The 1.4x Teleconverter Doesn't Cut It!

[Space Face]

I agree with the above.

I always expose for the subject, particularly for birds and shoot wide open almost all of the time. The general rule too is your shutter speed needs to be at least equal if not greater than your focal length ie if you are at 800mm then the SS should be at least 1/800. I also use the lowest ISO I can get away with to give me the SS I desire which I find is around 1/1000 - 1/1200 for the lenses detailed below.

This can vary depending on how steady your hand is, how good your stabilization (if your lens has any) and whether you are using a tripod/monopod but it's a good generalisation to consider.

I have a Canon 500mm f/4L, the Sigma 800mm f/5.6 and Sigma 150 -600C and the slightest twitch at my end results in a lot of camera/lens shake at the fat end and subsequently almost unusable IQ.

I don't have any of the newer TC's but have a Canon x1.4 MK1 and never use it due to the loss of IQ.