I have had little to no experience with actual printing, so I have a question for the experts.

Whenever I see an image posted that is going to print, the resolution/image size is some insanely high size. Why is this? What constraints are there that cause the image size to be so high when it comes to printing something?

Thanks! :)

You want the long version or the short?

Short version:
Images have to be at least 300 dpi for quality printing.
(that's a period at the end)


Long version:
Offset presses print using a dot screen. There are two type of dots screens, AM and FM. AM screening which has same sized dots with varying spacing is used by the majority of printers. FM screening uses different sized dots with the same spacing. FM screening technically is better and you can use a lot lower resolution images with it but it's a lot harder to print and spot errors on so it's not used very much. The higher the dot screen the smaller the dot. Good quality printers use a dot screen of at least 175lpi (lines per inch) and it can go up to 300lpi and beyond. Powers that be say that your dpi should be double that of the lpi to get optimum print quality.
150lpi = 300dpi
175lpi = 350dpi
200lpi = 400dpi

In actual practice you can skimp at little on the dpi, maybe down to 200dpi depending on the image. Images with lots of fine detail need a lot higher resolution especially if they have circles of diagonal otherwise you get evil pixelation a opposed to say a screened (lightened) background.

So if you want an 8 by 10 image printed you need at least a 2400 by 3000 pixel image.

Now don't just increase your dpi in Photoshop all this will do is make your image blurry (it will prevent the evil pixelation though). You need actual "good" pixels not filler or grain.

The reason the images look so big on screen is actually what I call an error in internet browsers and e-mail programs. The reason they appear big is that these programs display everything at 72dpi no matter what the actual image size is. So if I have a 300dpi 1-inch square image displayed in Internet Explorer it will appear 4 by 4 inches. The problem really comes when you mix the printing part with the internet part. Say you want to send a proof to a client (most clients only have e-mail and internet browsers, no Photoshop). They either view it on screen really large and it will print the correct size and be readable or they can view it on screen the right size and when they print it they can't read the text because it's pixely.

We could go into the whole thing about inkjets and the different print processes and why lpi isn't equal to dpi when you think it should be but I'll leave that for another post. (I have to catch up to SD somehow)


freakyclean

No one will catch up with me...muahahahah.

Thanks for the info freakyclean. :)