to a 18mm equivalent rectilinear equivalent perspective that weighs 100g, costs £/$50 or less and fits in my pocket and does not distort. However - when you look at the design spec' for such "ultrawide' rectilinear lenses, they typically have 15 or more lens elements, with many having costly high density glass, are very bulky and cost >>£/$500, you know that a cheap lightweight conversion lens from eBay or Amazon just cannot compete - it is just physics.
So this is why my advice for still shooters is to learn how to do panorama shots, or get a fisheye and learn how to "defuse" the image. Every other good solution is either very expensive or sacrifices too much in image quality to be worthwhile.
