A very interesting question on resolution and prints

Started Jun 3, 2013 | Discussions thread
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Some basics on print versus display
In reply to rovingtim, Jun 3, 2013

I spent several years developing raster image processors for color laser printers, during which I was exposed to the basics of how printed material is perceived.

First off, get a handle on resolution. DPI is largely a marketing red herring. What you are really interested in is what's called a pixel on an electronic display, and is known as a pel, or printable element, on a printer. It's the smallest possible dot of any color.

So while a high end inkjet may boast 2400dpi or even 4800dpi (interpreted), that inkjet will use an 8x8 or some times a 16x16 matrix to form a single pel. Within that matrix, the printer mixes dots of the four primary colors to achieve the desired color. In truth, that 2400dpi printer is a 300 ppi printer, after halftoning. The halftoning process is an interesting one, a variety of formulas to mix the dots in patterns that won't create moire, or in the case of some inkjets, the pattern is purely random, a process known as stochastic halftoning. There are also contone printers - continuous tone - that don't halftone but create pels from translucent layers of melted plastic. These are dye sub, and lately melted wax printers. While their resolution may seem low at 300ppi, they're actually the same as a 2400dpi inkjet using an 8x8 halftone matrix.

The real difference between display and print? Emissive versus reflective.

Electronic displays are emissive - they create light of specific colors, and direct that light straight at the viewer. Something like the telecentric ZD lenses. For that reason, they are a great deal more precise than printed media, and their resolution requirements to achieve a given level of perceived detail is much lower. In the old days, that is why slides in a viewer or projected onto a glass bead screen looked so much better than prints, definitely why they appeared to have much higher dynamic range: slide viewers are also emissive devices.

Printed media is reflective - it reflects ambient light, and it reflects that light in no particular direction, typically the inverse of the direction the light is coming from, but the color media can affect that. Obviously, there is a great loss of precision when comparing print to electronic displays, plus the colors laid down must be calibrated against environmental factors (temperature, humidity) and the color of the substrate, which will also affect perceived color. Color calibration was one of my specialties - you apply calibration by varying the halftone screens to adjust the primary colors laid down.

The color scientists I worked with taught me that perceived quality of a print is related to two basic factors: resolution of the print, and distance of the viewer, plus there are some tricks that can be used to fool the eye into thinking it's seeing something it isn't. In the case of reflective media, resolution tends to decay more over distance as opposed to emissive media, because the transmission of the reflected light is far less precise.

At average viewing distance for an 8x10 print, there is a cutoff point of around 150ppi, above which increases in resolution aren't as noticable - the scattering effect of reflective media tends to lose the extra detail. Obviously, as you go to 11x17 or larger, the detail drops, but the distance between viewer and print increases, so the scattering effect tends to mask the loss of detail. For lower resolution images, the printer will 'dither' the missing pels, drawing color hints from adjacent pels, to fill in the blanks.

To get back to the original question... you can see the difference between a 5mp and 16mp image on an electronic display, because that is a very precise emissive device, that aims precisely created colors directly at the viewer - you don't get as much 'distance decay'.

You don't see the difference as much on printed media, because that is a reflective device. It's not as precise in the first place, the transmission of light is not as precise or as directional. You don't lose as much detail, because it is lost in the reflection process in the first place. And you do get a scattering effect the further away from the print that you are.

In case you were curious, that is why electronic displays use RGB, while printers use CMYK: the difference between emissive and reflective transmision calls for a different color model.

So, yes, it is quite true that you can print 8x10 out of 5MP, and like what you get. One of my favorite family shots, printed on my old Oly P400 dye sub printer, came out of a 3mp Oly P&S, and you'd be hard pressed to tell it from a 10mp print, at an average viewing distance.

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