Optimal file resolution for Epson printers
Optimal file resolution for Epson printers
- Subject: Optimal file resolution for Epson printers
- From: Dave Creek <email@hidden>
- Date: Sun, 16 May 2004 10:31:06 -0700
Here's an article by Arthur Bleich that makes mention of file
resolution for Epson printers. While the article is a bit dated, it
matches what I've been told by Epson tech support.
The resolution trio
By Arthur H. Bleich
(8/21/01)
Camera resolution is a huge part of the decision-making process when
you're buying this equipment. Now, prepare to change how you measure
that resolution. Starting next month, the Japan Camera Industry
Association (JCIA) is being a little more honest with what cameras
can do. JCIA members, including most of the leading digital-camera
manufacturers, will advertise only the number of "effective" pixels
that their digicams capture. Some 10 to 20 percent of the total
number of pixels on a CCD perform color calibration and image
processing functions and will, therefore, be excluded from the final
count. Although the lower numbers may make the average digital camera
appear slightly less powerful on the store shelf, ultimately, you'll
get a more accurate gauge of what you get for your money. But none of
this megapixel discussion adds up to much unless you have a solid
grasp of the realities of image resolution.
Yes, it's true that higher-resolution digicams have the pixel power
to produce bigger, better-looking images. But once you purchase such
a camera, understanding the concepts of image resolution and printer
resolution become equally important; in the end, they determine the
quality of your prints.
Image resolution: ppi
Unlike camera resolution, which is a static number that will soon
count the number of effective pixels on a CCD, image resolution is
measured in pixels per inch (or ppi) and changes with the size of
your photo. As an example, imagine that a digicam with a camera
resolution of 1.8 megapixels prints a 4x5-inch image on a piece of
rubber. If you stretch the rubber to get an 8x10-inch picture, each
of those 1.8 million pixels becomes bigger. As you keep increasing
the size of the photo, the pixel size also increases proportionately.
You'll soon be able to see each separate pixel, and the image will
begin to look jagged and blocky.
To extend the example, let's arbitrarily assume that at 4x5 inches,
300 pixels are lined up along each inch of a photographic print,
giving us a total of 1,200 pixels per line on the 4-inch side and
1,500 pixels on the 5-inch side. But when you stretch the image,
doubling its size to 8x10 inches, the pixels get bigger, so now only
150 of them fit in an inch of space. Nevertheless, there are still
the same 1,200 pixels as before on the 8-inch side and 1,500 pixels
on the 10-inch side.
So while the number of camera-resolution pixels in our example stays
the same (1,200 multiplied by 1,500 give you 1.8 megapixels), the
number of image-resolution pixels per inch changes with the picture
size. Here's the bottom line: smaller-sized prints of a photo have
smaller-sized pixels and hold more of them in every inch (a higher
ppi count), while larger-sized prints have larger-sized pixels and
display fewer of them to the inch (lower ppi). And that's all there
is to image resolution.
Printer resolution: dpi
An inkjet printer reproduces an image's square pixels by squirting
round dots of ink at the paper--the number of dots laid down along an
inch of paper is called the printer resolution and is measured in
dots per inch (dpi). Most quality photo printers have different dpi
resolution settings to allow optimal output when using different
types of paper and ink. Settings at and exceeding 720dpi allow pixels
to be defined sharply and have their color accurately reproduced. Ink
dots at higher resolutions blend together a bit more seamlessly, so
you won't see them as separate specks.
But using a high dpi setting on your inkjet cannot improve the basic
quality of your image; only more pixels in the image can do that.
Remember our stretchy, rubber photo? If you try to enlarge a picture
too much, the pixels get bigger and bigger and start to become
noticeable; a high dpi setting on your printer may just make each
pixel more pronounced, so you'll see them at their worst. You must
have enough pixels in your image to begin with, and the pixels must
be close enough together so that they can't be seen individually.
Settings for the best results
How close should pixels be? Epson representatives say you'll get the
best print quality if you have an image-resolution value of 240ppi
when your printer resolution is set to 720dpi; 300ppi is recommended
for 1,440dpi or 2,880dpi settings. Higher ppi values are not
necessarily better, though; if the pixels are too close together,
unsharp images can result. However, you can often print at lower ppi
values than those recommended above and your photos will look fine;
let your eyes be your guide. If your camera doesn't have high enough
resolution to produce lots of pixels and you want to print with more
of them per inch, you'll usually just have to settle for smaller
photos.
And why is this? Because, as we've seen before, the picture size and
the number of pixels per inch are related: If you input a photo size
of 8x10 inches and your program tells you that it has 150 pixels per
inch, you can't just increase that quantity to, say, 300ppi in an
image editor. But if you enter that higher ppi number, the dimensions
of your picture will automatically move down to a smaller size.
That's the program's way of saying there just aren't enough total
pixels available for them to line up, shoulder-to-shoulder, at 300
per inch to form the larger image. Remember: With each change of
picture size, the number of pixels per inch will be automatically
recalculated by your imaging program. And if you enter the ppi value
of your choice, your photo will be automatically sized to print with
that number of pixels per inch.
Digital alchemy
But there is a way to get both your desired image size and a higher
ppi value. You'll have to use a bit of digital alchemy here, called
interpolation, to tell your imaging program to add more pixels to an
image (within reason) till the ppi number gets high enough to produce
a smooth-looking print. Simply unlock the picture size and ppi
relationship (this is typically done by checking a box called
Resample Image in your image editing program), then type in the image
size and the ppi values you want. Interpolation won't give you as
good a picture as you'd have if all the pixels were there to begin
with (because the program spins them out of thin air and makes an
educated guess as to what colors they should be), but with a little
image sharpening after they've been added, the results are usually
pleasing.
The concept of the differences in camera, image, and printer
resolutions is not an easy one to grasp; part of that is due to the
confusion generated by manufacturers (and even professionals) who use
the wrong terms. But once you understand how this resolution trio
performs, you'll have complete control over the quality of your
prints.
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