Sorry but I got to laugh. Your old Canon 70D. Is that model really that old? I'm using a t4i that I got as a gift in 2013. At that time Canon was issuing the 50D, or was it the 60D? Anyway, the sensors of the cropped sensor cameras are not remarkably different when it comes to ISO performance and handling of extreme dynamic range scenarios. A full frame high-end model does indeed afford better management of HDR and noise at high ISO. That even applies to much older FX models. Obviously, a fast lens lets you apply a faster shutter speed at a given aperture. That's the most important factor. A faster lens will very likely also deliver the swiftest AF performance. A brand lens will also have the fastest and most accurate AF, whether Canon, Sony or Nikon, or Olympus. A heavy long focal length lens will prove difficult to bring into position and support. The use of a TC would provide greater magnification without the heavier feature of a long prime, but the original glass would need to be superb and the AF able to overcome the TC glass. Something like a 300mm f2.8 with a 1.4x TC would give you 420mm @ f4 and still have a sharp image and a fast AF and shutter speed. Unfortunately, that kit will cost you thousands of dollars and is so far beyond my means as to effectively be property of NASA for all intent and purpose. Yet, having acknowledged that it remains what you could describe as "what you guys are using". The latest mirrorless camera models offer AF at f8 and IBIS (in-body-image-stabilization) to the tune of 5 stops. You could use a coupler for DSLR lenses (moves the lens into the right position for the sensor) and then use a 2x TC with an old non-IS 300mm f 2.8 (which should be a lot cheaper than the IS model and the IS II model) thereby offering you 600mm @ f5.6 and the superb image quality that was a feature of even the oldest lens model of that configuration. The 2x TC mk III will not significantly degrade the image of these amazing lenses. Of course, what you save in buying an old non-IS lens is then sucked up in getting a $4,000 mirrorless camera. Money makes the world go round, the world go round ... money makes the world go round ... I'm hearing Joel Grey after all these years.
