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Old Mar 23, 2004, 11:29 AM   #1
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Default Size vs Resolution

I never realized when changing resolution you changed picture size, probably because when changing size the resolution stays the same. When shooting pics on an 'old' nikon (880) the resolution was always 300. In changing to 72 for posting the photos were always resized. This may seem basic to all of you but please bear with me. All my Oly's automatically download at a resolution of 72 do I need to change the resolution when I'm going to print? I've never done it in the past and have gotton wonderful pictures but now that I'm paying attention to the numbers I'm confused (proving the saying a little knowledge is dangerous).
When enlarging the resolution for printing the size goes up and I know you can't create something that's not there. Could someone please clear this up for me?

Thanks, Suzan
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Old Mar 23, 2004, 11:58 AM   #2
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I'm not sure I fully understand your question, so let me ask one back:
When you say "resolution" do you mean "dots per inch" or DPI?

If so, that shouldn't effect the size of the picture on the screen at all. That is only an instruction to the printer of how many pixes to put within 1 inch of paper.

I could go on about this, but if I'm missunderstanding myself, then it's a waste of my time (and yours.)

Eric
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Old Mar 23, 2004, 12:50 PM   #3
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I thought dpi was Dots per Inch.
When looking at Elements 2 resize box it just states resolution and I don't know which it means. Anyone else with PS?

Suzan
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Old Mar 23, 2004, 2:41 PM   #4
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Photoshop gives resolution in pixels/inch or pixels/cm according to how you want to work. Pixels/inch is usually abbreviated PPI, and people usually use PPI to differentiate it from the DPI that printers use.

The resolution that you download the pictures at makes no difference in the size of the image. If you uncheck ďResample imageĒ you can change the print size and the resolution reflects the PPI for that size print based on the image size in pixels. The image size wonít change when you do that.

To print to a certain size you usually need to trim something off the 4:3 ratio image most cameras give you. If you wanted to print 4 X 6 you would select the crop tool, put 6 in the width and 4 in the height with NOTHING in the resolution box. It will constrain the crop to that ratio and change the print size using all of the remaining pixels. You can go Image>Image size to see what PPI you ended up with.
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Old Mar 23, 2004, 2:48 PM   #5
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Hi Susan,

Perhaps dpi/ppi are one of the most misunderstood of all terms used in digital processing. Primarily this is because the terms dpi and ppi are often used interchangeably when they should not be.

Technically ppi refers to a display file size dimension in Pixels Per Inch (ppi) while dpi refers to a print density in "dots per inch". The problem is that they are so frequently misused that confusion reigns supreme here.

When your camera captures a frame, the sensor has an array of photosites (also called pixels - another place for confusion). These photosites are arranged in a matrix of so many wide by so many high. When you multiply the number of pixels in the width by the number in the height you end up with the "megapixels" of your camera.

When you "display" the image on the screen, the display card (video card) in your computer is set to display so many pixels across by so many wide. For example, a typical display may be 800x600 or 1024x768. This means that no matter how many actual pixels there are in the image matrix, the computer will remove enough of them temporarily to display the full image at whatever the display size happens to be set for.

In the old days of computing, most graphic and photo work was done on MacIntosh computers. The "standard" graphics display at the time was 72 pixels per inch. Adobe (PhotoShop - Elements, etc) simply adopted this as a more or less standard for display. Many camera manufacturers also simply set the defaults so that when you open an image it shows the width and height of the picture at a display of 72 pixels per inch. Obviously, this is often way larger than will fit on the screen, so PhotoShop shows the image as a "percentage" of the true size (at 72 ppi) and improperly refers to it as DPI.

When you get ready to print an image, the printer - depending on brand and type - has an "optimum" print density which is given in "dpi". This means that the printer expects the file to also have a density in pixels per inch which relate at 1:1 to the dots per inch print density. PhotoShop has the ability to "interpolate" or add pixels, or take away pixels so that the print density in dpi is matched by the file size in ppi.

For the most part, you don't need to be concerned with these things. What you might consider doing is purchasing an inexpensive but powerful program like Qimage (link below) which will take care of all this for you. You simply select the print size you want and Qimage communicates with the print driver and selects the proper print density and prepares the file (image) accordingly. Perfect results and you can forget worrying about the details.

Best regards,

Lin

http://www.ddisoftware.com/qimage/
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Old Mar 24, 2004, 7:16 AM   #6
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Although Lin has explained everything quite well, I think it is good to have a general knowlegde of resolution and how it relates to document size and pixel dimensions. Here is a very small, quick example to illustrate. If you have a file that is 300 pixels high by 300 pixles wide, then those numbers are your pixel dimensions. If you never plan on printing, then you don't need to know anymore. But, if your dpi or ppi, ( I will use them as if they were the same thing for this explanation ) is set to 300 dpi, then you document size is 1 inch by 1 inch because you are printing 300 dots in 1 inch and your file is 300 pixels wide, so 1 inch. Now, if you changed the resolution to 150 dpi, half of 300, then you could now print the file 2 inches by 2 inches, becasue you file is still 300 pixels wide. It still looks the exaxct same ion screen. This is assuming you have not resampled the image when you changed the resolution ( photoshop has this box that you must check or uncheck ) Now printing at half the resolution at double the size will give you a less desirable result. You cannot simply increase a files pixel'sl dimensions with good results, otherwise there would be no need for cameras that capture more pixels, you would simply just increase them yourself in a program like photoshop.
Now, if you took a picture with a good camera that is 2272 pixels wide by 1500 pixels high, and you wanted to email it to someone, then in this case, you should resample the image downwards, you click resample, and you change pixels width to 800 or something that can be viewed on screen withou t scrolling to the right or down. In this case, resolution is not important, because when viewing a file on screen, you view pixels at 72 dpi all the time. This can be very confusing but play around in file size and resolution to see the effect of the change. I am not familiar with elements, so I do not know if the image size dialog box is the exact same.
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Old Mar 24, 2004, 10:38 AM   #7
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To add something else, the proper "dpi" is partially picture dependent. Some pictures (like landscapes) have lots of detail and benefit from higher DPI. Other pictures (like portraits) can look good with a lower ďdpiĒ because your brain fills in the details that might be lacking.

