ppi help, I'm stupid

[Dmitri]

Yes I am stupid, but at least I know it.
Now, that out of the way, here's my problem:

I just now noticed that when I import raw files into photoshop as 240 PPI it will allow me to print it at 16x10 (image size), whereas when I import it as 320 PPI it only allows me to print it at 12x8 (image size).

ok, can someone explain this to me in small words? I get that there's probably some math thing going on - but why would someone choose a higher ppi if you can only print smaller sizes?? Since I started I had been importing as 320 (because I read it somewhere that it was the best to do), and now I'm wondering if I shouldn't just reduce it to like 100 or something if it allows me to make bigger prints without any degradation

So yeah, dumb it down for me please.

 

[Dwig]

...I'm wondering if I shouldn't just reduce it to like 100 or something if it allows me to make bigger prints without any degradation...

It doesn't.

You images are importing with the same number of pixels regardless of the PPI you choose. Its the number of pixels that determines the file's image quality. PPI is meaningless.

Only when the image is printed does PPI come into play. For example, if you have a 1200x1800 pixel image and print it as a 4x6" print there will be 300 pixels per inch (PPI). This doesn't change regardless of the PPI set in the file. The file's PPI is mearly a "preference" that tells the printing software that the creator of this file wants it to be printed at 300ppi, or whatever, and the default print size will be calculated based on that number.

If we take the same 1200x1800 pixel image an print it at 8x12 there will be only 150 pixels per inch on the paper. If we scale it to 12x18" there will be only 100 ppi. Each of these larger prints will be lower quality than the 4x6. It doesn't matter whether the file's PPI preference was set to 300, 150, or 100.

All the PPI setting does it to set the default display or print size. In Photoshop, its inconvienient to set the PPI too low (<200ppi) when dealing with modern larger digital files (10mp+) as the initial default print size is overly large for most uses. Regardless of the PPI you choose when processing the RAW file you can easily change it later with out actually altering any pixels. Its a simple mater to use Photoshop's Image Size control and uncheck "Resample" before changing the PPI value.

 

[Dmitri]

Ok so how do I figure out the maximum size I can print an image before it starts to get splotchy? I had used the "image size" in photoshop to tell me (which is why I get confused when it gives different sizes when 240 or 320 ppi).

 

[AnaBo]

well If you want to go to a bigger size than your file can handle you can always reduce the PPI and increase the size .. I wouldn't go smaller than 240..
when you import at 320, and you want to go bigger go to 300, depends on the size.. you shouldn't let PS scale sizes for you.. you can make custom sizes and if you can make test prints then all the better for you... if not, then just make sure you don't go under 240... I aaaalways print 300 but I know 240 is still really good!..

 

[Dmitri]

ok thanks. I'll just leave it at 240 and never give it another thought then. Thanks

 

[Garbz]

False!

You need to understand where the notion of quality comes from. 300ppi is the limit where someone with perfect vision will not be able to resolve the pixels of a photo held at arms length. Not everything is held at arms length.

240ppi or even 200ppi is good for a standard small size, but the goal is firstly not to resample (i.e. a 10mpx camera should produce a 6x4 print at something like 640ppi so if you're printing a 6x4 then don't reduce the ppi since larger is better), and secondly to take into account the final picture size and viewing distance.

The 40" wide image on my wall is printed at 100ppi and looks fantastic. There's some billboards on the road side that are printed at 25ppi and look fantastic. There is no finite line for ppi that you shouldn't cross. When in doubt, print a proof.

 

[Dwig]

Ok so how do I figure out the maximum size I can print an image before it starts to get splotchy? I had used the "image size" in photoshop to tell me (which is why I get confused when it gives different sizes when 240 or 320 ppi).

PS's Image Size will tell you the size that would be associated with a particular PPI. If you want to print at 300ppi, for example, setting 300ppi in Image Size with Resample unchecked will show you what size print you will get. Conversely, you can set the image size and it will tell you the resulting PPI.

What final PPI you need in the print is your decision. With most of the better printer and for prints that will be viewed closely, you generally want 300ppi or higher, 240ppi at a minimum. For very large prints that will be viewed at a distance you may find that you get decent results with final PPI's as low as 150.

 

[Dmitri]

okay I think I'm starting to understand. So it's best if I leave it at 240 ppi unless I plan to do something special with it (like a very large print or so)? OKay this I can grasp.

 

[Garbz]

It's best to ignore it completely until you go to print, and when you print let the number guide you as to the quality you can expect.

 

[kosherstock]

What final PPI you need depends on the printing device you're using.
You should make some tests but usually 150DPI is more than enough to get perfect quality on an ink-jet printer.
On printer devices using screens (e.g laser printers) you need your PPI to be between 1.5 and 2 times the screen ruling (screen ruling is the number of lines of ink dots that will be printed per inch on your sheet and is abbreviated as "LPI" - Lines Per Inch and you can set it on any Postscript printer. I don't know about non-Postscript printers). As Garbz said before you can print billboards at 25PPI provided you set your screen ruling to about 15LPI. You will then get only 15 printed dots of ink per inch (big spaced dots) and it will look fine if you look at it at a distance.