Dumb Lens Question #2

[Adam Less]

So here it is. I received a couple of "backup" lenses but they aren't marked at all. They are different sizes. How do I tell what size lenses they are? (Ie what precision). Is it based on the actually mm thickness of the lens?

[Scott Anders]

To find the focus hold it above a desk under a light (any light even a flouro will do) you'll see the image of the light source on the desk. When the image is at its sharpest line a ruler up beside it and the focus length will be (I can't remember if measured from the top surface or bottom of the lens) that distance.

[Gary Hair]

You can hold it over some fine print and move it until the print is sharp, measure the distance. Since most lenses are in increments of .5" you don't have to be very precise.

[Kev Williams]

I just focus on my thumbnail, then move my head to have a look, it's easy to see if the lens is 1.5", 2", 3" or 4" away from my thumb! :)

[John Noell]

If you want to know about the lens you have installed, and do not want to remove it, engrave a ramp and measure from sharpest point to the lens. Very quick and easy.

[David Somers]

Adam,

Just to add a bit to everyone's suggestions. When you have a 2" lens that number is referring to the distance from the lens to its sharpest point of focus. That is why folks have been suggesting you hold the bare lens away from something and move it back and forth until that object, seen through the lens, comes into sharp focus. The distance of the lens to that object is its focal length. So a 2" lens will have roughly a 2" distance from the underside of the lens to the object you focus on. A 4" lens will have 4" between them, etc.

Also remember that the number is from the underside of the lens to your focal point. When you put the lens into your lens tube there is some distance from the underside of the lens to the bottom of the lens cone on the tube. That is why the little focus gauge your laser company probably provided you with is shorter than the focal length of the lens. That gauge is actually acting as a spacer between the underside of the cone of your lens tube to the object you are working on.

That is also why you should probably do the little test John Noell mentioned when you get a new focal length lens or some new lens tubes. Take the time to do John's test and you will have accurately determined the correct distance you need between the underside of the tube and the object, and then you can cut a new spacer to use with that tube and lens combo. Personally, I would also do that test with a new laser to verify the spacer provided with it is accurate, and cut a new spacer if it is not. Then you know for certain your spacer is on the money.

And since I am babbling with you....you often hear us talk about depth of field. If you are into photography you are probably familiar with the concept. If you are not familiar with it.....think about a magnifying glass. You can go out in the sun and use the sun and focus the glass on a sheet of paper by moving it up and down. You will hit a point where the beam of light from the glass is focused enough to burn a hole in the paper. We all probably did this as kids countless times. If you go a step further and raise or lower that glass until it stops burning the paper you will now have an approximate idea as to its depth of field. In terms of burning something with a magnifying glass the depth of field is the distance above or below its point of sharpest focus where it ceases being effective at burning. Then think of the single lens you have in a laser cutter. It is essentially a magnifying glass that is being used to focus a laser beam to a point. That point is where the beam is focused to its smallest diameter and is most effective. As you take it out of focus the beam becomes less effective until you hit a point where the beam is not focused enough to do any real work for you. That is the rough depth of field. Because a laser lens has such a short focal length its depth of field is really short so it doesnt take being out of focus much to make the beam ineffective, to exceed its depth of field.

A side note on depth of field. There are reasons to go out of focus a bit with a laser. You might find that when engraving you are not getting a dark enough burn in your material to be effective. Taking the lens out of focus a bit may help darken the engraving. Or.....if you are cutting a thick material you may not want to focus on the surface of the material. You may want to set the focal point halfway through the thickness of the material. Think of your laser beam below the lens as being a pair of cones. Coming down from the lens the cone is point down. And once you go beyond that focal point the cone expands again and is point up. Where the points meet is that point of focus. So on thick material, if you focus on the surface you have an upside down cone to the shape of your beam and that will shape the edge of the material you are cutting. If the material is really thick you might even find the bottom of the material is beyond the depth of field of your beam and isnt being cut well, if at all. But if you focus in the middle of the thickness of that object you minimize the effect of that double cone shape. There are a number of threads in the forum that talk about this along with diagrams if you want to get a different take on this. I am afraid I dont have enough time to do up fresh diagrams for you right now. There are plenty of examples though if you do a search.

