Quote Originally Posted by Steve Smith View Post
That's exactly what they are.

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!

Awesome, I always wondered how they would quantify the figures, in that case is the focal length of that +1 filter 1000mm?

So am I correct in saying that I have added a 50mm lens with a 25mm lens to produce a 166mm lens (negating the reversed +1 lens)?