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Scanner dpi: How much is enough?


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TrippiN's Avatar
TrippiN TrippiN is offline
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05-Jul-2003, 06:25 PM #1
Question Scanner dpi: How much is enough?
I work at a retail store selling computers. I always have questions about Scanners and just how much dpi is enough for certain projects.
For example; If I were to take a 4"x6" photo scan it, and print a 8"x10" on a inkjet photo printer(HP 7150 or the like) would I notice a difference in quality between a 1200x1200 dpi and 2400x2400 scanner? What is the general rule of thumb here? I've heard the human eye can not distinguish between 300dpi and the extra dpi is useful for enlarging. Such as, a 600dpi could double the size of your scan without losing any quality. 1200dpi quadruaple and so on. Is this true?
One other thing, is there a real simple way to explain what is the difference between a 2400x4800dpi scanner and a 2400x2400 scanner. I know that the smaller number of the two is the true dpi it will give you, but what does the other number mean?

Hehe, I know this is pact full of questions. Any comments/answers are very much appreciated.
Thanks in advance.

Ryan
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05-Jul-2003, 07:33 PM #2
A very good photo has 300 PPI of information. If you scan at over 300 PPI you are not increasing the content but just adding extra pixels. The scanner is inventing those pixels by guessing at what they should be based on neighbors. You can do that at least as well in a good image editor and more quickly by just scanning at 300 PPI and resampling to higher for large blowups. This is a good read Ė go on to the next page where he runs tests on a photo taken with a fixed focal length Nikon on a tripod: http://www.scantips.com/basics08.html

Real world objects like jewelry or mechanical parts can be scanned at higher resolution as long as you donít have one of those skinny CIS scanners that get their power from the USB. CIS scanners have no depth of field and will only scan the surface of something completely flat on the surface of the glass..

If you scan in two digit B&W (lineart) higher resolutions will help eliminate aliasing or jaggies in text and line drawings. You end up with large files for what should be small files but it is worth it for some uses. I never go above 600 PPI for anti-alising but you will benefit from even higher with deminishing returns. Scanning a full page document at 2400 PPI in a standard consumer scanner would take a LONG time. Only the smaller number really matters. A 1200 X 2400 scanner does 1200 pixels per inch and the stepped motor can stop 2400 times in an inch. The 2400 is basically meaningless.

Film and slides can benefit from up to 4000 PPI. The dedicated film scanners are better at a given PPI because of the concentrated light source and optics. Flatbeds with lightlids built in and 2400 optical (2400 X 4800 etc) can do a decent job on film but not great. The ones with the film and slide attachments that sit on the glass are useless. The Epson 2450 and especially the 3200 flatbeds do an excellent job on film and slides but they are pricey.
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bikerchick4G bikerchick4G is offline
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10-Jul-2003, 09:37 AM #3
Thumbs up great 411
thanks for the info man!! woohooooo As a novice PS7/scanner user/photographer, i have over 10,000 negatives that oughtta be making revenue for me... I have learned a lot thru trial n error and I mean a lot of ERROR... but without fundage for tutorials/seminars/classes, I am left to my own devices - with or without a scotch and a helmet!@$%^!
I appreciated this post.
nancy
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jonhiker jonhiker is offline
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Location: Portland, OR
11-Jul-2003, 03:19 PM #4
Slipe:
Excellent response, I learned a lot. Great link!
Thanks!
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