View Full Version : Right distance ?

Hannu Rinne
11-05-2010, 7:10 AM
This is propably a stupid question, but it's unsolved for me... For examle, when talking about 2" lens, what is the right spot from the lens to measure the distance between the surface of the material and the lens - from the attached picture; A, B or C ?

Regards, Hannu

Dan Hintz
11-05-2010, 7:17 AM
Since lenses are bidirectional, the focal point is generally measured from the center of the lens mass and is equidistant on both sides. In your case, measurement 'B'.

Note: This is not always the case (very specialized lenses), but holds true for anything you are ever likely to run into.

Hannu Rinne
11-05-2010, 7:20 AM
Thanks Dan, case solved :)

- Hannu

Dan Hintz
11-05-2010, 7:29 AM

I probably wasn't clear in my initial post... this only holds true for symmetrical lenses (as you show in your diagram). If you're talking about non-symmetrical, like what lasers use, the focal point can very well be different for each side. I mention that as it's probably more what you were after.

Lens manufacturers will try to reduce the mass of the lens as much as possible, which means the plane from which the focal point is measured is not always the center of mass, so be careful. For our purposes (lenses that measure a few mm in thickness), you'll have a difficult time seeing a difference, but it's something you should be aware of for thicker pieces.

Hannu Rinne
11-05-2010, 8:04 AM
Dan, thanks for the information ! I made the drawing quicly with the ellipse tool but I ment the hollow shaped lenses as in our lasers. I will ask from the manufacturers ( in my case GCC and Rabbit ) how they'll do the measurement with the original lenses ( just curious as always :) ).

- Hannu

Dan Hintz
11-05-2010, 8:36 AM
If you want to be exact, here is the Thick Lens Formula and the Thin Lens Formula:
Those equations will help you solve for pretty much any lens shape.

If you're talking about a plano-convex lens in particular, the focal length is measured from the back principle plane H" (that's pronounced H two, I can't do superscripts here). Many consider this the same point at which the lens changes from the convex curve to the flat side of the lens, but these are rarely the same planes. Once you have the principal plane, you can use Gullstrand's equation to determine the focal length.

Rodne Gold
11-05-2010, 10:14 AM
You should have a tool with your machine that gives you the exact focus distance - a blue anodised rod that slots in a hole in the flying head. You can use this to fine tune AF..I think the menu on your lasers display allows for AF fine tuning.

Hannu Rinne
11-05-2010, 10:56 AM
Rodne, I have the tool ( got it with the laser and was totally out of focus ). My question is kind of theoretical as I'm trying to understand the "soul" of the lasers and laser optics ( with a poor success so far :D )

- Hannu

Richard Rumancik
11-06-2010, 8:29 PM
Rodne, I have the tool ( got it with the laser and was totally out of focus ). My question is kind of theoretical as I'm trying to understand the "soul" of the lasers and laser optics ( with a poor success so far) - Hannu

Hannu, the focus tool isn't necessarily 100% bang-on due to tolerances in the lens, lens holder, carriage assembly and the focus probe itself. But it should be close.

If the lens is "totally" out of focus then something else is wrong. They mounted a 1.5" FL or 2.5" FL lens in the 2.0" holder by mistake? (If the lens is not a 2" FL lens and that is what you want, I would assume that GCC would exchange it for you.)

You need to find out which lens you have, by changing the focal distance and observing your line quality until you get optimum (thinnest) line. Some people use black anodized alum for this test as it is more obvious what you have. The line should laser wider as you raise or lower the table from the "optimum".

If the probe does not match the lens by a small amount, then I suggest you make your own probe. You can get some straight rod and a matching shaft collar. Once you have optimized the focus iteratively, insert the rod into the probe hole, slide the collar down to rest on the focus tool flange, and tighten the setscrew to lock in the distance. Then mark the probe to correspond to the lens.

