You have a good question, Marc, on a subject that is fundamental to executing any and all all design concepts. However, I think you're focusing on the wrong end of the DPI elephant.
In this post, I will use DPI and PPI interchangeably. All linear units are for discussion purposes only.
Resolution, as stated by DPI is a device construct. The device could be a printer dealing with ink-on-paper or a flat panel monitor with glowing LEDs arranged in a matrix. A device needs to match the virtual components of a file with the real-world dimensions of the device. On the device side, resolutions expressed in dpi do not always transfer equally between devices.
iPad with Retina display
For example, Apple's original iPad display had 1024 pixels by 768 pixels. The Retina iPads display 2048 pixels by 1536 pixels, but the device
viewport is "still" 1024x768. "Normal" resolution artwork (132ppi) is doubled both horizontally and vertically, while text is rendered at the higher (264ppi) resolution. "Retina" resolution artwork is displayed at the native 264ppi.
My Big Laser Printer
Most of my design work is destined for paper, printed at 1200dpi on a $75,000 laser printer... but the printer's 1200dpi is dramatically different than Photoshop's dpi. The printer uses a 16-dot square (4 pixels on a side) composed of 16 dots each of cyan, magenta, yellow and black to express a single RGB Photoshop pixel. Even here there is no apples-to-apple comparison!
Trade Show Banners
I occasionally create vinyl banners for trade shows. The wide inkjet printers might have print resolutions higher than my big laser printer, but they only need 100dpi files to produce a good-looking banner.
In the end, Photoshop really doesn't care about devices, resolution or inches. Photoshop's engine is pixel-based. All the functions really compute pixels. Nothing else. The pixel-to-inch ratio only works when you use the file somewhere else, and usually only exposes a lack of understanding... when you do it right, almost no one sees the "good".
Back to my print design... let's assume I'm building a two-page spread measuring 17.25" wide by 11.25" tall. I need to supply the big laser printer's 1200dpi print engine with raster files at 300dpi. We'll discuss two files:
Raster File A
- I design a "synthetic" paper color and texture to print on white paper. The paper file in Photoshop is 17.25"*300dpi x 11.25"*300dpi, or 5,190 pixels x 3,375 pixels.
- When I save the finished background file, the file header contains the hard resolution in pixels, but only a "suggestion" for the dpi of the file.
- When I place the file in InDesign, it reads the header and sizes the file to 17.25"x11.25".
Raster File B
I shot a picture with my iPhone 5. The 8MP image's pixel dimensions are 3,264 x 2,448. Digital cameras default the "resolution" to 72dpi.
I drop this file into InDesign as a 45" x 32" photo.
Had I set the resolution to 300dpi in Photoshop, the file would have placed as a 10.8" x 8.2" image instead.
Regardless of the resolution I set in Photoshop, I can scale the file to whatever size I desire... let's say just about 5" x 4", or about 650dpi.
Answering your question
"Ok, Tom. I'll cry 'Uncle!' What's up with this dissertation?"
Correct me if I'm wrong, but your question is about perceived quality--right, Marc? Within reason, I can make a blend look good on any device. Perception is all in the output. My definition of a good blend is a smooth transition of color and does not display any visible banding.
Creating a good blend requires
- an image in RGB colorspace. Even with "one fewer color", RGB utilizes a larger gamut (total number of colors) than CMYK.
- a minimum of a 25% difference between the start and end values
- the blend is in at least one CMYK color channel (I'm not good enough to think through individual colors in RGB!)
- the blend covers approximately 300 linear pixels
- and can be assisted or "fudged" by introducing a little anarchy (3% to 5% Gaussian noise, with or without a 0.75 pixel to 2.25 pixel Gaussian blur in the blend layer)
There's a lot there. Following the guidelines produces a blend that exhibits no visible banding for about 300 pixels to 400 pixels.
If you are trying to blend a 35% black to "white" (we'll end at 10%K for a bit more "ummph!), you can extend the blend's length by varying the following...
- replace your original K35 with C10M10Y20K35 is "still" gray
- replace your original K10 with C3M3Y0K8 is "still" almost white
- You end up with more overall color data to calculate each step of the blend!
Adapting the recipe for a longer blend follows these rules...
- increasing the difference from the start color to the finish color
- increase the difference across multiple color channels in the start color tithe finish color
- increasing the Gaussian "fudge" factor
Making Photoshop Work Better
Regardless of resolution, file size or DPI, Photoshop works best when you throw more pixels at it. Anything you can do to a file that is 72 pixels on a side will look better when processed on comparable visual information that is 720 pixels on a side.
Photoshop is an extremely complex beast of a program. What starts out looking simple may have many intricate dependencies that can affect the outcome. Controlling theses interdependent variables is where the true magic happens.