Re: [spam score 9.00/10.0 -pobox] Digital Cameras - Megapixels vs. DPI

>I am primarily in the public relations business, and am often asked to
>provide newspapers and magazines with photographs.  I'd like to purchase a
>digital camera so I can get newsworthy photos to media outlets ASAP. 
>
>Most papers/magazines will not take anything less than 150 DPI (and usually
>prefer 300 DPI).  How does this translate into megapixels (assuming it
>does), and what digital cameras currently provide that kind of quality?
>
>Thanks!
>
>Jim Campi

Resolution is expressed in various ways. With digital cameras, the 
bottom line is the number of pixels in the sensor. For example, in my 
3.2 megapixel (3.2 million) pixel camera, I can take pictures which 
are approximately 2200 pixels by 1500 pixels (in round figures). (If 
you multiply 2200 x 1500 you'll get the 3.2 m pixels.)

That means, the unaltered image, shown at 100% view, will occupy 2200 
pixels across my monitor, by 1500 pixels high. If I'm running my 
monitor at a resolution of 800 pixels x 600 pixels, then the image 
will be almost 3 times the width of my display, and I'll have to 
scroll to see it all. (BTW, in terms of actual size on monitors, they 
generally display between 72 (typical Mac) and 96 (typical PC) ppi 
(pixels per inch).


You don't really care how this works out on the screen, though. 
You're interested in how it gets translated to the printer. Well, if 
you're going to print it at 300 pixels per inch, then the 2200 pixel 
dimension translates to 7-1/3 inch (2200/300) and the 1500 translates 
to 5 inches (1500/300). In other words, the the image can produce a 
7-1/3" x 5" printed picture at 300 pixels per inch. If you want an 8 
inch by 10 inch print, you'll need to lower the ppi to something less 
than 300 ppi.

Keep in mind that you often will be cropping a photo, so the number 
of pixels available is less than 2200 x 1500.

Often, the terms ppi (pixels per inch) and dpi (dots per inch) are 
used interchangeably, although printer dots are really something 
different. To add to the confusion, scanning is sometimes described 
in dpi, although it really should be spi (samples per inch).


For offset printing, where halftones are used, linescreens become 
involved: e.g., a linescreen of 135 lpi (lines per inch). The 
rule-of-thumb is that the photo in ppi should be 1-1/2 to 2 times the 
linescreen. So if you're using a 135 lpi screen, then you want an 
image somewhere in the 200 to 270 ppi range.

You probably want to look for a 3 or 4 megapixel camera.

Also, it would be nice to get a camera that has the option of 
recording the images as uncompressed TIFFs (Tagged Image File Format) 
as well as JPEGs (Joint Photographic Expert Group). The TIFFs have no 
data loss, while the JPEGs use lossy compression. At low JPEG 
compressions (e.g., 4:1, you probably won't notice the loss). 
However, with a 3 megapixel camera, a low compression JPEG will have 
a file size of about 900K, while a TIFF will have a file size of 10 
MB. The newspapers probably will want the smaller size JPEGs. Some 
magazine may opt for the larger size, higher quality TIFF.


I suggest you browse through the reviews at Digital Photography Review:

	http://www.dpreview.com

Jeff






-- 
___________ Jeff Kane ___________
__ Digital Imaging for the Web __
_______ www.oceansong.com _______

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