To ballpark the focus of a lens, I remove the lens coverplate and measure with a steel rule from the surface of the material to center of the aluminum lens holder. I'm not sure exactly where on the actual lens that would be - probably close to the center - but it will at least give a starting point for optimizing further. (I could not actually measure directly to the optic surfaces anyway, plus I would not want to risk scratching the coating.)

You can do the math if you want, but in reality you just want to find out where this particular lens focuses the finest and then make a probe to record that setting.

Rodne Gold
11-07-2010, 12:09 AM
The probe supplied has an adjustable collar (apart from mercurys which has a fixed length) - easy way to test is just AF in the middle of a slightly tilted strip (glue a piece of black ally on a shallow wedge) and laser along the axis of the tilt , you will soon see what amount you need drop or raise the table to be exactly accurate.

I measured the exact amount the probe needs to protrude in another thread and posted the results.

Mercury: The length of the thin portion of the shaft is 55mm.
Explorer :The length of the thin portion of the shaft is 70mm.
Spirit:The length of the thin portion of the shaft is 74mm.
Spirit GX might be different , could be 77.5mm

Pete Bejmuk
11-07-2010, 3:10 PM
the hard white plastic focus tools from ULS do occasionally go out of focus (mine was when I got my used system). there's a little screw in the tool that you can loosen to adjust the height of the post. the focus distance on the tool should be from the bottom of the tool to the top of the "notch". tighten the screw again when you're done.

Hannu Rinne
11-07-2010, 5:27 PM

My situation was as Pete's - the collar was sliding freely on the probe ( the screw was untightened ) and I had to find the focus manually - and I found it with using black aluminium sheet :). In my laser I have 2" lens - I've ordered also 1,5" lens but seems that I'm never going to get it from my dealer ( I made a material and device order last August and still waiting the delivery - prepaid, worth over $ 3,5k ) - but I think this was a good deal to him, nice money and easy to double when he perhaps sell the items to someone else :D ( not very funny, but I can't state here how I feel about this case... )

Regards, Hannu

Richard Rumancik
11-08-2010, 12:31 PM
Is this the same dealer that you bought the laser from? Is he still in business and answering your communications? If he is not delivering maybe GCC can put some pressure on him - they probably want to ensure all their dealers are reputable.

Hannu Rinne
11-08-2010, 5:10 PM
Yes, this is the same dealer and he is still in business but for some reason he don't answer me anymore. I'm also worried about the technical support and my warranty, because GCC turns my questions to the dealer... Possible I can get some support from other dealers but not for free - I afraid. This is what makes me angry because what I've read about Epilog's support, they seem to give some great service to all Epilog owners. Anyway, I like this guy ( my dealer ) and I wish a happy ending for this case, but in some point I have to ask ( or have to try ) some official help - sad, but what else I could do :(

Regards, Hannu

Garrett Nors
11-08-2010, 6:05 PM
As far as Epilog support goes, I have never, ever purchased anything in my entire life that came with better/more helpful tech support. Epilog's support is pure win, no contest.

Dan Hintz
11-08-2010, 7:02 PM
Why on Earth would you like a guy who takes over $3k of your money and walks away? If I didn't get my product after a few weeks I'd be calling corporate and telling them one of their distributors is stealing customer money.

Richard Rumancik
11-08-2010, 7:20 PM
Yes, you need to explain the situation to GCC. They will tell you the dealer is supposed to provide support, but if he is not supplying support or product they need to know. They have a vested interest in having reputable dealers. GCC will turn on the heat if he is treating a customer unfairly. Hopefully you get your product in the end.

Hannu Rinne
11-09-2010, 8:43 AM
Garrett - that makes me jealous :)

Dan - good question, perhaps because everything has been just fine with him before this case and I still don't want to believe the worst scenario ( but I also rely on the lottery - every week :D )

Richard - obviously I have to talk to GCC ( and to Rowmark, JRS... )

Regards, Hannu