PhotoGrav and spot size? .05, .07 etc for chinese lens.

[Clark Pace]

Anyone here know what a 50.8 or 63mm chinese lens would be equal to as far as spot sizes on photograv?

[Dave Sheldrake]

Spot size is a product of focal length - incident beam Clark, without knowing the size of the input beam a mean spot size cannot be calculated. Incident beam is different depending on the power of the tube, the type of tube and the age of the tube.

cheers

Dave

[Michael Reilly]

When I was at training for my Vytek and we covered photograv, they told me that in order to get fine detail, it's necessary for the substrate to be visible, thus the dots should form a grid and touch but not overlap one another. Once they overlap, you're just going to see it as white. So the recommendation for dpi was 1 inch divided by the spot size in inches. So 1 / .005 = 200dpi. You could work it out by varying your dpi until you get the dots touching but not overlapping and then do 1 divided by that value to get the spot size. 50.8mm is 2" and that commonly works out to .004 in other machines, so you can start around 250dpi.

[Dan Hintz]

When I was at training for my Vytek and we covered photograv, they told me that in order to get fine detail, it's necessary for the substrate to be visible, thus the dots should form a grid and touch but not overlap one another. Once they overlap, you're just going to see it as white.

I wouldn't call this bad advice, but I would call it poorly worded. This is true for some substrates, but others will require overlap to get a good image... it's up to you to make that determination. For example, overlap in glass is (mostly) a bad thing, but overlap in wood can often be great.

[Michael Reilly]

Well, it also depends what you're engraving. If you're doing primarily line art, then overlap is going to give you a deeper engraving in a single pass. But if you're doing halftones, then overlap isn't going to allow you to see the variation in dot size. But yes, the training was focused mostly on doing marble and glass, though we generally don't do either of those.