What DPI for grayscale image?

[Prosperi-Tees]

I am going to practice my first grayscale photo on a white shirt black ink. I am running Simple Seps and it asks what DPI to set the grayscale option to and what halftone. The default is 300 dpi grayscale and 600 dpi for halftones with a 45 lpi and 22.5 angle. Is this a good start?

[Frog]

Unless you are looking for a certain "big dot look", your lpi is usually a product of your screen mesh or vice versa. To reduce chances of moire interefence patterns, the rule of thumb is at least 4.5 times the lpi determines your mesh count. (or dividing your mesh count by 4.5 gives you your lpi)
Using 5 rather than 4.5 is a luxury, but reduces chances for moire even more. For instance, your choice of 45 lpi, would dictate the use of a screen at least 200. I might go with a 230
On the other hand,  some times folks have luck with numbers way below 4.5, and have great results.

[Prosperi-Tees]

I will be on a 230 mesh so 45x5= 225 so im good.

[Denis Kolar]

It depends on how detailed you want it to be, which screen mesh you want to use, how good is your exposure unit and emulsion. 45 might be OK, but you van also go up to 55.
22.5 angle and round dot should work too.
I'm sure more people with more experience with screen printing separations will chime in.

[Denis Kolar]

I think that this was 45 on ........... yeah, it does not make any sense, ........ on 155 mesh

[Prosperi-Tees]

I just shot the screen, 230 mesh, washed out nice. I will print it in a couple hours and see how it turns out.

[Colin]

I am going to practice my first grayscale photo on a white shirt black ink. I am running Simple Seps and it asks what DPI to set the grayscale option to and what halftone. The default is 300 dpi grayscale and 600 dpi for halftones with a 45 lpi and 22.5 angle. Is this a good start?

As for the Dots Per Inch to set your photoshop design at:  Your Lines Per Inch can not hold the "resolution" of more than 2.5 times itself.  This means that when you set a design to print out haltones at 45 lpi, it will not hold any more detail than if the image was set at 112 DPI.  At 55 lpi the max holdable detail is 137 dpi.  I still set my sim-process and greyscale designs to either 200 dpi or 300 dpi depending on the text involved.

When you are using Photoshop to create your halftones, it is highly recomended you set the resolution to 600 DPI in order to make your halftone dots as smooth as possible.  There is a visible difference, albeit a small one, between a 300 dpi haltone generation and a 600 dpi one.

** Do NOT ** go above 600dpi for halftone generation however.  Photoshop's math gets funky and will increase the LPI due to some math fluctuations.  At 1200 dpi a 45 lpi conversion ends up at 50 lpi or so.... it's really funky.

[Sbrem]

Just to add to the confusion... is Simple Seps using Photoshop to do the conversion? When I convert a grayscale to a halftone (Image/Mode/Bitmap) my conversion type is "Halftone", and I set the angle and line count to my preferences, usually 22.5° and 50 lpi. I change the output resolution to 1200 PPI, which makes the dots very, very clean and sharp. If you're printing a grayscale image, and your ppi is more than 2.5 times the line count (pixels per inch, not DPI, which is a printer function) then Photoshop will tell you that the PPI is too high, but it will still work, unless it's way high, and you get the issues previously reported. So, an image to be printed at a 50 line screen only needs 125 PPI for image resolution.Though it's used a lot to describe the resolution of an image in your computer or scanning resolution, DPI is a printer function, PPI is for images in the computer/scanner, and LPI is the halftone line count. So when someone asks for an image to be "300 DPI", we all know what they mean, but it should be "300 PPI" to be technically correct. God, I love this kind of talk...

Steve

[Gilligan]

I need to understand all this better.

I've recently been playing with ghostscript again and I noticed that I can make the dots "invert" essentially.  They become donuts kind of.  Looks terrible.

The other issue I see is blocky bitmap stuff in photoshop's halftones.  I try upping the resolution of the PSD file (before doing the bitmap conversion) and it still looks terrible.

[Prosperi-Tees]

Didn't come out to bad. Now I have to figure out how to manipulate photos. Oh BTW this was done out of Corel and Simple Seps, not PS

[Gilligan]

How does one make that even more photo realistic?

I saw a grayscale Marlyn Monroe being printed at a shop that looked pretty damn good... even in the screen you could tell it was gonna look good.  This shop isn't very good... vacuum lid single point halogen exposure unit... probably coating 1:1 because they don't know much better.  Probably on a high 305 mesh.

It blew my mind that it looked that good and then it blew my mind even more so that this shop was turning it out!

[ebscreen]

A lot of it has to do with the quality of the original photo. If it ain't great to start with, it's only going
to get worse.

Next, most greyscale work should be printed with at least 2 and preferably more colors. Trying
to hold the gamut from black to almost white with one screen and one color ink is difficult
to say the least.

We won't go below 45 LPI for halftones, most is at 55, some at 65. the higher the nicer the outcome.

Prepping greyscale for output is an art all it's own, and will be tailored to each individual shop.
the curves function in Photoshop is your friend.