UPDATE: Since first writing this I discovered I can get the EF-EOS M adapter from Canon for $199. Although having to add this to the camera won't really help to make adding lenses to the EOS-M affordable, it would give me the option to use the telephoto lens from my existing EOS 600D with the M100. Assuming it actually is available at that price, it would have fallen sharply in cost from the original pricing which was around $500-600 for the adapter.
One of the obvious factors about owning an interchangeable lens camera is that we expect to be able to change lenses when we need something like a telephoto or wide angle instead of a standard lens. Since I own a Canon EOS M100 camera, I'd hope to be able to fit it with a tele zoom one day and be able to take photos at considerably greater distance than the lens that comes with it, which is a Canon 15-45 mm collapsible zoom. The Canon zoom lenses for the EF-M mount, however, are very expensive, priced around $600, which is almost as much as I paid for the camera itself, making it inevitable I'd want to take a look at a third party lens.
One of the obvious factors about owning an interchangeable lens camera is that we expect to be able to change lenses when we need something like a telephoto or wide angle instead of a standard lens. Since I own a Canon EOS M100 camera, I'd hope to be able to fit it with a tele zoom one day and be able to take photos at considerably greater distance than the lens that comes with it, which is a Canon 15-45 mm collapsible zoom. The Canon zoom lenses for the EF-M mount, however, are very expensive, priced around $600, which is almost as much as I paid for the camera itself, making it inevitable I'd want to take a look at a third party lens.
Industry commentators have long remarked on Canon's extreme reluctance to threaten their profitable D-SLR market segment with mirrorless cameras, given the less than inspiring design and high pricing of the early EOS M models. Canon has insisted this is not the case and have brought out a much wider range of EOS M models with lower pricing in recent years. However, there is only a very limited range of extremely expensive Canon lenses and accessories produced for the EOS M models, compared to what is available for the EOS D-SLR in the standard EF mount. The cheapest Canon EF-M zoom lens costs around $550 retail. It's very clear that Canon is only prepared to position the EOS-M series as high-end compacts with deliberately strategisation to discourage end users from looking for an interchangeable lens camera at a cheaper price than their DSLR range.
Current pricing on the Tamron AF 70-300 Di LD lens is around $287 which is a very low price for a lens (the actual range of the zoom may be somewhat different as it will vary depending on the crop factor of the body it is fitted to, this is unclear at the time of writing). At that price it is made of plastic like the Canon kit lenses. There are a few other limitations that you would expect with a cheap lens like a front element that rotates (tricky with some types of filters that are angle dependent), slow autofocus, no image stabiliser, sticky zooming, blurry corners, some distortion, telephoto fringing / softness. The results will depend on the type of camera it is attached to, with a smaller APS-C sensor the edges of the lens are missed which gives better quality in many respects, but does not overcome the mechanical / design limitations.
The main issue is that I am finding it difficult to discover if this lens is actually available for the EOS M. If it should turn out that Canon has placed restrictive licensing on third party lenses for the M mount it will achieve their strategy of deliberately strangling the market for EOS M lenses. So far it looks like Tamron's only offering is the much more expensive 18-200mm F/3.5-6.3 Di Ⅲ VC, which whilst being a better quality lens, ends up costing practically the same as Canon's lenses. It must be apparent that Canon has placed severe restrictions on the production of affordable third party lenses for its EOS M cameras.
