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View Full Version : Depth of field for 2" and 2-1/2" lens



Colin Jenkins
01-16-2015, 6:07 PM
I saw a while ago that someone posted a rough guide to the depth of field for a variety of focal lengths and have been searching on the forum for 45 minutes and can't find it again. In laymen's terms, I'd like to figure out how much of a taper in a cylindrical item is allowed before losing focus on a 2" lens and a 2-1/2" lens - if that makes any sense.

Mark Sipes
01-16-2015, 6:34 PM
If you jack up the item so the taper is level, then there is no variance. Now the difference in Circumference plays a role. but you can compensate for that in the graphic.

Colin Jenkins
01-16-2015, 7:37 PM
If you jack up the item so the taper is level, then there is no variance. Now the difference in Circumference plays a role. but you can compensate for that in the graphic.

Thanks for your response but I maybe wasn't clear. I'm trying to figure out how to determine the engraving area for a cylinder that is tapered and in a rotary fixture so jacking up one end isn't possible. I'm trying to figure out if it is worth getting a 2-1/2" lens to use instead of my 2" lens and whether doing so will buy me more flexibility and roughly how much.

Rich Harman
01-16-2015, 8:11 PM
I'm trying to figure out how to determine the engraving area for a cylinder that is tapered and in a rotary fixture so jacking up one end isn't possible.

What prevents you from tilting the entire fixture?

Gary Hair
01-16-2015, 9:34 PM
The difference between a 2" and a 2-1/2" is negligible.

Dan Hintz
01-16-2015, 9:51 PM
There are no hard and fast rules as it depends upon the power density you need. That said, expect +/- 1/4" to be a relatively comfortable range.

But I'm with the others... place a book or other item under one end of the rotary and get rid of the taper.

Mark Sipes
01-16-2015, 9:56 PM
My Trotec Rotary fixture has a built in ratchet mechanism that adjusts for taper / ....??

Dave Sheldrake
01-17-2015, 8:14 AM
D = Diameter of input laser beam from tube.
F = Focal length of the lens.
W = Wavelength of the laser beam.
DOF = Useful depth of field for a given optical set up.

The equation goes as follows:

DOF = 2.5 x W x (F/D) ^2.

Mike Null
01-17-2015, 9:11 AM
I feel that =/- .125" is the maximum you can work with.

Ray Scott
01-17-2015, 10:10 PM
Get a book.. Prop up the low end of the rotary device. The result is going to be a more even laser texture.