Trying to understand resolution and pixels...help please!

[Kristine]

I have read numerous articles and tutorials regarding image quality though I still can't quite get my head around it so thought I'd join a forum to ask a specific question so an expert could help clarify things for me - hopefully!
I am not new to Photoshop though I use it sparingly and am not a particularly techie-minded person so please bear with me.
I am an interior designer and often download free personal use images (usually vintage illustrations) to print and frame as wall art. Once I download an image, I open it in Photoshop to check the image size and resolution. Depending on the source this can vary greatly, anything from 72ppi to 300ppi and 5x5cm to 150x150cm.
My main confusion comes from understanding how an image will look once printed, particularly if I enlarge it. If I select View>Print Size, will the quality it appears on my monitor reflect the quality it will appear once printed?
I previously printed a large-scale map which was only 72ppi and it came out great, though I've often heard it said that 300ppi should be the minimum?
One of my main desires is to create large-scale art from free-to-use images though I don't quite understand how enlarging an image will effect its print quality. As an example, I recently found a great website with hundreds of images. I downloaded one, opened it in Photoshop and saw it was 300ppi and 12cm wide x 18cm high. Just to see what would happen I changed the height to 150cm (I left the resample image box ticked - whatever that means). I then changed my view to Print Size - it still appeared clear (well, as clear as a vintage poster needs to). Is this how it will look once printed? When I changed the view to Actual Pixels it was reasonably blurry.
Can anyone offer any insight into this baffling topic for me? Or advice on the best way to go about enlarging images.
Thanks so much (and sorry for the long post).

 

[RTContent]

You need to think of your monitor and printer as two separate pieces of hardware. What you see on the screen is hardly ever what you get in terms of color accuracy and print quality. You get the colors to match as close as possible by calibrating your monitor to your printer.
300dpi is typically the limit the human eye can see at a couple feet, which is why it is often used as a baseline for print quality.
Overall it depends on the quality of your monitor as well as the quality of your printer. I would recommend printing test strips to see what the result will look like without committing to a full print.

R

 

[ibis]

if you download such images from the internet as you say, there is an option in google images search where you may define results image size. i always check show only images greater than 2mb (that is the way google separates them) and you should be good with resolution issues.

 

[hawkeye]

Resolution all comes down to the number of pixels available.

For instance, let's say you download an image and it's 50" X 50" at 72 PPI has 72 PPI X 50" = 3600 pixels available each direction . To make a good print you'd like to have 250-300 pixels per inch (depending on viewing distance it could be even less) 3600 pixels /300 PPI = 12 Therefore you could make a good 12" X 12" print.

Example 2) You download an image which is 20 X 20 at 150 PPI. 150 X 20 = 3000 pixels
3000/300 = 10 inches

 

[ibis]

Resolution all comes down to the number of pixels available.

For instance, let's say you download an image and it's 50" X 50" at 72 PPI has 72 PPI X 50" = 3600 pixels available each direction . To make a good print you'd like to have 250-300 pixels per inch (depending on viewing distance it could be even less) 3600 pixels /300 PPI = 12 Therefore you could make a good 12" X 12" print.

Example 2) You download an image which is 20 X 20 at 150 PPI. 150 X 20 = 3000 pixels
3000/300 = 10 inches

it is truly the quality measurement.
you may see this in action after you import an raster image into vector program such as coreldraw. when you resize image here, you may see how it recalculates resolution, like when you make it smaller it increases dpi resolutioon and vice versa...

any raster image is described by the number of pixels on x and y axis.
any vector drawing is described by mathematical formula and recalculates each time it has been resized.
as simple as that.