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Old Jul 13, 2004, 6:35 PM   #1
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I have a new HP R707. I've found that every picture taken, regardless of the resolution setting, is set a 72 dpi. A picture with higher resolution (based on the camera settings) will have more pixels and consequently be larger, but I'm thinking that setting a dpi at a higher resolution will result in crisper prints. Right? Wrong? Do all digi cams use 72 dpi for all their pictures?

Thanks in advance
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Old Jul 13, 2004, 7:02 PM   #2
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This is a common misconception. The cameras don't take photos at 72dpi by default -- that's just how your software is displaying them (it's a size that most image editing packages have standardized on, even though most monitors use a display resolution that's slightly higher).

For example: A5 Megapixel Image with your HP camera will be taken at around 2592 x 1952 pixels.

So, with a typical 5MP Digicam, if you are displaying an image with software, at 72dpi, you get a very large image size on screen (2592/72ppi =36 inches, 1952/72 ppi = 27.1 inches). So, on an older model monitor (as the 72dpi display standard was supposedly based on), a 2592x1952 pixel image would appear to be around 36 x 27 inches large (with monitor size/resolution entering the equation, too).

But, when you send this same image to a printer, the pixels per inch sent, depend on the print size.

For example: if you wanted the "perfect" 300 pixel per inch print , a 2592 x 1952image would translate to an 8.64" x 6.5" print (2592 pixels/300 = 8.64, 1952 pixels/300 = 6.5).

However, most users won't see any difference between 200ppi and 300ppi (unless you look at the photo under magnification).

The printer driver should use all of the resolution (as long as you don't have any resize/resample boxes checked).

Actually, what it's really doing, is taking the image you send it, then converting it again, into the actual DPI printed by the printer model. It knows nothing about what your software is displaying it at -- only the actual resolution of the image in pixels (around 2592 x 1952, in the case of your HP).

If you are using a 3rd party printing service, ask them how many Pixels Per Inch (minimum) is needed for the desired print sizes (most don't even care, with the printer software automatically interpolating -- but for very large print sizes, this can be a problem with some printer types).

As far as print sizes, many will argue that 150 pixels per inch is plenty of resolution, with anything more a waste, since the human eye won't be able to tell the difference at normal viewing distances.

Others will argue that 200, or even 300 pixels per inch is necessary for the best quality. Again, I've found 200PPI to be plenty.

Here's a chart that may help. You'll probably find that the "good" column is all you need (unless you're going to examine the print under magnification).

http://www.cordcamera.com/products/digital/aspect_ratios.html

IMO, anything above about 180PPI is fine for prints up to 8x10", and even less is can be used for larger prints (because you'll be viewing them from further away). The printer used can make a difference, too -- as many ink jet printers "optimize" the input they receive for photos. Dye Sub printers may require more pixels (300 ppi is best for some dye sub printers).

Here's another chart that takes popular digital camera image sizes, and shows how many pixels per inch you'll be sending to the printer driver for popular print sizes:

http://home.earthlink.net/~terryleedawson/dcnotes/tables.htm#ppi

Also, you can interpolate an image using software (you shouldn't need to, unless you are printing VERY LARGE prints). This does not increase the detail captured originally, but it does add pixels (based on the value of adjacent pixels). This lets you print larger images without pixelaton.

A good free package is irfanview. It has a very sophisticated Lanczos Filter based interpolation algorithm (you'll find this option under the resize/resample menu option). You can download the software (free) from this link:

http://www.irfanview.com

Also, you may want to be aware that standard print sizes can result in cropping of your digital image. Refer to this chart, to determine the percentage of your image that will be used, at popular digital camera sizes, for standard prints:

http://home.earthlink.net/~terryleedawson/dcnotes/tables.htm#frameutil

Some printing services (like Photoaccess.com) offer "digital size" prints (no cropping of your photos). Others don't. So, you may want to crop your photos for standard 4x6" prints. But, don't mess around with DPI count (or check any resize/resample option boxes). More resolution than needed is fine to send to a printer.

Now, there is a flag for dpi in the image header, but changing it is only changing a byte in the image header and doesn't really change the image at all. But, I'd advise to never try and change this setting with Image Editing Software, as some software may try to resize an image based on this flag. Printing software ignores it.


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Old Jul 13, 2004, 8:01 PM   #3
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Thank you so much. I've been playing with the numbers and trying to get my head around what they all represent. I have a tentative grasp now. I also d/l'd the graphics program you recommended - thank you for that also.

style="BACKGROUND-COLOR: #000000"With regard to the resizing advice you gave me: I'm thinking that is meant for photos that I intend to print; I experimented and was able to resize one of the 2+Mb pics down to a 800x600 size for viewing on the screen and it took the size down to 90K. I did not notice a discernable degradation in image either - of course my eye is not practiced. Still, I understand why you would not want to do this for photos to be printed. I think! lol


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Old Jul 13, 2004, 8:14 PM   #4
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jbarmstrong wrote:
Quote:
With regard to the resizing advice you gave me: I'm thinking that is meant for photos that I intend to print; I experimented and was able to resize one of the 2+Mb pics down to a 800x600 size for viewing on the screen and it took the size down to 90K. I did not notice a discernable degradation in image either - of course my eye is not practiced. Still, I understand why you would not want to do this for photos to be printed. I think! lol

Right... For onscreen viewing, lower resolutions are just fine. However, if you try to print an image, without sufficient resolution, pixelation (where you can actually begin to see the individual pixels) will begin.

Be careful when modifying your images, too. Never overwrite the originals. Use the "Save As" option and give it a new filename if you make changes. That way, you can always start with the original image again, if the changes are not toyour liking (or you have something different you want to do with an image).

Be careful how much JPEG Compression is being applied to images, also. Too much compression will introduce undesired artifacts, and reduce detail. Most image editing packages let you set the JPEG Quality when you use their "Save As" function.

Also, it's a good idea to try and make all desiredchanges at once, before saving the image. This is because each time your resave a JPEG image, some degradation will occur.


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Old Jul 13, 2004, 8:47 PM   #5
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Is the Canon S9000 a "dye sub printer"?

Now for the Newbie Question

A pixel is a square, a dpi is a dot. Are they virtually the same?


Jim C, great post The best explaination I have seen


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Old Jul 13, 2004, 9:44 PM   #6
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obrien040362 wrote:
Quote:
Is the Canon S9000 a "dye sub printer"?

Now for the Newbie Question

A pixel is a square, a dpi is a dot. Are they virtually the same?
No, the Canon S9000 is an inkjet printer; and no, ppi and dpi are not really the same. You often see PPI incorrectly called DPI. Even one of the charts (from cordcamera.com) in my first post, incorrectly refers to PPI as DPI.

Actually, it can take many dots from an inkjet printer, to represent one pixel in an image.
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