Hi,
while mounting new air assist I have discoverd that (sotck) lens is little chipped off. I am currently in search of replacement. Does anybody here has expirience with cheap lenses from ebay? I know thjat they definitely won’t as good as expensive ones, but I have it only for my hobby projects so I don’t really want to invest much.
So are the ebay lenses acceptable trade off, if not, how bad they are?
As long as it doesn’t move in sure you’ll be ok for a while. I have used cloudray before from aliexpress and they may be on eBay to.
Use that lens for as long as it lasts. When you buy another, spend a little extra to get a decent quality lens.
thanks @Paul_Mott , do you have any first hand expirience with using cheaper quality? Burn increases, or power requirements?
My lens was chipped worse than that. I ordered a new ZnSe lens from LightObject last night. Obviously, I haven’t tried it yet. But I’m optimistic.
@Ales_Tomecek_Alandra Good quality lenses (as opposed to the cheap far eastern variety) will enable smaller focussed spot sizes and this really does make a significant overall improvement to the work quality (My lenses have all been purchased from II-VI Infrared).
@Paul_Mott thanks for clarification. Would you mind sharing what lenses exactly have you bought? I am honestly little bit lost on which type should I look up. Only suitable diameter I have found is in plano-convex lenses, and that doesn’t seem shaped like the stock ones.
@Ales_Tomecek_Alandra I have;
2 x ZnSe plano-convex of ~2” focal length.
1 x ZnSe plano-convex of ~4” focal length.
1 x GaAs plano convex of ~2” focal length.
1 x GaAs meniscus of ~2” focal length.
I can easily and quickly exchange lens (in it’s holder) for different applications and as a comparison the GaAs meniscus is the best, producing by far the smallest spot size.
#K40Optics
@Abe_Fouhy The ‘focal length’ of a lens determines it’s ‘depth of field’ and it is sometimes handy when cutting deeper into a material such as when producing lithophanes. Increasing the depth of field has the downside of increasing dot size but almost everything laser is a trade-off of some sort.
Increasing spot size decreases energy density so there is less
cutting power.
Increasing depth of focus increases the length of the waist of the beam and can reduce the hourglass effect in thick materials.
http://www.engraversnetwork.com/files/Choosing-the-right-Laser-lens-tip-sheet.pdf
…
http://support.epiloglaser.com/article/8205/42831/focus-lens-101
https://www.troteclaser.com/en-us/knowledge/tips-for-laser-users/selection-of-the-right-lens/
youtube.com - Laser cutter Advanced Tutorial 04 How focal length changes affect the cutting effect
https://www.youtube.com/watch?v=YNfNyOqUG0c
Note subtitles 
Some math for fun: https://www.rp-photonics.com/focal_length.html