I am using the Samyang 500mm for video. Picked up a used one for about $75. IMHO it provides good value for money. Image quality is good although one should not expect Canon/Sigma quality at this price point. This lens does not have image stabilization, so on windy days it is very hard to get steady images, even when used on a tripod.
Crop vs teleconverter
Not sure what 'very long' means but I've used this Vivitar (Tokina) 70-210 AF lens on my GH2. The results are generally good although it tends to lose contrast in bright sunlight.
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