What is the advantage of different focal lengths of the focus lens. I see them vary alot on EBAY. I have been using a 50.8 focal length. Is there an advantage to using a 38.1 or even a 25.4?
I think you're too lazy to search for the answer of your question. If you read my previous post you'll know that longer focal point will make it easier to cut thick material and shorter one will give you a great engraving resolution. There is a thread in this forum which gives a very good explanation about the lens focal point, how it relates to the quality of cutting and engraving. I don't know which thread but I'm pretty sure it' somewhere in this forum, because I have a problem with the lens and find the anwers here.
They all have different applications. Think of the focused light as a pair of cones, or an hourglass shape.
The longer focal length lenses make a cutting beam that has a longer useful area (the angle is less severe), but focus down to a slightly larger spot. Good for cutting thick material, not great for engraving.
The shorter lenses focus down to a finer spot - more power density, and finer details when engraving. However, the spot diameter increases more rapidly as you move away from the focused point, so you lose some of your ability to cut thick materials.
I run a 50w ULS machine with a 2.5" (~63.5 mm) lens, and it's a good all purpose lens. Engraving is fine, cuts up to about 1/4 inch acrylic, and will also cut 3/4" foam. My machine doesn't really have enough power to warrant the longer lens, or at least for what we do. I've also seen people buy them to engrave in things like the bottom of bowls, where the head would strike a side wall with a shorter lens.
Focal length
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