Quote Originally Posted by m1tch View Post
Ah, the close up lense I have are +1, +2, +4 and +10, could those be the Dioptre values?
That's exactly what they are.

Quote Originally Posted by m1tch View Post
With regards to stopping the lens down, I had a Kodak autographic brownie which had the aperture iris in front of the lens rather than behind
Whilst that's easier to keep it all light proof, I don't think it's optimum.

Some of the early cameras only had one lens element so there wasn't the option of putting the stop in the middle.

I have read that if you keep it symmetrical, any errors are cancelled out.

However, please don't think I'm an expert in this, I am just repeating things I have learned from others on this forum who are more expert than me!