The ďgolden ruleĒ of dpi used to be ď300Ē. But Iíve seen people here say they get good results at 200 or even 150 dpi. I donít know what they were printing, but it could very well be true. So assuming you arenít making huge expensive prints, then you can probably get away with something in the 150-200 dpi range and be happy.

Eric
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Old Mar 24, 2004, 11:36 AM   #8
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Quote:
When you get ready to print an image, the printer - depending on brand and type - has an "optimum" print density which is given in "dpi". This means that the printer expects the file to also have a density in pixels per inch which relate at 1:1 to the dots per inch print density. PhotoShop has the ability to "interpolate" or add pixels, or take away pixels so that the print density in dpi is matched by the file size in ppi.
I doubt you mean to say that. A modern 2 picoliter photo printer uses 64 dots to represent a pixel. They give DPI numbers like 4800 and 5760. You certainly donít want to match the pixels to the printer DPI 1:1.

Through version 5.5 (I didnít use 6) Photoshop gave you a message that it was sending the image to the printer at 300 PPI if you printed a higher resolution image than 300 PPI. Version 7 and higher no longer gives the message Ė I think to avoid tech support calls from people who think their printer is being starved of pixels. I suspect Photoshop is still sending the image to the printer at 300 PPI. If not, it is limiting it to a reasonable size to keep the printer spooler from taking all day to digest image data it doesnít need.

Back when 10 picoliter dots were state of the art Epson gave a figure of 240 PPI as the highest PPI you should send to a photo printer. 10 picoliter printers usually used 16 dots to represent a pixel. The size of the dots has gone down and the number of dots per pixel has gone up at about the same rate the resolution has increased. I think that 240 PPI number is still probably valid. 4 and 5 picoliter printers use 32 dots for a pixel and 2 picoliter printers use 64 dots.

I have a Canon wide carriage photo printer that uses 4 picoliter dots. I canít really see image improvement feeding more than 180 PPI to the printer. Someone whoís opinion I have come to value came up with the same number after testing on his Epson 2200 photo printer. I donít think there is an inkjet on the market that needs over 300PPI.

Interpolation causes some image degradation. Not a lot with a good filter like bicubic or Lanczos. Programs like Genuine Fractals and setting Photoshop up for SI will do a little better, but there is still some loss. Qimage technical data lists Lanczos as their best quality resample. You should do some tests on blown up sections to save paper and come up with a good idea of where adding more PPI doesnít improve the image and not upsample unless you donít have that PPI.
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Old Mar 24, 2004, 12:10 PM   #9
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First, we need to realize that injet technology is but one of numerous technologies presently used in the printing business. Your discussion assumes that the pint will be done by inkjet. Though inkjets are wonderful instruments for producing extremely sharp and colorful images, the mainstay of professional printing is still done with Durst Lambda, LightJet and similar continuous tone processes.

Don't confuse input ppi with print dpi. Even though injkets (we use Epson 9600 ourselves) use multiple dropletts to create a single pixel, it's the "pixel" itself taken as a unit which is matched to the file size at the 1:1 ratio which the print driver expects in most cases. Some printer hardware such as Epson require several different imput densities depending on the media (paper) used for output. For example, glossy film settings on the Epson inkjet line request an imput of 750 ppi and regardless of what is "furnished" by the software, the print driver will interpolate to get to this requirement. Each pixel may indeed consist of contribution from numerous dropletts of ink, but this output is still printed based on the 300 ppi information furnished by the file. This is why PhotoShop reported 300 dpi in prior versions.

Much of the confusion over "printing" at lower dpi and seeing excellent results comes from not understanding that the print driver is going to furnish the printer with with what it requires in the way of input resolution matched to the media selected. When the user sends a 180 ppi file to be printed the file is interpolated up to the system requirement by the driver. The reason people see little differences in many cases between say 150 ppi and 300 ppi files sent to their printer is simply that the printer driver equalizes the file input by interpolation. The only visible differences in output are those differences revealed by the amount of interpolation used by the driver. In most cases they look very similar.

Qimage no longer uses Lanczos as the "preferred" default for interpoltion. For some time now Lanczos interpolation has been surplanted by Vector interpolation which produces smoother output.

Best regards,

Lin
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Old Mar 24, 2004, 3:16 PM   #10
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So your suggestion to ImKayd1 to match the printer DPI with the PPI resolution 1:1 assumes he prints with ďDurst Lambda, LightJet and similar continuous tone processesĒ? It is my understanding that DPI in that sense doesnít properly refer to continuous tone printing anyway.

Quote:
the Epson inkjet line request an imput of 750 ppi
Do you have a reference for that? If a printer is using 32 dots to make a pixel, 300 PPI should provide sufficient information for 9600 printer DPI. I donít know of one that is rated that high.

What Iím suggesting is that 180 PPI will provide enough digital information for 5760 PPI on a 4 picoliter printer, and that is currently the highest on the consumer market.

Interpolation isnít magic. An upsample using any method canít provide information that isnít there. A 120 PPI image is upsampled in the printer driver to whatever is needed to provide the requisite number of dots, but you can sure see the difference between a 120 PPI print and a 180 PPI print.
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