"Resolution for Dummies" (pixels vs. dpi)

[Trenton]

Hi all-

I'm a film user thinking about making the jump to digital. (I shoot Canon, so I'm thinking about the D30, but this question is "brand independent", I think)

I'm also a graphic designer by day who works mainly in print. As such, I live in a dpi world, (dots per inch) not a pixel world. I know Photoshop well - I'm in it every day it seems. But I'm a little confused about resolution in terms of pixels. Tell me you have a 5" x 7" 300 dpi photo, and I know in my head how sharp that will be. Same goes for an 8" x 10" @ 72 dpi. Or a 13" x 19" @ 150 dpi.

But say I have a 2,000 x 1,000 pixel image. (like you might read in digital camera specs) What physical size is that in terms of inches? What dpi would that translate into? How do I know what resolution that will be in terms of dpi? You can have a 2,000 x 1,000 pixel image that's 72 dpi or 300 dpi. Pixels measure physical dimensions, don't they?

When you open up digital camera pics, what dpi does it come in as? Does it differ by camera? Are there industry standards?

I was playing around with a blank Photoshop file. I made a new one that was 2,000 x 1,000 pixels @ 72 dpi, and that translates into a 27 x 13 inch file @ 72 dpi. I made a file that was 2,000 x 1,000 pixels @ 300 dpi and that was only a 6 x 3 inch file. How do you know what dpi an image comes in at?

See what I'm getting at? I hope I explained that clearly. Maybe I'm making things too hard.

Thanks,
Trenton

 

[BobTrips]

Hi all-

I'm a film user thinking about making the jump to digital. (I shoot
Canon, so I'm thinking about the D30, but this question is "brand
independent", I think)

I'm also a graphic designer by day who works mainly in print. As
such, I live in a dpi world, (dots per inch) not a pixel world

Hi Trenton,

I'll give you a simple answer. Then maybe someone can come along and give you a better one.

Basically take the pixel output of the camera and divide it by the dpi output you want to print.

For a 2 meg, 1200x1600 pixel camera you can print an 8x10.5 at 150dpi, a 4x5.3 at 300dpi.

Think of pixels as tiny squares of color that you can spread across the paper. (Works for me

The number of pixels aren't the only factor to consider. The Nikon D1 is a 2 meg camera but can out produce many cameras which have a higher pixel count.

 

[Darrell Spreen]

Hi all-

I'm a film user thinking about making the jump to digital. (I shoot
Canon, so I'm thinking about the D30, but this question is "brand
independent", I think)

I'm also a graphic designer by day who works mainly in print. As
such, I live in a dpi world, (dots per inch) not a pixel world. I
know Photoshop well - I'm in it every day it seems. But I'm a
little confused about resolution in terms of pixels. Tell me you
have a 5" x 7" 300 dpi photo, and I know in my head how sharp that
will be. Same goes for an 8" x 10" @ 72 dpi. Or a 13" x 19" @ 150
dpi.

But say I have a 2,000 x 1,000 pixel image. (like you might read
in digital camera specs) What physical size is that in terms of
inches? What dpi would that translate into? How do I know what
resolution that will be in terms of dpi? You can have a 2,000 x
1,000 pixel image that's 72 dpi or 300 dpi. Pixels measure
physical dimensions, don't they?

I think Bob's example will probably help you, but let me add a point
here. Your example of a 2000 x 1000 pixel image should be thought
of as an amount of information. Obviously, the larger the numbers, the
more information will be contained in the image. How much is enough
can be argued at length, but Bob's examples tell you how large a print
you will get at the ppi resolution you prefer - 150 ppi, 300 ppi, etc - without
any interpolation. In every case, the information content will be the same,

but you will see the pixelation in the larger pictures. Interpolation algorithms
can smooth out the pixelation effects, but add no real information. That's
another topic.

