Opticallimits

This review of the Sigma 100-400mm F5-6.3 DG DN OS C...

https://sonyalpha.blog/2020/07/21/sigma-...temporary/

... reports a statement from Sigma:

Quote:Regarding your question on the teleconverter for Sony E mount, yes, your question is quite understandable. As you noticed, yes, we released a new TELE CONVERTER TC-1411/2011 for L-mount camera, but not for Sony E-mount camera, which must be unreasonable for you. As you may already know, we are licensed from Sony to design and produce Sony E-mount lenses, and it must be done in accordance with Sony E-mount specifications and license agreement. In the specifications and agreement, there are no statements on teleconverters. So, this is why we cannot design and produce Sony-E mount teleconverters. So, at this point, we have no concrete plan and schedule for making TELE CONVERTER TC-1411 in Sony E-mount. 

It's clearly my fault, but I understood that E-mount specs were open and third-parties didn't need a license for developing lenses.

I think it's normal that you have to sign a contract in order to obtain the license - even if it's free of charge.
As a manufacturer, you have to commit to comply to the specification and to be allowed to use the trademark.

Without licence, manufacturers appear to have no issue labelling lenses as being for Nikon F-mount or Canon EOS/EF-mount...

As simple as that:

https://support.d-imaging.sony.co.jp/www...etail.html

FWIW, I just read that Zeiss never received an official license for X-mount. Quite surprising actually - they released lenses in the very early days there.

Maybe Sigma just needs to ask Sony for a clarification. No reason not to allow making teleconverters if they're allowed to make lenses.