Understanding Photoshop: Resolution, PPI, Printing Resolution and DPI

Pixels

To understand Photoshop, you first have to understand pixels and resolution. This is because Photoshop is a program that is based heavily on pixels and resolution. Another term for it is a “raster image” program. “Raster” is just a fancy scientific term that basically means the images are made up of many different squares, usually multiple millions of them.

These individual squares are called “pixels,” which is a hybrid word formed from “picture elements.” These tiny pixels are an orderly way for a computer to break the image up into a format it can understand, manipulate, display, and duplicate. It builds the image like a mason would build a brick wall, except that these picture elements are in a perfect grid, whereas a wall has staggered rows. (There are some LCD screens on cameras that do stagger rows like brick rows, incidentally, but that is neither here nor there.)

You can contrast that with film. Film (isn’t that the stuff that you get on your teeth when you don’t brush?), is made of film “grains” that are arranged more like a mosaic tile array than bricks. Some photographers like what they refer to the “pleasing” look of grain. In the end, most photographers would rather not see the grain or pixels, and that can be achieved if there is enough of either.



Resolution

That is where the resolution comes into play. Resolution is basically a measurement of density. If the photograph has at enough pixels in any given square inch, then it is increasingly difficult to see the pixels. Resolution is measured (for us Americans) in the simple formula:

Resolution = Pixels / Linear Inch

PPI

The term is expressed most commonly as Pixels Per Inch (PPI). I added the “Linear” bit to point out that while resolution is an expression of pixel density, it is expressed in a one dimensional format, and photographs are (at this point) 2D.

So any expression of resolution is giving the number of pixels per inch in any direction along a one-pixel path. It therefore goes without saying that an image with 100 ppi has a true density (measures in an actual area) of 10,000 pixels per square inch.

But this portion of the discussion has been mainly theoretical, since the industry (and Photoshop) talk in PPI not PPSI (per square inch). However, it does explain how images so quickly get to millions of pixels. Getting a little ahead of ourselves, I’ll tell you that (depending on the printer you use), maximum (optimal) PPI is between 300 and 360 ppi. That means that (we’ll use 300 ppi) if you have a 4x6 print at optimal resolution, you started with (4 in * 300 p/in) * (6 in * 300 p/in) = 1200 * 1800 pixels = 2,160,000 pixels.

Apply the prefix “mega” (for million) to the number and you get 2.16 mega pixels. To get an optimal 4x6 image, you therefore need around 2 million pixels. Now, as you’ll learn, there is “optimal” and there is “as decent as you would ever need, and as good as a normal person would expect.” At 150 – 180 ppi you get decent prints that no normal person (which means I am excluding photographers) would ever see as “pixilated,” “low resolution,” or “blurry.” By about 80-120 ppi, a “lay” person is definitely starting to see the image as “grainy,” “digital-looking,” or “fuzzy.”

This is where the resolution understanding is vital.



Starting Resolution

If you are wanting to edit a design with an image as a background, and the design will be 8x10 inches when you are done with it, you must start out with both an image that has enough resolution to get by as an 8x10, as well as a file (to design in, sometimes called a “canvas”) that has enough pixels per inch.

Here is a non-scientific chart of resolution:

Final Print Size 4x6 – 8x10

72 ppi - Screen Resolution
100 ppi - Fuzzy, but workable
150 ppi - Decent for all prints
240 ppi - Good for all prints
300+ ppi - Optimal for any output


Final Print Size 11x14 – 24x36

72 ppi - Fuzzy, and not too workable, but getting there
100 ppi - Decent for most prints
150 ppi - Good for most prints
240 ppi - Excellent
300+ ppi - Optimal for any output

Explanation: The distinction between the two categories of print sizes is because enlargements at around 11x14 and larger are not viewed as critically as smaller photos in terms of viewing distance.

Let’s say you have a 5x7 photo with 150 ppi and a face on that photo is one inch squared. You have 22,500 picture elements (150 squared) making up that face. It may sound like a lot, but it is a decent amount.

An 11x14 print is (going by width) is 2.2 times as large. So that print has the same face covering 2.2 inches squared. If the photo is 100 ppi, then you have 10,000 picture elements per square inch. Multiply that by 2.2 square inches, and you get 22,000 pixels. That is almost the exact same pixels making up the same face.

Now the face has (for all intents and purposes) the same number of dots and therefore detail. The detail has just been spread over a larger area. If you look closely at the two, the 5x7 will appear to have more detail overall. This is because the 5x7 does still have more pixels in any given inch.

So as far as pure density and detail per square inch is concerned, the 5x7 wins (after all it had 150 ppi to the 11x14’s 100 ppi. However, they are almost at a tie in the pixels per area of interest. The face is an area of interest that is non-scientific to one degree. We are programmed as humans to see faces, recognize them, and look critically at them.

So in both cases the area of interest is equally detailed, and now we look at the other factor that makes these two prints more equal in one aspect.

That factor is viewing distance. The 11x14 can be appreciated from a greater distance than the 5x7 can. If the subject matter is a small family portrait of 4 people, you have to look much closer at the 5x7 to recognize the people than you do at a large 11x14 on the wall.

Basically this means that a viewer will come closer to a 5x7 to view it than an 11x14. And we discussed that the 11x14 is less clear when viewed as closely as a 5x7. Therefore, an 11x14 can (mostly) get away with 2/3rds the resolution (ppi) as a 5x7.

PPI vs DPI

Now if you are new to Photoshop then I am sure much of this discussion will take time to sink in. If you are not new, then you might be confused at this point concerning DPI vs. PPI. You have probably heard of DPI in the wrong context and usage.

Here is a simple way to see it:

PPI (pixels per inch) = Cameras, Software, Computer Files, any electronic version of the image

DPI (Dots Per Inch) = Prints, basically any way of putting the image to paper.

So next time someone talks of a digital image in DPI, they are wrong unless they are discussing a digital image that is now on paper. DPI is NOT therefore interchangeable with PPI (as so many people wrongly think it is). DPI is a much higher number to get the same quality. Think of an inkjet printer. How many inks does it have? The answer is between 4 and 12! For a standard 4-ink printer, you are using as little as one dot (ink droplet) to replicate a pixel, and as many as 4 (and nowadays the droplets vary in size if you need one color to be stronger than another).

So the printer has a potential of printing 4 drops of ink for every picture element it is trying to duplicate onto paper. If the printer has a native input of 300 ppi (300 ppi is the maximum, for every pixel there is a group of dots made, any more ppi and the extra pixels are "thrown away" becasue the printer converts the image down to 300 ppi), then the DPI (potential) output of that printer is 300 pixels/ inch * 4 dots/ pixel = 1200 dots/ inch.

For a 4-color printer, a 1200 dpi resolution output will (theoretically) reproduce a 300 ppi image at optimal pixel --> dot coversion.

And you thought the 5600 dpi Epson printer was AMAZING resolution? No, it just has 8 inks at any given time, and sometimes the variable dot pattern will have a single ink color put multiple dots down for that color reproducing one pixel. In the end it is getting better color tonality (more colors and dots sizes), but it is still just reproducing at around 300 pixels per inch (your eye can’t see much more detail than that anyway).