When you open up digital camera pics, what dpi does it come in as?
Does it differ by camera? Are there industry standards?
I was playing around with a blank Photoshop file. I made a new one
that was 2,000 x 1,000 pixels @ 72 dpi, and that translates into a
27 x 13 inch file @ 72 dpi. I made a file that was 2,000 x 1,000
pixels @ 300 dpi and that was only a 6 x 3 inch file. How do you
know what dpi an image comes in at?

Almost all digital camera files will be displayed at 72 ppi when downloaded -
the resolution of most monitors. When you want a high quality print, you must

adjust the image quality you send to your printer to 300 ppi (or whatever you choose).
You don't really see any improvement in the picture quality on your monitor when
you adjust the picture to 300 ppi, because of the inherent limitation of

monitor resolution -- you just see the size reduction in the picture. But the quality
will be obvious in a print, as you know. The basic point is that, as long as
you don't change the file size (by resampling), you can make a picture any size
you choose (with image quality differences) without affecting image content.
Once you scale down or up (WITH resampling) you have changed the image
content -- but that may be useful if you want a big print and don't want to see
any pixelation. Confusing -- I hope not.

See what I'm getting at? I hope I explained that clearly. Maybe

I'm making things too hard.

I complicated matters a bit by distinguishing between ppi and dpi. When you send

an image to the printer at 300 ppi (pixels-per-inch), you will likely print it at 720 or
1440 or 2880 dpi (dots-per-inch) depending on how finely you want the printer

to lay down the ink dots. This is independent of the ppi image resolution -- you
are simply telling the printer you want each pixel represented by some number
of dots in the printing process.

Thanks,
Trenton

 

[Redbreast [UK]]

Nothing to add to the theories outlined above - they'e said it all.

But the big question I found (and left open above, for very good reason), is: How many PPI to send to your printer. And it is because of the variables at the priner end that I have not yet found any definitive guide "out there" that says "use this many PPI".

However it is no coincidence that the previous contributors suggested a range of 150 - 300. In my researches no "guru" has ever suggested more than 300, and some have said that 150 is sufficient.

Here's what I did:

I identified a suitable image, one with a good spread of colours (both flat and graduated) and also with fine detail. I then made four duplicates, and resized each to the same physical" dimension (5.5" x 4") but different PPI - i.e. 150, 180, 240 and 300. Thus the pixel-based sizes were 825 x 600 for the first, 990 x 720, etc.

I then printed all four on one A4 sheet of quality photo paper at my printer's best (1440 dpi) settings, and tried to spot any difference. I couln't! So I now use 180 PPI as my normal setting when cropping and resizing images for printing.

I suggest that you experiment in a similar fashion and find the optimum setting for your printer, paper and ink combination.

Regards,

Robin [Redbreast]

 

[Gary Grundy]

I like your test. I assume you mean that when you print 4 pictures to a sheet you use 180ppi. Have you done this test for an 8x10?

Nothing to add to the theories outlined above - they'e said it all.

But the big question I found (and left open above, for very good
reason), is: How many PPI to send to your printer. And it is
because of the variables at the priner end that I have not yet
found any definitive guide "out there" that says "use this many
PPI".

However it is no coincidence that the previous contributors
suggested a range of 150 - 300. In my researches no "guru" has ever
suggested more than 300, and some have said that 150 is sufficient.

Here's what I did:

I identified a suitable image, one with a good spread of colours
(both flat and graduated) and also with fine detail. I then made
four duplicates, and resized each to the same physical" dimension
(5.5" x 4") but different PPI - i.e. 150, 180, 240 and 300. Thus
the pixel-based sizes were 825 x 600 for the first, 990 x 720, etc.

I then printed all four on one A4 sheet of quality photo paper at
my printer's best (1440 dpi) settings, and tried to spot any
difference. I couln't! So I now use 180 PPI as my normal setting
when cropping and resizing images for printing.

