Bair Art Edition's Tutorial on:
Epson Professional Printing Workflow
Resolution for Optimal Printing Quality
Resolution is key to outputting a quality photographic image. By resolution we mean the image's base resolution, or the resolution it has before its resolution is artificially increased (interpolated). You can take any thumbnail and increase it's resolution, but after a certain point, you are just blowing up square pixels.
Optimal base resolution varies by the end print size. Smaller prints will be viewed from a closer distance, and therefore require greater resolution, larger prints can squeeze by with lower resolution, as they will most often be viewed from a greater distance.
So let's start talking about resolution in technical terms.
PPI vs. DPI
PPI (Pixels Per Inch, usually written in lowercase ppi) and dpi (dots per inch) are two similar concepts, but are still different.
Pixels per inch denotes how many pixels are in each inch of your image at the current printing size. This is one dimensional! If your image has 360 ppi, for every inch of your image, it is 360 pixels wide and one pixel tall.
Most of us like to think in terms of square inches (two dimensions). Resolution, however is usually given in one dimension. For a reference, 360 ppi can be understood in two dimensions as 360 x 360 pixels per square inch.
Dots per inch is how many droplets of ink are sprayed in a given one dimensional inch as well. Often dpi is given in that one dimensional notation (1440 dpi) although it's horizontal and vertical resolution are not always equal like they are in ppi. For example, 1440 dpi is usually 1440 x 720 dpi. This means that in a square inch, it is 1440 dots wide and 720 dots tall.
In high quality photographic printing, dpi is always going to be a larger number than ppi.
Why? - If you think about it, each pixel has it's own color. That color is defined by a number (actually three numbers of Red Green and Blue). The computer interprets the number and displays it as a single pixel with that color. Pretty straight forward.
With an inkjet printer you can't just squirt one drop of ink out, made of the specific color of that pixel. Most of the pixels' colors must be represented by many ink droplets of various sizes and colors. For example, to get pure green, it takes cyan and yellow (two droplets) whereas there was only one pixel needed for that color. As you get into various tones and shades of green, the amount and sizes of droplets increases.
The graphic on the upper-right (by Epson) shows how many dots are laid down for full color. You can see red, green, and blue show up as a result of ink droplets laid on top of each other. Red is a product of yellow and magenta, and blue comes from cyan and magenta.
Optimal Base Resolution (ppi)
Again, by base resolution we mean the pixels per inch that exist in your digital file without any *interpolation. To find your base resolution at the size you want to print at, open Photoshop's Image Size... window located under the Image menu.
In computer terms this generically is any "guessing" the computer does to create extra pixels or color information that wasn't there to begin with by looking at the existing pixels and color schematics as a basis for filling in the blanks.
Do not select "Resample Image" in this window, at least yet. If you do select it, it won't tell you how much resolution you have in your image. Instead it will give you whatever resolution you tell it to have.
Here is an example of what resolution I have with a 6 MP camera at 8 x 10 inches (I have cropped 20% of the image in order to have the standard 4x5 "professional" aspect ratio):
As you can see, I have 252.75 pixels in every inch of that image. Each square inch has 252.75 x 252.75 pixels in it, or about 64,000 pixels per square inch. Although square inches make more sense for some of us, we stick to one dimensional resolution as an industry standard.
252 ppi is pretty good. It could be better, but it isn't bad by any standard.
Here is a table of the minimum resolutions for common print sizes. Again, these are (subjective) minimum acceptable base resolutions, higher resolutions are always better. The maximum ppi you'll ever need is 360.
4 x 5"
5 x 7"
8 x 10"
11 x 14"
16 x 20"
20 x 24" and up
Optimal Output Resolution (dpi)
This discussion should really go with the printer settings module, but I put it here, because it also goes with the concept discussed above, ppi.
Above, we discussed the resolution you should use in terms of ppi (pixels per inch), which is in relation to the digital file, now let's determine your optimal dpi (dots per inch), which is in relation to printing the file on an inkjet printer.
PPI was limited by the camera or scanner you have, but dpi is limited by the printer you own. This tutorial assumes that you have a printer capable of 2880 dpi (actually 2880 x 1440 dpi) printing resolution. That is very high, regardless of what HP claims they can do! You rarely will need to print at a resolution higher than 1440 dpi (actually 1440 x 720 dpi).
Let's just go through some standards, and you decide what is right for you. First, let's look at viewing distance:
3 ft. and closer
6 ft. and closer
Farther than 6 ft.
1440 or 2880
Those are by no means set in stone, but it gives you an idea of what to shoot for. 2880 is overrated, and completely unnecessary unless you have "precious" (to you at least) shadow details, or customers toting loupes. Besides, it takes twice as long as 1440 dpi and requires more ink.
If you want to get ideas on print speeds, visit that module.
Next we'll look at subject matter. This one is more subjective (get the pun?), but it is important. If you are reproducing a photograph in which you want amazing details, and you have a base resolution of around 300 ppi, then you might go with 2880 dpi. Although 1440 dpi is probably still be acceptable. The issue of base resolution, and the sharpness of your image (are you using a good lens? It makes a huge difference) are very important - what's the use of upping the printing resolution, if you lack the base resolution?
If you are reproducing artwork, then 1440 dpi is always adequate. We've found that oil paintings on canvas, even those with great detail, still look great even at 720 dpi. One reason is that it is painted, where finer detail is harder to come by, and the other is that the image is being reproduced on canvas. Canvas "hides" poor resolution to a degree (even on photographs), this is partially due to the weave of the fabric and also because of what we have come to expect from canvas images.
If you want to make a poster (not a "fine art poster," but something like a retail store poster) or graphic for display in a courtroom, etc. and it won't be scrutinized closely, then 720 or even 360 dpi will do just fine.
In the end you'll have to mess around to see what suits you best.
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