question about resolution

[joeq]

it's my understanding that the resolution of an image is determined by the measurements of its pixels... length x width. but, when it comes to exporting from lr and printing, don't we need to set the proper dpi for the output device?

i always thought the rule of thumb was 3'' dpi (at the final image size) for high quality output - true?

 

[Victoria Bampton]

As long as you've got the right number of pixels overall, it shouldn't matter in the least.

 

[joeq]

really? so a file that's, say, 4256 x 2832 will print the same whether it's output at 3'' dpi or 1'' dpi?

i'm not sure i understand how that's possible.

 

[Victoria Bampton]

Imagine you've got a pizza... whether you split it up into 4 big slices or 8 small slices, if you eat all of the slices, you'll be equally full up, regardless of how you cut it!!!

If you feed a printer 4256x2832 pixels, and tell it you want an 8" x 1'" print, it will decide how to split it up.

 

[dj_paige]

The number of ppi that is embedded into a file’s metadata is really a meaningless number.

 

[joeq]

nothing like a good pizza analogy to clear things up... thanks.

i guess what has me still a little confused is that i'm coming from a print/graphic design background and i was always taught to scan something at 3'' dpi and at the size of the final output.... making me think that the two are related to each other. not true?

 

[Victoria Bampton]

The 2 are related... now we've done pizza, we're on to a maths lesson!

Say you were scanning to print at 8" x 1'".

If you were scanning at 72spi, 72x8=576 and 72x1'=72' - so you'd have a file 576x72' pixels. That's not enough pixels to print well.

If you were scanning at 3''spi, 3''x8=24'' and 3''x1'=3''' - so you'd have a file 24''x3''' pixels. That's much more like it! You were doing that to make sure you had enough pixels to work with.

Now think about your camera. If you set it on small jpeg, you don't have enough pixels to do a big print, whatever size imaginary slices you split it into. If you set it on large, then you've got a lot more pixels to play with.

At the end of the day, as long as you've got enough total pixels, your dpi/ppi setting is meaningless.

 

[Kiwigeoff]

The 2 are related... now we've done pizza, we're on to a maths lesson!

Say you were scanning to print at 8" x 1'".

If you were scanning at 72spi, 72x8=576 and 72x1'=72' - so you'd have a file 576x72' pixels. That's not enough pixels to print well.

If you were scanning at 3''spi, 3''x8=24'' and 3''x1'=3''' - so you'd have a file 24''x3''' pixels. That's much more like it! You were doing that to make sure you had enough pixels to work with.

Now think about your camera. If you set it on small jpeg, you don't have enough pixels to do a big print, whatever size imaginary slices you split it into. If you set it on large, then you've got a lot more pixels to play with.

At the end of the day, as long as you've got enough total pixels, your dpi/ppi setting is meaningless.

Not quite meaningless in the case of scanning etc. Kind of like a small vs large pizza. Try stretching the small pizza to cover a big one, will spread pretty thin. Now try stretching a small pizza with loads of topping and a thick crust (more ppi to start with) it will make a large pizza satisfactorily.

LOL:lol::lol::lol::lol:

 

[joeq]

well, i'm not sure i understand this any better... and now i'm starving on top of it all!

 

[Kiwigeoff]

Joe, in my experience scanners haven't given out put in pixels per se, they give output in dimensions and ppi.
This results in say a 1'x8 inch file at 3''ppi
This file can the be resized to 2' x16 inches at 15'ppi
What I haven't seen scanners do is allow an output of 2''' x 3''' pixels only

 

[joeq]

ok, i'm starting to get it now. i just export two images - one at 3''dpi and one at 1''dpi. the 3'' one measured 9.44 inches by 14.187 inches while the other one is 28.32 x 43.56... and, lo and behold, the numbers work out to be the same if i were to, for example, make the 3'' one 1'' while keeping the overall file size the same.

interesting. thanks to all for the help.

 

[Kiwigeoff]

By jove, I think he's got it !!:lol::lol::lol:8)

 

[parintele]

joe, is very simple.
in order to print high quality u need as much definition/pixels on the square inch.
3'' dpi mean u will have 3'' pixels/inch,which mean a good definition, enough pixels in order to obtain a good quality.
24'dpi is less pixels, less quality but overall good.
less pixels mean u will have less but "bigger" pixels on the same surface, a square inch...
72 dpi mean u only have 72 pixels of information printed on inch of paper



photoshop pr LR are just trying to help u with the calculation..tehy divide the total resolution of file to the density per inch and calculate the phisical dimension of the print for you...
or divide the total resolution of file to the physicall dimension of print in order to calculate how many pixels will be spreaded on peper inch...
what interest you is to have enough pixels for the desired phisicall output.
3'' is great, 24' is good enough, 2'' is better than nothing, especially if u print large and u do not look at the print from 1'-15 inches away...


for example a 9'' x 6'' pixels image...to print best quality, 3'' DPI, u divide those numbers with 3''...
that is 3 x 2 inches print best qualty
if u need to print 6 x 4 inches , u need 6x3'' by 4 x 3'', that is 18'' x 12'' inch resolution picture...
if all u have is the 9'' x 6'' and u set the print dimension to 6 x 4 inches, the app will calculate the resolution per inch for you, dot per inch, DPI...and it will spread the amount of pixels of the picture to cover the printed surface...

