Laser “Depth of Field” (focussing range)

I’m not sure of the correct term, so I’ll use the photographic terminology…

When using my laser (3.8w J Tech with high res lens) how much leeway above and below the focus point should I reasonably expect?

I realise the effects of going out of ideal focus range, but I was wondering things mostly in relation to lasering curved surfaces. The idea of engraving things such as pens etc had crossed my mind…

I’ll make some tests using dowelling, but I thought I’d see if anyone has a quick answer already.

David.