Focal length details

[Uros Sovilj]

Hi

I had ordered new lenses from chinese suplier. I ordered 50, 40 and 55 mm lenses. when i tried to do some testes with them obvius something wasn't right since resoults of engraving were disaster. First thing on my mind was that they packed wrong lenses so i searched the net and find simple test to determine focal lenght http://www.youtube.com/watch?v=v0znprj3xsw . I mesured folowing:
50 mm was ok
40 mm was 45 mm
55 mm was 37 mm
I know that it's not wery accurate test but if i set lenses to their focal lenght the image on paper was blur.
OK. they mixed something up but when i checked lens i am using in laser which i never remove before i noticed that it's also not 40 mm but 34 mm. I can live with but i am wondering what is with quality of engraving i do.
Also i found that lens was insert with flat side facing to last mirror.
Questions:
1.) Focal lenght is mesured from optical center(which is midle of lens thickness)?
2.) Lens should be insert whit convex side facing the last mirror and flat side facing to working table?
3.) If i make spacer let say for 40 mm lens it should be than 40 mm plus 1/2 of lens thickness minus height of laser head to lens?

Regards
Uros

[Rich Harman]

1. Don't know
2. Yes
3. I don't understand the question.

Easy way to check focal length is to engrave a straight line on a piece of material that has one edge raised off the table slightly. Adjust the table height so that the focus should be in the center of the piece. The line you engrave will be sharpest/thinnest when in focus.

[Uros Sovilj]

Thanks Rich

I have done another search on forum and found all answers in old thread.

Regards