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Old Nov 26, 2003, 9:39 PM   #11
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Guess I had some of that wrong. We'lre still worried about DPI since we often run large photos and need them to print well When you work for a broadsheet newspaper, this is an issue. Still, it's good to learn from you guys.
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Old Nov 26, 2003, 10:11 PM   #12
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Please forgive me, but I am still confused. As I read through the posts, some seem to be saying that there is a difference between pixels and DPI. However, JimC seem to equate them.

I am sorry for re-asking the question, but what IS the difference (or relationship) between pixels and dots? Are pixels the "dots" on the camera's image sensor and DPI the dots the printer makes on paper? If so, is there a standard conversion between them or is it related to pixel density as a function of the area of the camera's sensor? Do the pixels become dots when printed?

On a practical level, let's say one wanted to produce 300 DPI prints in 8x10 and 11x14 and 16x20 sizes, How many megapixels would a camera need to have in order to accomplish that resolution in each of those respective sizes?

Or to ask it another way, what is the largest 300 DPI print that a 5 megapixel camera can make? What about an 8 megapixel?

If a camera had 90,000 pixels, does that mean that the largest 300 DPI image it could make would be one square inch (300 x 300)?
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Old Nov 26, 2003, 11:53 PM   #13
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Quote:
Originally Posted by exagorazo
Please forgive me, but I am still confused. As I read through the posts, some seem to be saying that there is a difference between pixels and DPI. However, JimC seem to equate them.
No, there is a difference. Sometimes you see the terms used interchangeably, but they are different.

You want to look at the PPI (pixels per inch) being sent to the printer (based on the image resolution). However, you still need a printer that's good enough for quality output (that's why some of the newer inkjet printer models are rated at 1400, 2800, or even 4800 DPI (dots per inch) -- so that it can mix the colors easily with finer dots, to match the data in the individual pixels being sent to it.

However, a dye sub printer usually works much differently (that's why you see them rated at around 300 dots per inch). Instead of using multiple colors from ink tanks, they can get a single color in a single dot.

In the case of a most printers, the printer (or printer driver in software) takes the image, then converts it into dots per inch.

Quote:
Originally Posted by exagorazo
I am sorry for re-asking the question, but what IS the difference (or relationship) between pixels and dots? Are pixels the "dots" on the camera's image sensor and DPI the dots the printer makes on paper?
It's a little more complex, but basically, yes. That's a very good way to look at it.

Quote:
Originally Posted by exagorazo
If so, is there a standard conversion between them or is it related to pixel density as a function of the area of the camera's sensor? Do the pixels become dots when printed?
Depending on the printer type, one pixel may become many dots. However, you still need to make sure that you are sending the printer enough Pixels per Inch for Good Quality. Otherwise, you'll get what is known as "pixelation"

This is when you begin to see the individual pixels separate. Even if you are using a printer with 1400 dots per inch, it's still trying to represent the pixels per inch being sent to it accurately.

So, if the density of the dots (pixels per inch) is not high enough, the image will begin to show pixelation.

Now, with some modern printers and software drivers, the printer may try to interpolate the image (adding pixels that aren't really there). You'll see options in some printers to "optimize" the output with lower resolution images. Basically, they're interpolating the image. However, this will degrade quality, too.

Quote:
Originally Posted by exagorazo
On a practical level, let's say one wanted to produce 300 DPI prints in 8x10 and 11x14 and 16x20 sizes, How many megapixels would a camera need to have in order to accomplish that resolution in each of those respective sizes? Or to ask it another way, what is the largest 300 DPI print that a 5 megapixel camera can make?
See the charts in my previous post. You'll also see a simple way to compute it (I included an example). Here it is again: if you wanted the "perfect" 300dpi print , your 2560 x 1920 image would translate to an 8.53" x 6.4" print (2560 pixels/300 = 8.53, 1920 pixels/300 = 6.4).

I should have noted ppi versus dpi. But, this is also dependent on printer type. Basically, all you need to do to see the maximum size at 300 pixels per inch, is use the formula above (substituting the resolution of your camera for the 2560 x 1920 (typical 5MP image size) that I used in the example.

Or, just use the charts in the links in my previous post.

However, most users find that anything over 200 pixels per inch, is not noticeable to the human eye anyway. I've seen some studies on it (viewing distance versus the ability of the eye to discern the difference between 200ppi and 300ppi, but I'd have to dig around to find them again). Heck, I've seen pretty darn looking images on inkjet printers, with only 150 pixels per inch being sent to the printer driver.

Again, this can be dependent on printer type, too.

You may also find this recent thread interesting. See some more calculations here:

http://www.stevesforums.com/phpBB2/v...?p=76400#76400

Quote:
Originally Posted by exagorazo
If a camera had 90,000 pixels, does that mean that the largest 300 DPI image it could make would be one square inch (300 x 300)?
Right!
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Old Nov 28, 2003, 8:57 PM   #14
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bwilly,
I've been meaning to post an explanation of this resolution confusion for a while now. I've posted my 2-cents worth in a new thread for newbies..."resolution confussion". Hope it helps!
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Old Dec 3, 2003, 6:49 PM   #15
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Thanks everyone for all the replies. I wasn't expecting such a dramatic response, but this all helps.

Sounds to me like I need to find a better photo lab.

Thanks again
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Old Dec 4, 2003, 11:04 PM   #16
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Read this....it seems to debunk the 72DPI monitor res theory.

http://www.scantips.com/no72dpi.html
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Old Dec 5, 2003, 5:30 AM   #17
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I'm posted a new thread on this called "Resolution Confusion". I hope it's worthy!!
- Mike
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Old Dec 5, 2003, 9:10 AM   #18
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I never change the DPI of my photos unless I'm printing them on my own printer (which I rarely do, because it sucks). When I take them to a lab, they're automatically printing them at the highest DPI allowed by the size of the print and the number of pixels in the image.

By the way, my 15" LCD monitor is 12" across. 1024/12 = about 85 DPI.
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