What is PPI and DPI and how do they affect printing?

...While high fidelity professional inkjet printers can render a DPI (Dots per Inch) of 1440 to 2880, this is not how a file should be prepared. Let's take an example. You have a file that measures 20X30" at 150 PPI. If you want to print on a fine art paper, this is all is needed. The confusion comes as of why it is sufficient to have a file at 150 PPI when the printer is imaging at 1440 DPI. The answer is that PPI and DPI are two very different measurements.

...PPI refers to the number of pixels per inch in a file. It is an electronic figure, purely digital. PPI is important because it affects print quality and even the way an image displays on a screen. While about 72 PPI is all is needed for web images, it is generally too low for printing. The result would be jagged edges and pixels showing in the print. Giclee printing in particular is demanding because professional grade nozzles are not very forgiving of low resolution files. Generally speaking, textured materials such as canvas or watercolor paper will print well at a lower resolution. Smooth, photo type papers will need a higher PPI.

...DPI on the other hand refers to the printer resolution. One pixel does not equal to one dot. One dot represents one color while one pixel will render 4 or more. DPI is an important measure of quality for printers but not the only factor. There are expensive machines that only have a 300 DPI capability but produce extremely high end because of the screening technology employed.

...Interpolation of a file to boost PPI before printing can help but must be done with care. Generally an interpolation factor of about 200% will create a good print. Be aware not all files size up with the same quality. Digital photographs are easier to enlarge because there are no dots to start with. A film scan from a 35mm negative will not be as easy to interpolate because of the inherent film grain. A well-exposed image with good values is also a better candidate than a poorly prepared file.

..."Garbage in, garbage out" is true. Start with a good resolution file, proper highlights and shadows, and you will be able to produce outstanding prints. A poorly prepared image can’t be salvaged to create an equally beautiful reproduction

Image Interpolation

.It is a fact of life that many digital cameras do not produce enough native (true) resolution to print large format giclee.

...It is possible to increase the file size of an image by interpolating. Interpolation can be done in many imaging programs and it increase the file size by adding pixels to make up for the lack of resolution.

...However, In terms of quality, all pixels from various digital cameras are not created equal.

...The way interpolation works is by averaging adjacent pixels in the file and "guessing" what values to put in. By doing so, the image will soften a bit and detail is not really gained. What is gained though is the chance to reduce or eliminate pixelation (providing the interpolation is performed correctly and within limits). Photoshop has 3 ways to interpolate: Nearest neighbor, Bilinear and Bicubic. For most purposes the latter two are fine. Nearest neighbor is better used for line work.

...A picture taken with a pro level camera will interpolate better that one taken with a point and shoot camera even if the two have the same resolution. This is because a high quality file can hold better when inflated. It is a good idea to do an unsharp mask after interpolating to reduce the softening.

...We have seen 5MP cameras that produce good large format prints. This is because the photos were taken with quality cameras and skill.

...We do not generally recommend to increase file size more than 200%. It is unrealistic to expect to see a 2 or 3MP producing an acceptable 30X40 giclee.

...Some images will also interpolate better than others. A very sharp image (not a sharpened digital file) with a good histogram without any clipping in highlights and shadows is a good candidate.

...If you have doubts about the quality of your file, please have us take a look at it. At no cost to you, we will analyze it and provide advice.

What is the suggested DPI for quality print reproduction?

We suggest a minimum of 150 PPI for all papers and if possible 200 for our Photographic papers.