that will be 15' dpi quality of the print...


by default, camera "sets" 72 dpi because that number is "traditional" display resolution. allthough displays since a long time ago may have various pixel densities per inch ... that number mean nothing, what is important is the total quantity of information (pixels" u have available in a file, that total info u can spread over any surface...
3x2 inches print or 3'' x 2'' inches print...first will be great quality, second would be extremelly poor and usable only if u look at that prin from 2 miles away..

important note: higher the DPI , better the quality/definition of the print BUT please consider printers can not tight more than 3'' pixels on one inch...
so if u print 6'' DPI and your printer can only put 3'' ink dots in one inch, u will loose half of the information ... 3'' of them will be dumped or 6'' will be averaged/mixed/interpolized into 3'' and sent to machine for printing...



is extremelly simple, no pizza involved :lol:

 

[Kiwigeoff]

Valid and appropriate comment Parintele.
Additionally my experience shows that most printers will interpolate the ppi (not dpi) to the printers print resolution which may be quite large e.g. 144' ppi.
I understand it is best to set a ppi that is a whole fraction of the printers resolution or the printers "native" resolution e.g Epson use 24' and Canon 3''...

 

[parintele]

indeed, modern printers have higher native PPI capability.
the same thing like in LCD monitors, the same as using zoom tool inside any photo processing app.. in all these cases u need to use a 2 multiplicator, positive or negatie, in order to "see'" the best quality.

the same with printers but still, with such great density of pixel printing capability, human eye can not really see the "interpolation"...dots are so small that u can;t really see any difference most of the times so no need to be concerned....

to simplify, imho best printing quality is something between 3'' and 24' ppi...that is appropriate for close examining the picture, let's say looking at the picture holdedin hand...1'-2' inches....

for wall pictures u do not examin very close 2'' ppi is just fine, maybe little under...

really, it all depends on the use of the print and in the end what is really important is the resolution of the image file, bigger the better.

 

[Mozella]

Valid and appropriate comment Parintele.
Additionally my experience shows that most printers will interpolate the ppi (not dpi) to the printers print resolution which may be quite large e.g. 144' ppi.
I understand it is best to set a ppi that is a whole fraction of the printers resolution or the printers "native" resolution e.g Epson use 24' and Canon 3''...

I'm gonna' pick a few nits. The printer in question may be able to lay down 144' DPI, but that's not the same as printing 144' PIXELS in an inch. That is to say, you aren't fitting 144' pixels from your image file into that same inch of paper. Ppi (a file parameter) and dpi (a printer output parameter) are NOT the same thing because an image pixel is not made up of a single ink droplet.

Typically, modern multi color printers lay down many ink dots in nearly the same place in order to form one pixel.

In the case of an Epson printer, divide 144' by 6 and you come up with 24' and that's why sending a 24' dpi image file to your 144' dpi printer is often recommended. Sending an image file in some multiple of the "native" resolution is accepted as good practice for best quality.

Each manufacturer uses a diferent method to deposit multiple ink droplets to make up a pixel using multiple passes, various kinds of ink blending, special papers, etc. in an effort to product the best product. When you think about what's going on, even on the cheapest photo printer, and then consider just how good the end result is, I'd say the process is astounding.

 

[parintele]

what u say about rpinters is true, as many producers as many technollogies.i used the above anallogy in order to simplify my examples as much as possible.
we could talk many pages about printers and priter technollogies...for example we all know about CMYK printing...well, i was amazed to read the white paper of my small Canon selphy 51' foto printer qhich appear to use RGB ink for printing instead of traditional ink colors...

many many interesting things, still, in practical terms not many people print higher than 3'', infact usual print resolution is 24' effective pixels per inch...

maybe we'll talk about printers and different printer technollogies on a different topic
would be very helpfull, especially if some of us would want to buy another better printer/

 

[Kiwigeoff]

I'm gonna' pick a few nits. The printer in question may be able to lay down 144' DPI, but that's not the same as printing 144' PIXELS in an inch. That is to say, you aren't fitting 144' pixels from your image file into that same inch of paper. Ppi (a file parameter) and dpi (a printer output parameter) are NOT the same thing because an image pixel is not made up of a single ink droplet.

Typically, modern multi color printers lay down many ink dots in nearly the same place in order to form one pixel.

In the case of an Epson printer, divide 144' by 6 and you come up with 24' and that's why sending a 24' dpi image file to your 144' dpi printer is often recommended. Sending an image file in some multiple of the "native" resolution is accepted as good practice for best quality.

Each manufacturer uses a diferent method to deposit multiple ink droplets to make up a pixel using multiple passes, various kinds of ink blending, special papers, etc. in an effort to product the best product. When you think about what's going on, even on the cheapest photo printer, and then consider just how good the end result is, I'd say the process is astounding.

Oops, my error there was meaning to say 144' dpi not ppi...... sorry!!

 

[joeq]

all good info - thanks everyone.

 

[Denis Pagé]

Years ago I buyed "A Few Scanning Tips" from Wayne Fulton... What I can say is that is a must read! You can read it for free on the web site or order a paper version as I did. The site is still alive and seem to be updated. There, one get to know and understand that ppi and dpi is not the same, that displays whow nothing about dpi (inches) and all the basic stuff...