I've been thinking about one of these to help with shooting hovering dragonflies. They often hover too close for the 400mm (MFD is 3.35m)
I realise the 300 f4 IS would be better but I allready have the 400 for birding.
It seems like a +1 diopter (77mm closeup filter) might be a useful and lightweight addition. I found this post, and using the same maths calculated a MFD of 0.76m with a +1 filter. I assume that these +1 closeup filters are actually +1 diopters.
I may have a go with an ultra elcheapo chinese one, I know they'll be optically awful but would suffice to test if it is a viable option.
Has anyone tried this, or have any thoughts?