dpi & pixels help

You are confusing two related but dissimilar items.
Pixels are screen dots (pixel=picture element).
Items measured in dpi are printer dots.
However, the raw picture is always the pixels, and you can have a dpi setting referenced in the data that is only used by the printing device as to what to use for best printing. In other words the dpi is not the actual data, only a reference point. You could have a 300x300 pixel picture that is about 4x4" on the screen (@75 pixels per inch nearly the typical 72 many screens are - not a hard number, though). With no dpi info in the picture the printer might print it 1x1" (with 300 dpi printer) or 0.25x0.25" (with a 1200 dpi printer) or anything else. But if a dpi number is recorded in with the printer then it will probably print at a size proportional to that setting. Meaning with a 75 dpi setting it should print 4x4", or 2x2" with a 150 dpi setting, etc.

You said you have an image needing to be 300dpi (evidently for printing purposes) and the size is 4x6 (inches, I assume). That means, if you want 1 dot to equal 1 pixel, the picture must have 1200x1800 dots of information.

But you said your picture is just 640x840 pixels... doesn't equate does it? (by the way maybe you really meant 640x480, or the typical VGA resolution?)

In any case you are going to have to expand your resolution by some multiplication from 2 to 3, which only causes distortion, and/or blockiness. And the ratio does not work out either, your picture is going to distort, UNLESS that is, if you really did mean 640x480 and you have the picture rotated 90 degrees.

Anyway, there are tools that can expand your picture to whatever dimensions you want, and/or they can also record the reference dpi number in it as to what the default dpi is.

My suggestion, is leave the picture alone, in size and just change the dpi info in the file. That way the picture does not expand or distort, and the dpi setting with be whatever you set it to.

You might be able to use your Photoshop to change the dpi value, I really don't have Photoshop to know its capabilities. I do know that the free IrfanView can do just that, I would not expect less from the well respected Photoshop.

What size is the original at 72 dpi in inches, not pixels?
As an example with a 4x6 inch photo file:Go to image/image size, with constrain and file size checked, you will see the sixe in width and height. If the size is in pixels, change to inches to view. Now change dpi to 300. You will see that the size of the image has changed to .096x1.44 inches. That is the maximun size you can print your image if your printer really is using 300dpi. or 150 line screen.
The normal newspaper prints at 85 line screen x2=170dpi the printer wants as the final size.
If a printer uses a 150 line screen they would want the file to be at 300dpi.
Meaning the original size at 72dpi would have had to be 16.666x25 inches in size. If you use a digital camera the image would be at this size or larger.
Bottom line: Shoot everything as max size and you will be fine. Forget about pixels and do all your measurements in inches and always think in inches. How tall are you in pixles? Who cares!!
I hope I was not too technical.

Kevin

Look, I am not going to debate the differences of what values you ought to set your images to in dpi (dots per inch), pixels (picture elements), screen lines, or anything else. The basic reference of all images is what the resolution of the picture is; and that is the number of picture elements there are, and that is pixels.

The more pixels, the more detail you can have in your picture.
For any one set size of printing, or viewing, how many pixels have you got? And that determines the coarseness (conversely, the fineness) of the picture printed or viewed.

I have one fairly nice, many colored, picture that is an American flag with an eagle over it, with some fine text along the bottom of the image. (I can send the pictures to you if you tell me where to send it to.) I have made six copies of it changing only the dpi in the image to six different values; 1 dpi, 72 dpi, 100 dpi, 144 dpi, 300 dpi and finally 1000 dpi. All six pictures are the same byte size (108 KB), since they are all the same resolution and color depth. All six pictures appear on the screen EXACTLY the same, because the screen doesn?t care about dpi. The screen shows pixels, and the pixels are 1024x768. The printing of the pictures MAY be identical too, it all depends upon your printing tool.

If your printing depends upon the dpi, then the pictures will print in six different sizes, from miniscule (at 1000 dpi) to gigantic (at 1 dpi). IrfanView can print following your desires ? to follow the dpi, to follow the printer dpi (using the pixels to printer dot conversion), or just follow your desired final output size (in inches or centimeters).

Chances are that the printing of your images will depend upon the size you stretch your image to in your document. It won?t matter what your dpi is. All that will matter is how many pixels you?ve got to spread over that size and is that going to be fine enough?

I am not going to try and throw in some arbitrary data about screen lines because that can really get confusing. There is no hard and fast rule that states 2 pixels = 1 screen line. That is only the theoretical limit you do have when attempting to use screen lines. I can take a 1000 pixel wide picture and print it with a screen of approximately 500 lpi (lines per inch ? very fine) or 200 lpi (fine), or 85 lpi (newspaper quality) or even 53 lpi (throw-away paper quality).

Screen lines are a method of breaking up an image so that some printing methods can print solids and grays. You only worry about screen lines if your printer or scanner has some solid transference limitations. Screen lines throw in a set pattern of interruptions in your raw picture. Screen lines are a method to get past limitations in these other mediums. You should not concern yourself with screen lines at this time.

Here is some good information, that I hope you read:

Scanning tip basics
http://www.scantips.com/basics1a.html

Debunking the 72-DPI myth
http://www.nicholsonprints.com/Articles/dpi.htm

DPI, Pixels and Printing
http://www.acdsystems.com/English/Community/ColumnsArticles/TechTips/tech-2002-04-13.htm