I suggest that you experiment in a similar fashion and find the
optimum setting for your printer, paper and ink combination.

Regards,

Robin [Redbreast]

 

[Redbreast [UK]]

I assume you mean that when you print 4 pictures to a sheet you use 180ppi.

No, I mean that my printer appears not to need more than 180 PPI whatever the print size, since even with 300 PPI I can detect no difference under normal visual examination.

I printed my test sheet 4-up so that I could easily compare all four of the different-PPI pics.

Have you done this test for an 8x10?

Yes, with the proviso that the image file has to contain the right number of pixels at that PPI resolution (viz. 1440 x 1800).

Conversely at this PPI resolution a 3MP image will just about allow an un-resampled print size of nearly 12 x 10 (2160 x 1800 -> 3,888,000 pixels).

The exact figures will rely on the effective pixels produced by your camera, since all cameras reserve some of the native CCD pixels for technical data purposes. Many review sites often quote the effective pixel values alongside the nominal.

Regards,

Robin [Redbreast]

 

[Jeffrey Behr]

Trenton--all of the previous postists have answered your questions very well, more or less, but no one has made this point quite as bluntly...PIXELS HAVE NO DIMENSIONS. As someone already said, a pixel is only a dot of electronic color information. The image from a digital camera (or scanner) has no DIMENSIONS (that is, SIZE) until you display or print it, and then YOU or your monitor must choose a rate of display or printing. PC monitors use 72 pixels per inch (ppi) by default, so an image 720 pixels by 360 pixels will be 10 inches by 5 inches (unless you're using software such as Photoshop that changes the display size, but its rulers will still tell you the pic is 10 by 5 inches). If you send that 720 x 360 image to your printer and tell it to use 360 pixels per inch, the image will be printed 2 x 1 inches.

BTW, ppi in printing is definitely subjective, but I think the center part of the bell curve of acceptable quality might have something like 200ppi at its lower end and 400 ppi at its upper end. Brands of printers make a difference too. Most Epson printers use 360ppi as a base number, and prints sent at whole multiples of 120 seem to look best from an Epson. Said another way, 240ppi will look better from an Epson than 270 or 300, because the printer must reinterpolate the 270 or 300 into something else, where it can use 240 'natively'. Canons like 300, so a print resolved at 150 is likely to look better than one sent at 180ppi.

I've done tests with my Epson 1270 (whose highest resolution is 720 x 1440), and 240 does indeed look sharper than 270 or 300 but not as good as 360. I use 240 for noncritical work (ie snapshots) and 360 for critical work.

Trenton wrote:
Tell me you

have a 5" x 7" 300 dpi photo, and I know in my head how sharp that
will be. Same goes for an 8" x 10" @ 72 dpi. Or a 13" x 19" @ 150
dpi.

Indeed. The key here is the RATE. You know a 5 x 7 at 360ppi will be sharp, not because of the 5 x 7 part, but because of the 360ppi part. You know that 8 x 10 at 72ppi will NOT be sharp if it's a print because of the 72 part, not the 8 x 10 part.

 

[Redbreast [UK]]

Most Epson printers use 360ppi

Canons like 300

Jeffrey,

Many thanks for such an informative reply, which certainly added to the quantum of my knowledge on this elusive subject.

I have two questions for you, if you don't mind:

1. Where did you get the above figures from?

2. I should love to know more about the underlying theories, so are there any web sites on this topics that you would recommend?

Many thanks,

Regards,

Robin [Redbreast]

 

[Mofongo]

Darrel,

Actually I think you simplified this. I always hear people talking about resizing a 72dpi file to 300dpi. That would be 72ppi or 300ppi. Alot of people mix these up and use ppi and dpi interchangeably...Bob

I complicated matters a bit by distinguishing between ppi and dpi.
When you send
an image to the printer at 300 ppi (pixels-per-inch), you will
likely print it at 720 or
1440 or 2880 dpi (dots-per-inch) >