Printing Resolution/size/dpi/ppi

[Jeff Mottle]

This is somethng that gets asked a lot on almost every graphics forum and I have spoken to many print houses that don't underdtand pixels either becuase they always work with inches and dpi, so you are not alone. I have considered writting a brief PDF document with graphics to help explain the relationship that exists between all the print variables, but I'm bored this afternoon so I'll do some of it here:

 

Basically when you are printing/rendering an image you have three variables to consider:

 

1. DPI/PPI (Dots per Inch/Pixels per Inch) Both are known as resolution.

 

2. Pixel size - measured in pixels (this is normally what you render to and is how monitors are measured.

 

3. Print size - measured in inches (can be anythign but I'll use inches for convinience)

 

1. DPI/PPI

 

First you need to know what resolution your printer can handle. Some will say that they go to 1200 dpi or 600 dpi, but in practice you should never need to go bigger than 300 dpi, even if you are printing a 60 ft wide billboard. Just like digital camera's non-optical zoom abilities, printers use DPI to claim superiority in the industry.

 

DPI is the number of dots of ink that are put down by your printer onto a page over a one inch line. The amount of resolution that a printer can achive is based upon how close together the print head elements or jets are positioned. Of course it is a bit different with a 4 color press, but for now I'll just leave it at that. You may also notice that your inkjet will say that the vertical resolution will is different than the horizontal resolution. This is becuase in one axis the printer is relying on the proximity of the jets to each other, whereas the oher diection is related to the sensativity of the rollers that are feeding the paper across the jets.

 

2. Pixel Size

 

I'm assuming that everyone is pretty familiar with pixel size as we deal with is on a daily basis. However, don't assume that your print house will be, because in many case they are not. You will usually need to give them an image size in inches based upon the resolution that their printer is capable. Don't try to explain it to them becuase they just won't get it.

 

Pixels are something that your monitor uses to describe the very small square dots (pixels) of light that are lit up by the guns of your CRT. (LCDs are different) Typically we say that a monitor is 72dpi.

A pixel translated to a page is usually represented by many dots of ink depending upon the print resolution.

 

3. Print Size

 

This one is pretty self explantory and is the image size on the page that your printer printed.

 

So you know what each one is but how do they relate to one another? As I mentioned earlier there are thee variables in this equation. To get one you must have the other two, so here are the possibilites:

 

For these examples let's assume that our printer is capable of 300dpi I am going to print an 8 x 10 image and my rendered pixel size is 3000 x 2400

 

You have Pixel Size and DPI and want image size:

 

This is useful when you want to know how big an image will print on your page if you do not allow the printer to scale the image at print time. (ie scale to fit).

 

===========================

 

Image Width = Pixel Width / DPI

 

Image Width = 3000 pixels / 300DPI

 

Image Width = 10 Inches

 

Image Height = Pixel Height / DPI

 

Image Height = 2400 pixel / 300 DPI

 

Image Height = 8 Inches

 

You have Pixel Size and Inches and want DPI

 

This one is not all that useful and I can't think of any reason to use it practially but here is it anyway.

========================================

 

Horizontal DPI = Pixel Size (Width) / Inches (Width)

 

Horz. DPI = 3000 pixels /10 inches

 

Horz. DPI = 300DPI

 

You can use the same equation for the Vertical DPI as well. Some printer are the same resolution in both axes.

 

You have Inches and DPI need pixels

 

This one is usful when you know that your printer can print 300dpi and you would like to print an image that is 8 x 10 and need to nkow how big to render your final image to do this. Of couse you could always render it bigger, but rendering time is precious so you never want to render more pixels that you have to.

 

=======================================

 

Pixel Width = DPI x inches (Width)

 

Pixel Width = 300 DPI x 10 inches

 

Pixel Width = 3000 pixel wide

 

Again the same formula can be used for pixel Height as well.

 

So now you know what each variable is and how they relate to one another. The biggest hurdle I suspect is how they relate to one anotehr visually, but that will have to wait for my DPF document.

 

Hopefully this helps some of you and does not confuse you even more. Even if you do not fully understand the why yet, you will at least have the formulas so you know how.

[robing]

This is a very useful tool.

 

 

 

Clic here ***DPI2PIXEL***

 

 

 

Enjoy

bye

[Jeff Mottle]

Photoshop's "Image Size" Works well too.

[dazz_downunder]

thanks jeff,

 

good info........well explained

 

dazz

[salf]

i just bumped into this thread, thanks for the info Jeff, its pretty handy, but if you still get free time, it would be great a PDF version with graphics.

[archinano]

Very good tutorial. Thanks.

[WAcky]

We have the same surname...wieeerrrd

 

thanks jeff,

 

good info........well explained

 

dazz

[will1113]

Thanks for tutorial!!

[nige777]

Nicely explained, thanks alot - i've always struggled to properly understand this topic.