Noob Question about DPI

[sjdsjdsjd]

Please have mercy on me.

So I'm making 24x36 posters for the office. Essentially, they're just images that we took so I opened the raw files directly into photoshop and scaled them. I took a look at the Image Size and noticed the DPI was 240... should I change the number to the magical 300? That won't do anything but make the printing output bigger and screw up my size right? They always say to make your print files at 300 DPI so I'm just not sure what to do.

Much thanks as always!

[Sketcher]

240dpi might be enough depending on the quality of the image. View it at 100% scale in Photoshop and see how it looks.

When you say scaled them, did you scale them up or down? Scaling them up is going to lower the resolution. Changing the number to 300 is not going to increase the resolution. A low-res image is going to be low-res no matter what number you put in there.

[hank_scorpio]

"They always say 300 DPI" is an absolute myth.

It is completely dependent on the output device LPI (lines per inch) the more lines per inch, the more PPI you need to fill those lines.

In newspaper print they use 80-120 LPI - and you multiply this number by 1.5 to get the PPI.

So it could be anywhere from 120ppi to 180 ppi - depending on what LPI they use.

For magazines it's around 150 - 175.

And for high end coffee table glossy magazines it could be around 200 LPI.


Most people multiply the 150 LPI x 2 to get the mythical 300 PPI for print, but it's wrong, in reality it's actualy 150 x 1.5 = 225 PPI is plenty for a 150 LPI output device.


And these numbers don't really matter when it comes to Printing Large posters - or sign work - PD is the resident expert around here.


But signs and posters, large prints rely on resolution dependent on how far away someone is from an image.

I don't know if you're talking inches here - but this formula has served me well in the past and it's done in inches to keep everything neat (seen as we're working pixels per inch)

1/((distance x 0.000291) / 2) = desired ppi

If your poster is being viewed from say 4 feet away - that's 48 inches

1/((48 x 0.000291)/2) = 143 PPI


143 PPI would be the Optimum resolution for the image.



And upscaling images from 240 to 300 ppi is a bad workflow. Technically you do get an upscaled image, that reads as 300 ppi, but it causes interpolation in the image - which can look really bad.

If this was going in a magazine I'd call the printers and find out what LPI they output at - 240 PPI would probably be fine.

[longboy]

I would quote everything hank said, but that would just be silly and make for a long post.

So, yeah...what he said. I often do my company's high-end printed calendar at 300ppi, but I do tradeshow booth panels at about 125ppi and they both look great. Different mediums and different viewing distances

[PrintDriver]

Too much math.
150 to 200 for a 24x36 is fine. If it's real.

[garricks]

Originally Posted by PrintDriver

Too much math.
150 to 200 for a 24x36 is fine. If it's real.

LOL! You're right about the mathy stuff.

The reason I tell my clients that I need a minimum of 8-1/2 x 11 photos at 300 dpi is because otherwise they send 1 x 1-1/2 inch photos at 72 dpi.

[hank_scorpio]

Not enough math - knowing what dpi/ppi is one thing, being able to work it out is a different kettle of fish.

[sjdsjdsjd]

Okay I should be fine guys that's what I thought. Thanks!

P.S. Never thought I'd be doing math as a designer. Ha, thanks hank!

[hank_scorpio]

You do maths every day. Part of the job. I do like doing maths, I like the technicalities of the job more than the designing part of it.

One of those things I like doing and learning about. I was always told by prepress mentor "Stick to the maths and you'll never go wrong".

[iambunny]

You need math for EVERYTHING. They really weren't lying in high school.

Good thing I always liked math.

Thanks for your Noob Question sjdsjdsjd, I got some of my own questions cleared up from the responses.