Hi all,
Back in Wed, 15 November 2006, Ali Shah gave us a long extract from two
pages of the http://www.design215.com website.
Shortly I'll repeat them below.
At the time that Ali sent us these I did not have the free time to look into
them, but I extracted his posting from the Digest and printed it out for future
reference, and that took place last night and now..
When Ali sent us these, his posting was quickly knocked sideways with
some forceful responses that cast doubt on the assertions in the pages and
seemed to kill off a lot of discussion. I thought and still think the matter
needs to be revisited.
I also recalled pages from my local photo lab that I had previously seen, so I
saved these, reformatted the pages so they would print out and put each
image entirely on a sheet of A4 paper, and printed them off. These pages
from Photo and Video (Merivale) seemed to reinforce the thesis of the
design215 people, and that strengthened my wish to raise this important
matter again..
A lot seems to depend on what you consider to be "adequate photo quality".
It seems to me that these pages accurately depict what ppi and dpi
represent and how digital images should be handled to obtain desired quality
in the chosen output medium.
So, with that long introduction, I repeat Ali's posting and invite discussion
again.
Brian
I begin with links to the Photo and Video pages as these involve only three
lines.
http://www.photo.co.nz/faq/print_optimise.htm
http://www.photo.co.nz/faq/screen_optimise.htm
http://www.photo.co.nz/webprints/webprints_intro.htm
Now, Ali's posting with the URL corrected to make it usable
Date: Wed, 15 Nov 2006 06:59:36 -0800 (PST)
From: Ali Shah <alizookoman@xxxxxxxxx>
Subject: [OM] Megapixels and printing photos
Some of this is above my head but I figured it is good info to pass along. You
probably cant see the chart, however if you click on the URL you should be
able to see it.
http://www.design215.com/toolbox/ megapixels. php
http://www.design215.com/toolbox/ print_guide. php
Megapixels Chart
v1.0, 2005.12.26
Each colored box represents a certain number of megapixels. The numbers
along the top and left side are print dimensions in inches at 300ppi (pixels per
inch). Most books and magazines require 300ppi for photo quality. For
example, the chart shows that you can make a 5" x 7" photo quality print
from a 3 megapixel camera.
inches @ 300ppi (numbers inside colored boxes are megapixels)
Notice that as the print size doubles, the megapixels required increases
geometrically. You can make nice 8" x 10" prints with a 6 or 8 megapixel
camera, but to make a true photo quality 16" x 20" print, you need
between 24 and 30 megapixels. Don't be fooled by manufacturers' claims
that say you can make 16" x 20" prints from an 8 megapixel camera. While
you certainly can make a print that size, it will not be true photo quality.
Here's why:
A megapixel is 1 million pixels. It's an area measurement like square feet.
A typical 8 megapixel camera produces images that are 3266 x 2450* pixels.
If you multiply 3266 by 2450, you get 8,001,700 or 8 million pixels.
To find the largest photo quality image you can print, simply divide each
dimension by 300:
3266 / 300 = 10.89 inches
2450 / 300 = 8.17 inches
If you are not publishing your images in a book or magazine, and you're just
making prints for yourself or your friends, you can "cheat". Good quality inkjet
printers can make a nice looking print at 250 or 200ppi. At 200ppi, the
maximum print size becomes:
3266 / 200 = 16.33 inches
2450 / 200 = 12.25 inches
If you know how to use image editing software like Photoshop, you can
"cheat" even more by increasing the image size, and even doubling the
number of pixels in the image. The quality of the camera and lense becomes
more important at this point bacause any loss of detail or sharpness is
magnified. If an image is enlarged too much in this manner, it will look
"fuzzy" or "pixelated".
Megapixels vs. Maximum Print Size Chart
Megapixels Pixel Resolution* Print Size @ 300ppi Print size @ 200ppi Print
size @ 150ppi**
3 2048 x 1536 6.82" x 5.12" 10.24" x 7.68" 13.65" x 10.24"
4 2464 x 1632 8.21" x 5.44" 12.32" x 8.16" 16.42" x 10.88"
6 3008 x 2000 10.02" x 6.67" 15.04" x 10.00" 20.05" x 13.34"
8 3264 x 2448 10.88" x 8.16" 16.32" x 12.24" 21.76" x 16.32"
10 3872 x 2592 12.91" x 8.64" 19.36" x 12.96" 25.81" x 17.28"
12 4290 x 2800 14.30" x 9.34" 21.45" x 14.00" 28.60" x 18.67"
16 4920 x 3264 16.40" x 10.88" 24.60" x 16.32" 32.80" x 21.76"
35mm film, scanned 5380 x 3620 17.93" x 12.06" 26.90" x 18.10" 35.87" x
24.13"
*Typical Resolution. Actual pixel dimensions vary from camera to camera.
