Help picking a lens

[Rich Harman]

Sure, other materials will also give you an idea of your focal length, but wood probably isn't the best option.

You only need to discern where along the line the engraving is at it's finest. The best option is to use what you have on hand that will do the job.

I suppose if you were burning the line at too slow of speed it could make it hard to read using wood. I use something like 600mm/sec at 70% power and it makes a nice, easily readable line.

I prefer to use a long piece, about a foot long or so, and prop up the end so that one side is a bout two inches below the nozzle and the other just clear of the nozzle. There will be certain distance along the engraved line where it all looks the same. That distance represents the depth of field, the longer the focal length of lens - the longer this distance. I then make pencil marks at the extents of this region and then take the middle as the focal distance.

[Dan Hintz]

Go to Home DEpot and pick up some gutter material... it's aluminum flashing with a polyester-based paint (black, white, and brown are common colors). Take a section (doesn't matter how long, but 10" is a good round number that makes the math easy) and put a 1" high block of wood under one end so the section rises from one end to the other. Draw a series of straight lines 0.5" apart, separated so each new line is drawn at a different height along the section.

Once engraved, one of those lines will be sharper than the others. That's the perfect focal height. Since you have a 10" run over a 1" rise with a line every 0.5", every line represents a 0.05" increase in focal height off of the table. Line #4 from the bottom would mean your focal point is now 0.2" above the current height of the table. Move your table up 0.2" and you are perfectly focused on the table. You can then move the table down, as necessary, to account for the thickness of the substrate you're going to engrave.