A photographic image or gradient must be converted to a B&W halftone to be exposed and screen printed. Halftone is the term for tiny dots (or lines) that give the illusion of a tonal range. The image may be used on it's own, or placed back into Illustrator/Photoshop and colorized for the final design.
It is also one of the settings that varies between screen printing studios based on inks, experience, and equipment. For my design projects, it often becomes an aesthetic choice where I go back and forth, save multiple versions, and place into the layout with different LPI/angles and/or dots/lines, before finalizing to print films.
Screen printing halftones requires practice and testing to print successfully.
A: original 10x10 image.
B: detail showing B&W halftone conversion using 1 screen.
B: detail showing CMYK conversion using 4 screens.
01: open or place in Photoshop, flatten image.
02: set the image to final size you are printing.
03: confirm image is set to 300dpi/Greyscale.
04: convert to Bitmap and follow the prompts.
Please Note: these settings are a general starting point but can be adjusted based on your screen printing experience, equipments, inks, materials (paper or fabric), etc.
In the end it's all about testing and practice.
Bitmap Dialogue Prompt
Resolution: output 300 pixels/Inch
Method: use halftone screen
Halftone Screen Dialogue Prompt
Frequency: 25-35 Line/Inch (LPI)
Angle: 45 degrees for spot colors. CMYK requires 4 unique angles requiring research and testing.
Shape: round or ellipse