# Help Understanding Halftone LPI vs. images DPI?



## ShaolinD (Aug 25, 2010)

Hi all, I would just like some info regarding Halftones. I have been printing a few months now and would like to start messing with some finer Halftones. I ran across a formula online a while back which was to multiply your halftones LPI by 3.5 and that would give you the minimum screen mesh you should use... EG: 65LPI x 3.5 = 227.5 so the closest would be a 230 mesh.

What I'm wondering is when people talk about halftones are they assuming your working with a 300dpi image? The reason I ask this is because it seems that when creating a halftone in photoshop (by converting to grayscale and then to a bitmap), that images may have larger or smaller halftone dots depending on the dpi of the image. For instance say I have a 200dpi image and I make it a halftone with say 35LPI, if I convert that same image to 300dpi. and render it at the same LPI then the resulting image would have a different dot size right?...

Am I right? If not please shed some light, I just want to understand the whole thing before I start messing with them.

Any help would be appreciated...


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## CherokeeDesign (Jan 31, 2010)

I have not heard of the formula you're talking about, I got my info from this: Screen Printing Four Color Process, How To Print Process, 4 Color Process Info

I don't have a ton of experience, but I have been creating my halftones the same way as you're describing in Photoshop (changing to grayscale then bitmap). The only difference I have noticed with different resolutions is that when I go to a higher resolution (I've experimented from like 300-1200 dpi) is that if you really zoom into the dots, the higher the resolution, the less pixellated they are when you zoom in. What you're saying about the dot size being different at different resolutions makes sense, although it's not something I have noticed in my own limited experience. Like I said, all I have seen is that zooming in on a 1200 dpi halftone, the edges of the smallest dots are a lot more perfectly round than they are at 300. Whether or not they are actually different sizes, I have not noticed but will pay closer attention next time.

The attached shirts were following the info on the Ryonet site, on a 305 mesh screen. The page describes setting up for CMYK and these were just a 1 color experiment, but I was very happy with the result.

305 mesh count. 100% Saturation. 
Half tone line. 55
Half tone angle. 22.5
Half tone shape. Either ellipse or round.(I used round)


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## jsf (Aug 4, 2009)

ShaolinD said:


> Hi all, I would just like some info regarding Halftones. I have been printing a few months now and would like to start messing with some finer Halftones. I ran across a formula online a while back which was to multiply your halftones LPI by 3.5 and that would give you the minimum screen mesh you should use... EG: 65LPI x 3.5 = 227.5 so the closest would be a 230 mesh.
> 
> What I'm wondering is when people talk about halftones are they assuming your working with a 300dpi image? The reason I ask this is because it seems that when creating a halftone in photoshop (by converting to grayscale and then to a bitmap), that images may have larger or smaller halftone dots depending on the dpi of the image. For instance say I have a 200dpi image and I make it a halftone with say 35LPI, if I convert that same image to 300dpi. and render it at the same LPI then the resulting image would have a different dot size right?...
> 
> ...


DPI refers to your image resolution. A higher image resolution will give you better presentation or output. In connection to your inquiry, converting your images to a 200 and 300 or even a much higher DPI with the same Frequency/LPI will give you the same dot counts. Now the only differs is the resolution results which the dots can be represented by squares @Low DPI and good to perfect ellipses @hiGH DPI. 










See this pic also..http://www.t-shirtforums.com/attachments/15796d1281082169-photo-print-untitled-2.jpg

Here's an article about halftone dots...IMAGES - THE JOURNAL FOR TEXTILE SCREENPRINTING ,EMBROIDERY, PROMOTIONAL CLOTHING AND GARMENT DECORATION


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## ShaolinD (Aug 25, 2010)

I see, so I guess it was probably just an optical illusion that was making me think the dots were a different size. thanks guys you've helped me a great deal...

Thanks.

Merry Christmas!


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## PositiveDave (Dec 1, 2008)

If you are specifying 35lpi then they should be the same size, if you are using the filters then PS asks for a no. of pixels in the postscript cell, that would give a different result at different resolutions.


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## tpitman (Jul 30, 2007)

JSF: Your attachment was an excellent, wordless explanation of how printer dpi and image ppi affects printed halftone dot quality. Sometimes it's hard to get the concept across with a verbal explanation. Thanks.


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