One last thought. If you buy a 1.5" lens you will likely find your existing lens tubes have too long a distance between the underside of the lens and the bottom of the tube's cone to work with the short focal length of that lens. The result is you BARELY have any distance left between the underside of the cone and the object you are working on. Your company can provide you with a modified tube that is designed for that really short focal length.

[Adam Less]

Adam,

Just to add a bit to everyone's suggestions. When you have a 2" lens that number is referring to the distance from the lens to is sharpest point of focus. That is why folks have been suggesting you hold the bare lens away from something and move it back and forth until that object, seen through the lens, comes into sharp focus. The distance of the lens to that object is its focal length. So a 2" lens will have roughly a 2" distance from the underside of the lens to the object you focus on. A 4" lens will have 4" between them, etc.

Also remember that the number is from the underside of the lens to your focal point. When you put the lens into your lens tube there is some distance from the underside of the lens to the bottom of the lens cone on the tube. That is why the little focus gauge your laser company probably provided you with is shorter than the focal length of the lens. That gauge is actually acting as a spacer between the underside of the cone of your lens tube to the object you are working on.

That is also why you should probably do the little test John Noell mentioned when you get a new focal length lens or some new lens tubes. Take the time to do John's test and you will have accurately determined the correct distance you need between the underside of the tube and the object, and then you can cut a new spacer to use with that tube and lens combo. Personally, I would also do that test with a new laser to verify the spacer provided with it is accurate, and cut a new spacer if it is not. Then you know for certain your spacer is on the money.

And since I am babbling with you....you often hear us talk about depth of field. If you are into photography you are probably familiar with the concept. If you are not familiar with it.....think about a magnifying glass. You can go out in the sun and use the sun and focus the glass on a sheet of paper by moving it up and down. You will hit a point where the beam of light from the glass is focused enough to burn a hole in the paper. We all probably did this as kids countless times. If you go a step further and raise or lower that glass until it stops burning the paper you will now have an approximate idea as to its depth of field. In terms of burning something with a magnifying glass the deptch of field is the distance above or below its point of sharpest focus where it ceases being effective at burning. Then think of the single lens you have in a laser cutter. It is essentially a magnifying glass that is being used to focus a laser beam to a point. That point is where the beam is focused to its smallest diameter and is most effective. As you take it out of focus the beam becomes less effective until you hit a point where the beam is not focused enough to do any real work for you. That is the rough depth of field. Because a laser lens has such a short focal length its depth of field is really short so it doesnt take being out of focus much to make the beam ineffective. To exceed its depth of field.

A side note on depth of field. There are reasons to go out of focus a bit with a laser. You might find that when engraving you are not getting a dark enough burn in your material to be effective. Taking the lens out of focus a bit may help darken the engraving. Or.....if you are cutting a thick material you may not want to focus on the surface of the material. You may want to set the focal point halfway through the thickness of the material. Think of your laser beam below the lens as being a pair of cones. Coming down from the lens the cone is point down. And once you go beyond that focal point the cone expands again and is point up. Where the points meet is that point of focus. So on thick material, if you focus on the surface you have an upside down cone to the shape of your beam and that will shape the edge of the material you are cutting. If the material is really thick you might even find the bottom of the material is beyond the depth of field of your beam and isnt being cut well, if at all. But if you focus in the middle of the thickness of that object you minimize the effect of that double cone shape. There are a number of threads in the forum that talk about this along with diagrams if you want to get a different take on this. I am afraid I dont have enough time to do up fresh diagrams for you right now. There are plenty of examples though if you do a search.

One last thought. If you buy a 1.5" lens you will likely find your existing lens tubes have too long a distance between the underside of the lens and the bottom of the tube's cone to work with the short focal length of that lens. The result is you BARELY have any distance left between the underside of the cone and the object you are working on. Your company can provide you with a modified tube that is designed for that really short focal length.

Fantastic feedback David. Thank you so much!

[David Somers]

Happy to help Adam!

Dave