Digital BW, The Print

There has been some discussion of this topic before herein but I guess
I failed to take real notice of it until the other day when I was
amazed to see a very rough edge to an angled black line on a print. 
This line appeared as a very smooth edge on my screen.  This effect
appeared on any high contrast edge that ISN'T parallel to or
perpendicular to the image edge – i.e. it slopes or is angled across
the image.  The line was actually the edge of some large text
(specifically the letter `A') that I'd added to an image in Photoshop.
 This was sufficiently bad to be visible in the print to the naked eye
– and once noticed, it was obvious.  It's not some micro effect that
only a loupe would reveal.  Once I'd noticed it in the letter A I
realised it was in all of the sloping parts of the text AND in the
image area wherever there was a black or very dark sloping edge
against a much lighter area.

I tried all sorts of things including checking the head alignment, and
eventually printing on two different printers.  I then realised it
must be an artefact of the conversion by the Epson Driver from the
image resolution (300ppi) to whatever resolution the printer uses. 
The thing that made me realise this was the fact that the exact same
problem was present when I made the same print from the same image
file sent to two different printers – a 2100 and a 4800.

For some images I've been using 300ppi as a standard for outputting
files to print.  But I remembered reading something about Epson
printers using multiples of 360ppi in the driver.  So I then tried the
same file resampled in Photoshop from 300ppi to 360ppi.  It totally
cured the rough jagged edge problem in the printed image.  I've since
confirmed by other tests that this was not simply as a result of the
resampling.  This problem seems to occur when you use some simple
fraction of 360ppi.  For example my 300ppi is 5/6 of the 360ppi, or
360ppi is 1/5 more than the 300ppi.  I haven't noticed this problem
when my image ppi is larger AND not a simple fraction of 360ppi.

I know that some people use 300ppi as it is often said that this is
the point at which pixels are no longer visible to the naked eye.  I
wonder whether your output to print is as sharp as it could be.  My
experience suggests that if you are printing from an image at 300ppi
using the Epson Driver, try resampling it to 360ppi in Photoshop for
printing existing images, and work at 360 ppi for future images.

I'm writing this up so that if there are others who've puzzled over
the cause of these jagged hard edges then at least there's a possible
explanation and cure available.  I somehow think that I can't be the
only printer who hasn't understood this and acted on it before!

I'm sure the maths used in resampling will explain this fully, but for
me avoiding it is what really matters.

Steve Gledhill
http://www.virtuallygrey.co.uk/