Solved

DPI vs. megapixel simple questions / scan old pictures at 'good quality, etc.

Posted on 2004-10-06
8
10,522 Views
Last Modified: 2012-06-27
This may be basic stuff for you guys, but please help me out!  I have a 4 megapixel Canon SD410 camera whose pictures at the highest / finest settings takes jpgs that are about 2 meg in size.

I have a scanner (HP scanjet 5100c) that I want to use to scan in 4 x 6 photographs so all my pictures are digitized.  When I scan at say, 300 dpi, the file size is a few hundred KB.  what is the 'native' DPI of a 4 megapixel camera?

And for digitizing 4 x 6 pictures taken with a traditional camera, what is the 'right' / best dpi.  storage space is not an issue.  But I don't want to scan at 4000DPI just to have a large file.  I've tried scanning on at 200 vs. 300 DPI and really can't see a difference when I zoom in.  May be the picture I am experimenting with is blurry?  

I'd hate to spend hours scanning in pics at 200 DPI when it really should be 600.  or scan in at 600 and find out later that 300 DPI is the effective quality of a 4x6 print (what is the 'weak link' in the scanning chain?  The print or the DPI you scan it in at?).  

I've heard people argue that it matters what you want to do with the picture.  I don't know right now.... so I want to get the best I can expect, starting with the 4 x 6 original print.  They likely will sit on the hard drive and go no where.  but I might want to make a poster one day and hate to hear that the scanned image yielded a lower quality poster than the 4 x 6.  Yes, I know - you enlarge a 4 x 6 too much and it gets grainy. I accept that and just want the same result from a digitized image...

THANKS!
0
Comment
Question by:spacecadet1234
  • 3
  • 2
  • 2
  • +1
8 Comments
 
LVL 30

Expert Comment

by:weed
Comment Utility
Cameras dont have a "native DPI". That's because DPI stands for Dots Per Inch, and is used only when printing material.

For scanning important photos anything between 300 and 600 is fine. BUT DPI (PPI) is only part of the issue. Also make sure you're scanning them at ORIGINAL size and arent reducing them to 2x3 (for example) because a 2x3 image at 600 dpi is still only 2x3...see?

The weak link is the quality of your scanner. A poor quality scanner will make poor quality scans. In a perfect world where scanners are accurate, the printer is the weak link.

Now, as for your poster question. In THIS particular case, you still want to scan it at 300 or more, BUT in the scanning process you want to enlarge the dimensions as well. You wont really lose quality by enlarging from the original because the scanner is just looking closer at the material it's scanning. It's not having to interpolate which is where the quality loss comes from. How big you want to enlarge it is up to you.

Here's a thought. Scan the images at 4x6, 300 dpi. If you ever need that size, you already have it on your HD and it's not sucking up much space. Keep the originals in a box, and if you ever need to do a poster, re-scan them huge.
0
 

Author Comment

by:spacecadet1234
Comment Utility
Not sure if I am catching on....   scanners are rated in DPI, right?  but as you say, dpi is for printing.  so why do they do that!?  I guess I am asking what do I need to do (or o I even want to do this)  to take a printed photo and scan it so it'll be equivalent to the file I get when I take a picture with the 4 megapixel camera set to the best settings / least number of pictures that will fit on the card?

that last paragraph you said confused me.... I throw something on the scanner and I thought the only control I have is the DPI setting - not the size of the reulting file?  I start with a 4 x 6 picture, I can only choose the DPI - not how big to make it.  when I go to print it I tell the app what size I want it resized to?  as I would with a file from the 4 megapixel - either print it as a 4 x 6 or poster?

I don;'t want to have to rely on the originals IF I need them later
0
 
LVL 17

Expert Comment

by:Lobo042399
Comment Utility
Hi Spacecadet,

Certainly what you're planning to do with the image is important. If you're not gonna have it up in a billboard you don't need to have it drum-scanned at 24000 PPI. But maybe later on you'd like to blow up a couple of your images and frame them, right? So you may want to scan at something in between 400 to 600 PPI.

Good Vibes!

Lobo
0
 
LVL 30

Assisted Solution

by:weed
weed earned 80 total points
Comment Utility
Scanners are not rated by DPI. Some scanners can scan at a higher DPI than others (some cheat and use interpolation. Interpolation=bad). DPI is NOT a measure of quality. Look at digital cameras for example. Super high end digital cameras generally dont have as high a megapixel count as some lower end cameras. BUT they make up for it in superb optics which means better color, less blurriness and less "grit". They also have a better CCD which makes a difference in the same areas. So as you can see, big numbers dont mean big quality.

ALL you need to do right now is scan your 4x6 image at 600 dpi. That'll give you some options down the road as well.

