Printing: PPI and DPI

[BryanC]

Printing: PPI and DPI


PPI and DPI are two different things. Sometimes, there's confusion because the term 'DPI' is often used when it's 'PPI' that is meant.

PPI, or 'pixels per inch' is the resolution of a digital image. DPI, or 'dots per inch' is a printer's resolution.


PPI

As we know, digital cameras come with different sensors rated at MP, or megapixels. A megapixel = 1,000,000 pixels.

A 10MP camera will capture an image at approximately 3872 pixels x 2592 pixels. (3872 x 2592 = 10,036,224 divided by 1,000,000 = 10.03MP)

Other Examples:

4MP = 2464 pixels x 1632 pixels

6MP = 3008 pixels x 2000 pixels

8MP = 3264 pixels x 2448 pixels

12MP = 4290 pixels x 2800 pixels

16MP = 4920 pixels x 3264 pixels

Using the same formula as above, (pixels x pixels divided by 1,000,000) equals the approximate megapixel of the sensor in the camera.

These numbers cannot be changed for the better, as in, you can't turn a 6MP image into a 12MP image. You cannot 'add' pixels to an image, only take away (as in cropping).

PPI or pixels per inch is the resolution of the image, whether it's at 50PPI, 300PPI, or whatever. It's still the same total amount of pixels in relation to your sensor size. What changes is the image size in dimensions. A 10MP image at 50DPI will be larger than a 10MP image at 300PPI. Same amount of pixels in each image, but the pixels will be larger in the 50PPI image than the 300PPI image.
Larger pixels mean more easily seen pixels, which means lower resolution. Smaller pixels mean not so easily seen, which means higher resolution.

Here's a chart showing the relationships between pixel resolution and print sizes.

MP's----Pixel Resolution----PrintSize@300ppi----PrintSize@200ppi----PrintSize@150ppi

--3----------2048x1536---------------6.82" x 5.12"------------10.24" x 7.68"------------13.65" x 10.24"
--4----------2464x1632---------------8.21" x 5.44"------------12.32" x 8.16"------------16.42" x 10.88"
--6----------3008x2000--------------10.02" x 6.67"-----------15.04" x 10.00"-----------20.05" x 13.34"
--8----------3264x2448--------------10.88" x 8.16"-----------16.32" x 12.24"-----------21.76" x 16.32"
--10--------3872x2592--------------12.91" x 8.64"------------19.36" x 12.96"----------25.81" x 17.28"
--12--------4290x2800--------------14.30" x 9.34"------------21.45" x 14.00"----------28.60" x 18.67"
--16--------4920x3264--------------16.40" x 10.88"----------24.60" x 16.32"----------32.80" x 21.76"


This chart is more of a rule of thumb because other variables can come into play such as sensor quality and viewing distance. A lower sensor quality can decrease the quality of these sizes. On the other hand, as the viewing distance increases, so can the size of the print. Again, this chart is more rule of thumb. It approximates 'photo quality' images at a few inches of viewing distance...(not being able to see individual pixels with the human eye). More often, print sizes can be increased with good results.

DPI

As for DPI, or dots per inch, it is the printer resolution. It doesn't change pixel count. It is the resolution that a printer prints at. The lower the number, the lower the resolution of the printing. The higher the number, the higher the resolution. If you print a 10MP / 300PPI image at 100DPI and the same 10MP / 300PPI image at 300DPI, the 300DPI will be at a higher printer resolution. Same image, better quality, more dots per inch.

[dcclark]

Good article, and it's good to know the technical distinction between these.

That said, the question which most people ask about, is what PPI can I print at -- and essentially every site which deals with printing photos calls this DPI. It's a misuse of terminology, yes, but it's important to understand what they're asking for. Functionally, we need to make sure that people understand what these sites mean and how to interpret this -- because, after all, "DPI" in Photoshop and "DPI" on Adoramapix are two different things, and Adoramapix's meaning is the one most people ask about.