What are the caveats for buying a teleconverter that is of a different brand than the lens?

Question

What are the caveats when choosing a teleconverter brand that is of a different brand than the lens it will be attached to in attempt to maximize image quality?

Lets assume the following to make the domain of this question more palatable:

1. The TC will fit the lens
2. The lens has a modern AF motor (AF-S, USM, HSM, etc.)
3. The lens is fast enough such that the TC will not prevent AF

The fact that modern lenses contain embedded computers which communicate to the camera body is giving me pause. On the face of it, it would seem logical that matching brands between the TC & lens would give the highest level of compatibility and functionality. However, not all brands are created equal when it comes to image quality and so one might be forgoing IQ in favor of functionality. The question is how large is that tradeoff?

The basis for this question is my current quandary of deciding if I should go with a Kenko, Nikon or Sigma TC for my Sigma 50-150mm lens on my Nikon D7000. Image quality, value and compatibility are the three variables that I see in play. Compatibility is my #1 priority and assuming they are all compatible (are they?) then I would be willing to pay 15-20% more for improved IQ.

Answer 1

The biggest concern would be compatibility of the electronics, but this is mostly an issue when using a non-camera brand tele-converter with a camera brand lens. The off-brand teleconverter may not know about some of the communication that the lens and camera do and future firmware updates for either the lens or the camera body may result in further issues if they update the protocol being used. It might also not work as well on future lenses unless the teleconverter itself is updateable.

As far as the quality of the optics go, a teleconverter is basically a magnifying glass that magnifies the image circle made by the lens. It it completely agnostic to the optics of the lens and the lens is completely agnostic to the teleconverter on a basic physics level as long as the teleconverter is properly designed.

Where it does matter is how the lens attempts to focus and what aperture settings are used. Since the teleconverter discards a large portion of the light that would normally make it to the image circle, it causes a drop in the speed of the lens, so the widest open setting goes down.

The compatibility problems occur when the teleconverter can not adapt the focus and aperture signals going between the lens and the camera to ensure they communicate correctly. The degree of compatibility will depend on the precise lens, camera and teleconverter in question though, so best to research for your bodies and lenses prior to making a purchase.