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[OM] Megapixels and printing photos (long)

Subject: [OM] Megapixels and printing photos (long)
From: "Brian Swale" <bj@xxxxxxxxxxxxxxx>
Date: Thu, 4 Jan 2007 11:24:14 +1300
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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