DPI is not the only setting you have when scanning. There is always a DPI setting and a Zoom setting. 50%, 100%, 150%, 200% etc. Usually you can enter your own percentage as well. Telling it how big to print it when you print it is a bad idea because somewhere you have to compensate. Either interpolate, or change the DPI. You dont want to be stuck with that.

0
IT, Stop Being Called Into Every Meeting

Highfive is so simple that setting up every meeting room takes just minutes and every employee will be able to start or join a call from any room with ease. Never be called into a meeting just to get it started again. This is how video conferencing should work!

 
LVL 17

Accepted Solution

by:
Lobo042399 earned 80 total points
Comment Utility
The problem is that manufacturers usually say DPI when they really mean PPI. Both are not the same. PPI (Pixels Per Inch) is a measurement of how many pixels it takes to describe an inch of an image. This is not really a physical measurement, though. Say you have an image that is 2x2 inches at 300 PPI. Measuring the image in pixels that will give you 600x600 pixels. That's your image size. Now, if you change resolution WITHOUTH resampling, say to 150 PPI, the image will grow larger because less pixels will be required for each inch. So at 150 PPI your image will become 4x4 inches. (600 / 150 = 4).

As Weed has said, DPI is a measurement for printing. It represents how many dots a printer will produce per inch to draw an image. It has nothing to do with PPI. You can take your 4x4 image at 150 PPI, and you can print it in your inkjet at 720dpi and it'll still be 4x4. It won't look very good, but it'll still be 4x4.

To fool the eye when printing it is recommended that an image be not less than 200 PPI at print size. That means a scan of your 4x6 photo should be not less than 800x1200 PPI if you want to print it at the same 4x6 size. It will look better if that resolution is higher, but if it's lower you won't get anything decent out of the printer.

So, depending on what you want to do with these scans, the lowest size I'd give to those photos would be (minumum intended size) x 200, in pixels.

For example. Say you want to print the scan at a nice 12x18 for framing. Then your minimum decent quality for the image would be 2400x3600 pixels, right? (12 x 200 = 2400; 18 x 200 = 3600). Now, How do you get that resolution when scanning? With an original of 4x6, scanning at 100%, that would mean... 2400 / 4 = 600; 3600 / 6 = 600. Your number would be 600. If you tell your scanner to scan at 600 PPI at 100% that should give you the size in pixels you require.

If you want a better quality printout all you need is to change that 200 to a higher number, say 300; and do the math again.

Good Vibes!

Lobo
0
 
LVL 3

Assisted Solution

by:RobMurota
RobMurota earned 40 total points
Comment Utility
If you divide 4 megapixels by 300 you "approximate" the 300 dpi setting on a scanner.  So a 4 megapixel image will yield a 16x12 inch image at 300 dpi. 300 dpi is the preferred setting for litho presses. That said, a jpg image is a compressed image with missing "critical" colour information to achieve a smaller file size. A scanned image saved as an eps or tiff will always be sharper than a jpg of the same file size. Because CCDs record only black and white (the colour is created by filters) it also stands to reason that the colour/grey proportions in a file will also change the file size in a digital camera but not on a scanner. The image produced by a 32 bit scanner has that much digital information describing each scanned pixel. The quality per pixel equation may be of interest.
0
 

Author Comment

by:spacecadet1234
Comment Utility
thanks guys... but why are my scanned 300 dpi pictures saved as jpegs so much smaller (600K), vs my digital camera jpgs (2MB)?  Different compression levels?
0
 
LVL 17

Expert Comment

by:Lobo042399
Comment Utility
Hi Spacecadet,

Yes, it's the compression level. When a camera saves an image as a JPG it generally uses a low compression level to  give you a good image. After that, recompressing the image even just a little more will reduce the file size.

Good Vibes!

Lobo
0

Featured Post

IT, Stop Being Called Into Every Meeting

Highfive is so simple that setting up every meeting room takes just minutes and every employee will be able to start or join a call from any room with ease. Never be called into a meeting just to get it started again. This is how video conferencing should work!

Join & Write a Comment

Suggested Solutions

Title # Comments Views Activity
Illustrator Print to Web 9 742
Quick online design 2 114
Graphics - jpeg format 3 47
Website cross platform development 6 29
Many programs have tried to outwit PowerPoint in terms of technology and skill. These programs, however, still lack several characteristics that PowerPoint has possessed from the start. Here's why PowerPoint replacements won't entirely work for desi…
Technology opened people to different means of presenting information, but PowerPoint remains to be above competition. Know why PPT still works today.
In this tutorial viewers will learn how to create blended and gradiated shapes in Illustrator using the blend tool Draw two shapes, one of them in a different color: Select both and create a blend by going to Object > Blend > Make: Blends can also b…
In this Micro Tutorial viewers will learn the basic shortcuts and functions of Illustrator. The viewer will learn about the paintbrush tool, anchor points, font sizing, and more.

763 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

9 Experts available now in Live!

Get 1:1 Help Now