[Leica] Home photo printers and the digital darkroom

["Anthony Atkielski" <anthony@atkielski.com>]

From: Robert Jones <drrjones@flashcom.net>
Sent: Tuesday, October 05, 1999 22:48
Subject: [Leica] Re: Leica Users digest V12 #41


> Optimum output from it is obtained when you set the dpi
> of your image to an integral divisor of the printer's
> output (1440); going above 480 doesn't seem to help, so
> I typically use 360 for larger prints and 480 for smaller
> ones.

That's because of the difference between "machine dots" and "halftone dots."
Halftone dots are dots of color of variable size that provide the different
shades of color in the photograph.  Machine dots are smaller dots of fixed size
that are combined to produce the halftone dots.  The smaller the machine dots
are, the smaller the halftone dots can be, but machine dots are _not_ the same
as halftone dots, and the DPI figure quoted for a printer is typically the
machine-dot size, not the halftone-dot size.

For example, if you want 256 levels of gray in a photograph, you need halftone
dots that can be of any one of 256 different sizes.  For that, you need at least
256 machine dots per halftone dot.  Now, since halftone dots are two dimension,
you'll need an area (a box, say) equal in linear dimensions to the square root
of the number of dots you require.  In other words, to print 256 different sizes
of halftone dots, you need a box that is 16 machine dots on a side (16x16=256
dots).

Given the above, the real resolution of a printer is equal to the number of
halftone dots per inch (the "screen frequency"), not the number of machine dots
per inch.  For halftone dots of 16 machine dots on a side, with 1440 dpi of
machine-dot resolution, the actual printing resolution is 90 halftone dots per
inch or a screen frequency of 90 lines per inch (to put it into conventional
printers' terms).

This being so, you don't need more than about 150 pixels per inch in your photos
to use the maximum resolution of your printer, since the typical resolution used
for printing is equal to the screen frequency of the printer plus 50% or so.

> As far as enlargements then go, you can back calculate the
> required scan: for a 10x enlargement (8x10) from a 35mm frame when printing
> at 360 dpi, you should scan at around 3600 dpi (for the longer side).

The proper resolution for a print depends on the size of the print and the
viewing distance.  Under ideal conditions, with perfect vision, a human being
can resolve details of about 30 seconds of arc in size.  For an 8x10-inch
enlargement, you'll need 380 pixels per inch to achieve this, or 3820x2546 for
the entire print.  You can achieve this with a 2700 dpi scan of the 35-mm
negative.  A scan of 3600 dpi won't hurt (it gives you a bit of headroom), but
it isn't actually necessary.

> This will minimize pixelation and give a clarity that resolves around
> 6 - 8 lpm as does silver based prints.

About 10 lp/mm, to be precise--which is far more than anyone will actually be
able to see at any normal viewing distance.

There is a tendency among photographers to dramatically overestimate the
resolution required for negatives and prints, as compared with the real-world
capacity of human vision under normal viewing conditions.

  -- Anthony