MORE ABOUT LENSES:
Yeah, the correct term i was looking for is
Depth Of Focus....
The definition is:
The depth of focus is the distance over which the focussed beam has about the same intensity, it is defined as the distance over which the focal spot size changes -5%~5%.
Basically, it defines the distance lengthwise to the beam, which the most intense part of the focus is spread over...
In praxis it looks like this:
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With any given lens, the closer to the lens you focus, the shorter the depth of focus is - the distance at which a laser will burn with the highest intensity is better defined.
With a VERY WELL defined focus, the laser will burn "only" at a specific distance, while leaving things just a bit further away unharmed.... (That's a part of the reason why they can do laser eye surgery without making you blind at the same time, for example!) Additionally, a less "spread out" (or better defined) focus will have a higher energy density in the focused spot - it has a "sharper tip" of the light cone....
- With the same lens focused further away, the depth of focus becomes more "spread out" over the length of the beam. The distance of the most intense spot is less well defined. The Depth Of Focus becomes "longer" (or "deeper"? :thinking
.... Because the focus is more spread out, it will also burn less well at longer distances, as anyone who ever played with a laser already knows....
- With a longer FL lens, the depth of focus is much better defined (far less spread out lengthwise) than with a shorter FL lens,
when both are focused to the same distance.... This is simply because the beam is fatter with a longer FL lens than with a shorter FL lens.
In reality, what changes is just the SCALE on which "things are happening"... A shorter FL lens focused on a shorter distance will give the same depth of focus as a longer FL lens focused on a longer distance.
Basically it's like taking a fat beam from a longer FL lens focused on a certain distance and then scaling it down - the beam becomes thinner, and the distance the beam is focused on with the same intensity becomes shorter..... (
I hope i'm explaining this well enough, but i realize it will require DRAWINGS for everyone to truly comprehend it all...)
- A shorter FL lens will have a less well defined Depth Of Focus at the same distance as a longer FL lens, and as a result, a lower energy density, if the power is the same, or even if the power is higher in some cases!
This is the reason people used to say that with an AixiZ acrylic lens an open can will burn better than with a Meredith lens, even tho it will only have 250mW instead of 300mW. Obviously if you use an AR coated acylic to increase the power to 280mW it will then burn even better. If it was possible to reach 300mW with the same beam diameter as an AixiZ lens produces, it would burn A HELL OF A LOT better than a 300mW laser with a Meredith lens.
- If you can get a fatter beam of the same power it will burn better than a thinner beam of the same power, when focused to the same distance from the laser. It's simply due to the Depth Of Focus and the resulting Energy Density, nothing else... (A very thin beam however will be able to burn even when focused to "infinity" due to the energy density being so high. But not as well as a well focused beam.)
The deeper (or longer) the Depth Of Focus, the more spread out the Energy Density becomes. And when it comes to burning, the power matters a lot, but the energy density matters even more. That's why people got the feeling that clear acrylic AixiZ lenses burn better than the Meredith, even tho they waste 20%!
In industrial or even military lasers, they use absolutelly MASSIVE beam diameters, so they can focus them more accuratelly for burning at a certain well defined distance and only there, and to make the cut as sharp as possible....
The further away you want to burn, the fatter the starting beam you need. Industrial laser lenses have diameters such as 25cm for example (10 inches), sometimes even more! Military lasers, which have to shoot down rockets kilometers away don't even use lenses for focusing, but gold plated mirrors instead - mirrors, that can change their shape for focusing, instead of moving the lens!
In praxis, the fatter your laser beam starts out, the further away you can make it "burn like hell", and the better defined your focus will be at any given distance....
But due to the modules we use in this hobby, the maximum lens diameter we can use is 7mm. More than that doesn't really fit anywhere.
Besides, we don't really like those fat beams that come out of AixiZ lenses that much....
AixiZ lenses have a 7mm diameter, but much of it is wasted on the edge, only just over 5mm of it is the actual lens, and the effective diameter is 5mm. As a result, the beam will have a diameter of 5mm in the fast axis (which gets clipped) and slightly less in the short axis (which doesn't).
Since the AixiZ acrylic also has a long FL, it has to be far away from the diode, and captures less of the light cone... As a result, the NA of an AixiZ acrylic is only 0.3 - and that is for reds. For BluRays it's even less, and the acrylic material is poor at passing wavelengths around 405nm. Due to the sharp transmittance cutoff at just this point, it also wastes a LOT more power with every nanometer less a diode has.
