[John]

I want to buy an extender for my 100/400 mm lens and I am not sure whether to go for a 1.4 times or a 2 times. I am aware of the 1 and 2 stops light loss respectively and that this will affect the ease of focusing. Due to a complete lack of experience I have no idea just how much more difficult focusing will be.
I would try it using AV mode and preview but I understand that one should not refocus when in preview. My inclination is to go for 1.4 times, but I think I will need all the extension I can get. I am mainly interested in kingfishers and herons which are usually on the far side of the river at about 150 m. Hopefully someone with experience of both converters will come up with an answer.