So his 5D lens is supposed to give him 20cm working distance which is very generous... but in practice he says it's more like 12cm.... I guess one has to take into account its particular eye sight problems as well to get the complete picture so to speak !
Well, it's more that if you drop the "at infinity" simplifying assumption you have to do some real physics to work out what's going on. As soon as you move closer than 'infinity' to one side of the lens you shorten the focal distance the other side of the lens - at least I think I remember it works that way, I've probably used the optics I learned at school only half a dozen times since.
That's why it's simpler to just measure it in real life rather than make assumptions based on theory... a ruler is simpler and faster and describes the actual reality without resorting to any math !
So, 12cm or so, said BU... it's waaaaay more than what I have right now with my crappy Optivisor thing of a bob, so that's all I need to know !
Wide field of view is also a huge and much welcome improvement ! Just checked, the lamp costs 60 Euros or so... it's OK, can do that. Add the lens on top of it, still affordable for something that works well.. and that you only have to buy once.
Definitely on my shopping list !