Over a year ago, when i started the Custom Lens Project, i didn't really know what to expect. In fact i did not even know what i am supposed to be looking for.
I made many expensive mistakes, and in the end i lost over $1k just to get to three good lenses (after multiple pieces of 7 different useless but no less expensive models, with the very first model, even 30 pieces of completelly custom made spherical lenses! :cryyy
.
I have not yet even seen a Meredith lens at that time! I had no clue what would happen or what it would make the beam look like!
After i learned a bit more, i started looking for aspheric lenses, trying to get a shorter FL for a thinner beam, and a higher NA for maximum light capture...
The final custom lens batch gave me three lenses, which i called v1, v2, and v3 at that time...
- v1 is what i now call the "Medium Custom Lens". It has a FL in between AixiZ and the Meredith and is made of a high 405nm transmittance glass with a broadband AR coating. It's a bi-aspheric lens (bi-convex). It has the maximum diameter possible (7mm), and as a result, even at the medium FL a reasonably high NA of 0.4...
- v2 is basically a broadband AR coated Meredith but with a larger diameter, possibly capable of capturing a bit more light than a regular sized Meredith, but not yet sure on this point. It has a NA of 0.55 and a FL identical to Meredith. Like the Meredith, it is a Plano-Convex lens (only one face has an aspheric curvature).
- v3 or the "Nichia Lens" is a lens designed to perfectly match specifications provided by Nichia, specifically for 405nm diodes, and is made to capture the maximum possible diode output, and waste as little of it as possible, using a wavelength specific coating, centered at 408nm. However this lens - in the same quality as the first two - cost almost twice as much. The one i have is a "low performance version", which means "refractometer rejects" or lenses that have failed the test of being capable of writing disks. I am not yet sure if this means it causes any abberations we could detect by naked eye or not, i will need it perfectly mounted by Jayrob first... It does however have the highest 405nm throughput of any lens anyone ever saw so far (or at least will, once properly mounted). It has a NA of 0.6 and a FL shorter than Meredith.
How exactly the NA's and FL's influence the beam/spot output shape will be nicelly explained by my drawings. I hope the drawings i am working on will make all this easily understandable for everyone.
But theoretically at least, each lens in the order i wrote them will have a "thinner" (and flatter) beam with a higher power (due to FL and NA being higher with each one). But for the same reason a less well defined Depth Of Focus and slightly lower burning capabilities due to lower energy density at the same distance...
Keeping in mind that we are limited to 7mm, the best burner lens would be one that would have the beam diameter of AixiZ lenses, but the power of the short FL glass lenses.
However this is impossible for obvious reasons. Since we can not increase the lens diameter, the only way to increase the beam capture is by using a shorter FL lens of the same diameter... The lens will then sit closer to the diode, and at the same diameter, it's NA will be higher - it will capture more of the light cone because of this... But as a result the beam will become thinner and flatter, and have a less well defined focus at the same distance.
Once the NA goes above 0.5, a lens captures the entire light cone of even a 405nm diode (since 405nm diodes have a NA of 0.5 or less in their fast axis), and the resulting beam/spot shape will depend purelly on the diode alone....
If you want to get a rounder beam, you could theoretically use beam-shaping lenses, but those require axial alignment to the diode, which means you can't turn them for focusing (you have to move them without turning). Another way would be to use special beam shaping optics first (directly on the diode), and then focus the already shaped output with a regular lens, which could then be turned for focusing...
The only other way to achieve any beam shaping is by using a lens with a slightly lower NA. This last method is actually the only one used in professional laser pointers, where a "pretty" round or at least roundish output is desired...
If the NA is lower just a bit, the fast axis of the diode will clip the lens (miss the edges of the effective diameter) and be "cut off"...
As a result, the fast axis will be "roundened off". This wastes some of the power (the bits that missed the edge of the effective diameter), but only the power in the very edges of the fast axis, while the center, where the output is the most intense still goes through, as well as the entire slow axis, as is evident in the photos of the spot (it looks completelly round from two sides and slightly flat from the other two sides).
Ideally - if i could afford it - i would experiment with even more different NA's, until i would find one, that does minimal beam clipping, and gets as much light out as possible, while still roundening the fast axis off a bit, while at the same time keeping the FL just long enough to keep the "splatter" hidden within the 405nm glare.
Maybe a NA of around 0.45 would be "ideal" - i don't know, i can't find such a lens anywhere...