**At 150ppi, printed images will have visible pixels and details will look
"fuzzy".
For an explanation of "pixels per inch" vs. "dots per inch" and why you need
300ppi for true photo quality, see our Printing Guide.
************ ********* ********* ********* ********
Photography Printing Guide
v1.0, 2005.12.27
dpi is NOT THE SAME as ppi !!
Even though "dots per inch" (dpi) and "pixels per inch" (ppi) are used
interchangeably by many, they are not the same thing. Traditional printing
methods use patterns of dots to render photographic images on a printed
page. While pixels on a monitor are square and in contact with the adjacent
pixels, printed dots have space between them to make white, or no space
between them to make black. Color photographs are printed using four inks,
Cyan, Magenta, Yellow, and Black (CMYK), and four separate dot patterns,
one for each ink. Dots per inch (dpi) refers to printed dots and the space
between them, while pixels per inch (ppi) refers to the square pixels in a
digital image. Keep in mind that many companies will ask for images at
300dpi when they really mean 300ppi.
Why do I need 300ppi for a "photo quality" image?
First, some background information is necessary. A digital image is what it
is. It is however many pixels wide by however many pixels tall. If you divide
each dimension by 300, you will have the size of the image at 300ppi. Now
think about 300 pixels in an inch of space. Each pixel could be black, white,
or any other color, but they are all next to each other with no spaces
between them. When a digital image is prepared for reproduction on a
printing press, pixels are converted to dots. Dots have spaces between
them. 300 pixels become 150 dots and spaces, so 300ppi becomes
roughly 150dpi. 150dpi is the accepted standard for printing photographic
quality images.
Printers usually refer to the number of rows or lines per inch (LPI). 150 lines
per inch is simply 150 rows of 150 dots per inch. 150 LPI and 133 LPI have
long been the established standards for the best quality reproduction of
photographs in books and magazines. Newspapers traditionally use 85 LPI
for photographs and detail is lost because the dots are plainly visible.
Viewing distance changes everything!
Everything is relative to viewing distance. 150dpi (or 300ppi) is accepted as
photo quality because the average person cannot see the "dots" at a few
inches away. A real photograph made from film in a darkroom has no dots or
pixels and therefore is the standard by which "photo quality" is judged. When
you move the viewer further away from the printed material, lower
dpi is acceptable. A huge billboard might be printed at only 40dpi but no one
notices because everyone is 50 yards away from it.
I always see "72dpi". Where is that used?
72dpi should really be 72ppi because most likely, it has nothing to do with
printing. Most of the time, 72dpi refers to output on a computer monitor. A 72
pixel by 72 pixel image should take up about one inch of space on the
screen. This of course depends on the size of the monitor and what
resolution it is set to.
When creating images for web sites, we've found it's best to determine what
screen resolutions will be used most often by that site's visitors, instead of
thinking about inches at all.
Megapixels vs. Maximum Print Size Chart
Megapixels Pixel Resolution* Print Size @ 300ppi Print size @ 200ppi Print
size @ 150ppi**
3 2048 x 1536 6.82" x 5.12" 10.24" x 7.68" 13.65" x 10.24"
4 2464 x 1632 8.21" x 5.44" 12.32" x 8.16" 16.42" x 10.88"
6 3008 x 2000 10.02" x 6.67" 15.04" x 10.00" 20.05" x 13.34"
8 3264 x 2448 10.88" x 8.16" 16.32" x 12.24" 21.76" x 16.32"
10 3872 x 2592 12.91" x 8.64" 19.36" x 12.96" 25.81" x 17.28"
12 4290 x 2800 14.30" x 9.34" 21.45" x 14.00" 28.60" x 18.67"
16 4920 x 3264 16.40" x 10.88" 24.60" x 16.32" 32.80" x 21.76"
35mm film, scanned 5380 x 3620 17.93" x 12.06" 26.90" x 18.10" 35.87" x
24.13"
*Typical Resolution. Actual pixel dimensions vary from camera to camera.
**At 150ppi, printed images will have visible pixels and details will look
"fuzzy".
For a chart of megapixels vs. maximum print size at 300ppi, see our
Megapixels Chart.
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