INCHES vs PIXELS question

Discussion in 'Buying Tips, Advice and Discussion (archive)' started by kwajaln, Jun 5, 2005.

1. kwajaln macrumors 6502

Joined:
Sep 18, 2004
Location:
CHICAGO!
#1
What are the dimensions of a standard 4X6 photo in pixels? Thanks!

Joined:
Oct 11, 2004
Location:
On the Left Coast - Victoria BC Canada
#2
It completely depends how large the pixels are you are measuring. The number of pixels per inch is called resolution. Think of a one foot by one foot chess board: you can have 4 x 6 inch squares on it, or 144 x 1 inch squares, the board is still 1 foot.

At 72 DPI (which is the nominal resolution of a Mac screen, vs. 96 dpi on a Windows screen) you would calculate 72 x 4" by 72 x 6" or 288 pixels x 432 pixels. This would be appropriate for Web display. But it is going to honk for printing, it is too low a resolution.

Typically, your print resolutions would be between 300 DPI and 600 DPI

So that would be 1200 x 1800, or 2400 x 3600 pixels on a 4 x 6 photo.

That's also why everything is smaller on a 17" LCD panel running at 1280 x 1024 than on a 19" panel at 1280 x 1024. Since there are the same number of pixels, the 17" has to pack the pixels in at about 94 Pixels per inch (1280 / 13.6" horizontal), whereas the 19" is a more spread out 84 ppi (1280 / 15.2")
Same number of pixels only bigger.

3. gwimby macrumors member

Joined:
May 6, 2005
#3
Typicaly the rule of thumb is when scanning, you want 2x the output resolution.
Generally when useing a photo printer 150 ppi is "ok".

Joined:
Oct 11, 2004
Location:
On the Left Coast - Victoria BC Canada
#4
More precisely, you are looking for 1.5 to 2 x the line screen frequency of the printer, when you are using the scan at 1:1 size.

Line Screen is not the same as the printer's dots per inch resolution, because many of the printer's dots need to be clumped together to make one halftone screen spot on the photo.

A 600 dot per inch monochrome laser printer typically prints 60 - 80 line screen on photos. Commercial printing would be 75 - 85 (newspaper) about 85 - 125 line screen for magazines, 150-200 line screen for glossy covers.

You have to do the math on enlargement or reduction; If the final output will be larger than the original, you have to scan at higher resolution.

Short course in PIXELS DISPLAYING, PRINTING, AND SCANNING DIGITAL IMAGES
http://www.shortcourses.com/pixels/

The development of very high resolutions in inkjet printers confuses the issue more: at 2880 dots per inch, theoretically an inkjet could produce a 250 line screen print... but that will never happen, because although the dots may be laid down on 1/2880th of an inch centers, the ink when it hits the paper spreads to be much larger than 1/2880" in diameter, so the printer cannot hold that resolution in the real world. 300 DPI as your input file is generally considered enough for photo printing. However if you have a superduper photo inkjet and are using the expensive photo paper, you can experiment with 400 or 600 DPI files to see if you get a better result. Just remember your file size goes up as the square of the resolution increase.

http://www.ltlimagery.com/resolution.html

5. tuartboy macrumors 6502a

Joined:
May 10, 2005
#5

You are very knowledgable about pixels and printing/scanning. Good work at giving a brief overview in such a short space.

6. kwajaln thread starter macrumors 6502

Joined:
Sep 18, 2004
Location:
CHICAGO!
#6
Thank you all so much!!! (especially Canada!)
The problem I am having is my Mom (75 years old and not computer savvy) receives the links to my .Mac homepages but when she prints the photos she only gets a small portion of the larger photo. I am in Chicago and she is in Arizona, and looooong phone calls attempting to resolve the problem(s) haven't been very successful. For now my solution is opening iPhoto first, selecting the photos there, and clicking "email", then choosing a moderate resizing option (usually 640 X 480). She is using a DELL laptop I bought her that runs Windows XP. I got that for her about a year ago, before I switched to Apple, otherwise she'd have an iBook G4. Oh well . . . Any other suggestions will be greatly appreciated. Canada, thank you very much for your assistance!!! I envy your knowledge of digital photography!

7. gwimby macrumors member

Joined:
May 6, 2005
#7
A much better explanation than I.