But i do have a lens with a NA of 0.4, which increases the power a lot. More even then my initial expectations, and produces a nice oval dot, with almost no side projections visible (unless your name is Traveller or you have his bionic eyes :angel
....
Thing is, if 405nm diodes would not have such flat outputs with nasty side projections, or rather if they were as advanced as reds, a short FL lens would give a nice round beam. If you have a short FL / high NA glass lens and have put it in a red, you already know this...
The weird side projections which people now call "splatter" were a completelly unexpected result for me. I didn't understand it at first and it worried me. I was worried what people would think about it. This was my own fault, as i still have not tested the Meredith lens kindly provided by Dave at that time! :yabbem: I did not realize, that people have already seen this "splatter" and are used to it, or don't even care....
To recap:
When it comes to lenses, power and beam/spot shape and diameter/size (after you have glass and AR coating taken care of), the only thing that matters is the NA and FL.
If one could manipulate with both NA and FL freely, one could make a lens with maximum power (NA of 0.55 or more) and zero visible splatter (by not using too short a FL)..
Except for Traveller of course.... 
But due to lens diameter limitations, the NA is limited by that lenses FL.
The only way to increease the NA is by reducing the FL, if the maximum lens diameter is fixed...
Obviously we want as high a NA as possible, as we all like seeing high numbers on our meters. In this i am no exception!
But if you reduce the FL below a certain point, the "splatter" will become larger and eventually come out of hiding behind the 405nm glare (even tho it is present with EVERY SINGLE LENS even AixiZ).
Additionally, due to the fact that 405nm diodes have a very poor aspect ratio (flat output), the higher the NA, the flatter the beam coming out of the lens will be. It will become thinner in the slow axis, but due to so much more fast axis coming out, it will seem almost just as wide there....
I was used to seeing round(ish) beams with AixiZ lenses, and while pursuing thinner beams of a higher power in my Custom Lens Project, i was foolishly expecting to get beams of the same shape, only thinner and with maximum possible power...
It wasn't until i started testing high NA lenses that i started realizing what was going on and what we will really get in the end.....
I have seen a BluRay beam as thin as 1-1.5mm once, and it was seemingly completelly round, and it even came with a high power increase! Obviously that lens was of EXTREMELLY SHORT FL, but it also had to have a lower NA, since it clipped the fast axis and made the beam round.
It is possible to make an ultra-thin round beam, but not without sacrificing some power. In addition this ultra-thin beam will have very poor focusing abilities (tested) and due to being so thin, also have a bad divergence.
Divergence is another thing to consider, when it comes to lenses. You can't really have a thinner beam, without also having a larger divergence, when focused to "infinity"... The two are directly related..
Jay: Maybe this is why you thought your lens was out of focus, when Franco took spot pics at 10m distance? The beam diameter of a shorter FL lens is smaller of course, but after a certain distance it will become larger simply due to divergence. The divergence itself is another example, where it is all just about the scale....
Anyway,
the lens i would like to introduce in the "Test-Buy" is the v1 or the "Medium FL Custom Lens"... (
And since we won't get any quantity discounts in the Test-Buy anyway, i could also offer some v2's for comparison... Even v3's in fact, if someone REALLY wanted to have one - like for "raw" power measurements....)
The reason for that is simply that i like the way it makes the beam and spot look - it is just
my personal preference. Other people's preferences may vary....
I just like how the beam is thinner than AixiZ, but still round-ish and still has enough volume to manipulate for very well defined focusing and burning.
If i put the v2 lens into my super-freak 6x, i get almost 280mW on my meter, but when it comes to apparent brightness i don't see a difference from 265mW, i do however see a difference in the beam shape, spot and splatter...
Like i said before, i am no exception, when it comes to wanting to see as high a number as possible on my meter. But most of the time, when i play with my lasers, i do it without the meter, and in those cases it is more important for me, what the laser's output looks like and how well it burns.
And when it comes to that, my personal favorite is the v1 lens.
People have different needs and wishes, some will want more power, others might want a prettier beam. Others again will want to have access to both.
This is why i believe a Test-Buy is the best idea. Let people get some and test them, and decide what they like the most for themselves. In the end the majority will likelly want both types...
As i said before,
the two lenses are completelly different, and do completelly different things. Each has it's own benefits and drawbacks...
The only thing that really matters in the end is personal preference of the individual laser hobbyist....
- Franco's testing was quite intense and he's got me sold, Jay sold, Ken sold and I'm sure a whole bunch of others too 
