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You say you want resolution? The whole world is watching these digicams.


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Old February 26th, 2004, 07:47 PM   #1
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splain somethin to me, DPI vs resolution

I apparently have a misunderstanding of dpi and resolution, could someone help splain it to me?

My neighbor needed a new flat bed scanner and doesn't know much about computers (apparently I didn't know as much as I thought). I checked pricing and read reviews and compared abilities and we settled on the HP 3970. He had quite a few negatives and slides and it had the ability to scan them. So I told him I didn't know how good negatives and positives would turn out on a flat bed scanner but the overall cost of the scanner was still very good. It was 2400 x 2400 and seemed as good as others. I had just built him a new P4 hyperthread so I knew he had the computer to deal with it. So, As a test I slid in a 35mm neg and set the resolution to 1200, it turned out (a single 35mm frame) to be a 653 megabyte file. It took the P4 about 3 mintues just to rotate it 90 degrees in PS7. The file was the size of an entire CD. Question, why was this file so big, where are my calculations wrong about resolution and file size? My thinking is 1200 x 1200 per inch, obviously wrong.

By the way scans were pretty good.

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Old February 26th, 2004, 08:24 PM   #2
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Hi Don,

What physical size did the scanner produce? It sounds like about 1200ppi (pixels per inch, not the same as dpi, but not important to this discussion) at an 8x10 size, 48bit color. In PS go to Image>Image Size and tell me the size in inches and resolution. Go to Image>Mode and tell me the color mode it's in.
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Old February 26th, 2004, 08:54 PM   #3
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NOT a graphics expert here and youíve got me stumped, too. I believe that a 35mm frame is actually 24x36mm, which is 0.9449 x 1.417 inches.

Multiply those together and then multiply by 1200 and 1200 again, and you get 1,927,559. Thatís the number of pixels.

For RGB color (8bitsx3colors), multiply by 24 = 46,261,417 bits.

Divide that by 1024 bits in a KB and you get 45,177 KB (or 45.2MB ).

If you saved in a format like *.psd, thereís an alpha (transparency) channel so the result would be another third larger (total 60.2MB). If you had scanned at max rez (2400 x 2400), then the file would be four times larger than that (about 241MB).

If I didnít screw up the math, thereís no way that you should be able to get that large a file from the scanner. And this ignores compression such as jpg. Anyone else got any ideas?
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Old February 26th, 2004, 09:49 PM   #4
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Most scanner software allows you to scale the scanned image larger than the original image. In other words the software creates a 8x10 or 11x14 at 1200ppi. The scanner software usually gives you an option of 8 bit (24 bit) or 16 bit (48 bit) which effectively doubles the file size.
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Old February 27th, 2004, 05:05 AM   #5
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You are right Jeff, when we went to print the next negative the photoshop was at 8x10. And at (I think) 600 or 300 dpi it still made a 6 megapixel file. I will have to look at ps7 to get the color mode, but it was probably on whatever the ps7 software comes up in default. And it is possible that the scan was 2400, I just remember jacking it way up to give it a try.

So is that what is called "up sizing".
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Old February 27th, 2004, 05:16 AM   #6
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Is there a suggested scale to use when scanning 35mm negs and pos. An enlarger of course uses light to enlarge a photo how does the scanner and photoshop do it. And then back to dpi ppi, dot per inch, pixels per inch, why aren't they 1:1?

Has anyone ever asked, how many dpi would film be considered? How many megapixels equal a 35mm.

Last but not least, are these little flatbeds coming close to the nikon film scanners?
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Old February 27th, 2004, 06:04 AM   #7
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A dedicated film scanner will always give better results than a flatbed simply due to their physical design. A flat bed is designed to scan flat objects sitting on the glass. When using the slide or film plates the film is actually sitting above the glass. You can get good results, and flatbeds are generally cheaper, but a dedicated film scanner would be better if he didn't need to scan prints or other documents. I have a Canon 9900F, a pretty good scanner that is sadly let down by poor software.

For an 8.5x11 to be printed by my Epson inkjet I generally scan at an output resolution of 300dpi. The 35mm frame is enlarged by approx 900% to give a final file size of around 25MB. These files are managable, even by my old PIII667 with 512MB of RAM, and come out very nicely from the printer that has a res of 2880x2880. Scanning at an output resolution of more than 300dpi is pretty much a waste of both storage space and processing time, as inkjet printers can't handle higher resolutions.

The 1200dpi you set was most likely output resolution. If I set my software to 1200 and 8x10 I also get a huge file.

I'm not sure if this is the best way but it seems to work ok for me. Jeff will probably have a better way, or probably tell me I'm doing it wrong. Anyway give it a try and see how you go.
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Old February 27th, 2004, 06:14 AM   #8
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It's real simple Don. For printing images, either commercially or at home with an inkjet, you need 300 ppi in the finished file you're printing. You can determine resolution by multiplying the ppi by the length and width. For example 8x300=2400, 10x300=3000. In order to make a photographic quality 8x10 you'll need a file 2400 x 3000. How many mega pixels is that? Well, multiply 2400x3000 and the your mega pixels are 7.2 (7,200,000).

So, it doesn't matter if you scan your neg at 2400 ppi at a size of 1 inch by 1 1/2 inch (35mm neg) or 8x10 at 300 ppi, the file will be the same.

35mm film is considered to be 4000 ppi at it's real size, 1:1 However, 35mm film is grain limited and scanning at 4000 ppi will produce a very large file.

Flatbeds, as a rule, do not produce as high a quality file as a dedicated film scanners, such as the Nikon Coolscans.
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Old February 27th, 2004, 06:26 AM   #9
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Thanks Jeff, Adrian and Pete, guess I need to go to some of Jeff's classes:)

P.S. Jeff what digital camera do you use for your widlife I you don't mind me asking? looks like I might owe you another lunch.

Thaks
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Old February 27th, 2004, 06:37 AM   #10
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Those are all shot with the Canon 10D. Yup, lunch and a Photoshop discussion would be in order.
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Old February 27th, 2004, 08:27 AM   #11
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>>35mm film is considered to be 4000 ppi at it's real size, 1:1

That is for a modest scan considering 50 line pairs/mm. With a good quality lens you can get 80 to 100 lp/mm which translates to around 7200 'pixels' on a line.
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Old February 27th, 2004, 08:49 AM   #12
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Rob, Jeff's refering to the resolving abilities of film I think. Yeah, a good lens can produce higher lines/mm but wouldn't you still be limited by what the film can reproduce. Either way, both are still way more than current affordable printing techniques can reproduce, especially inkjet printers.
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Old February 27th, 2004, 08:53 AM   #13
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Rob, those numbers only exist in theory and disregard diffraction limits and aberration limits. Canon shows MTF charts of all their lenses and their very best lenses get only slightly over 60 lines/mm. So, I'm not sure what 35mm lenses you're referring to that get 80 to 100 lines/mm. Most of the zooms are only getting 40 lines/mm or so.
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Old February 27th, 2004, 09:26 AM   #14
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One last question and this one is for a steak (from a decent steak house), What is the printing/developing process called that uses traditional chemical photographic paper and a laser to project the digital image onto the paper. And is that process any better (quality longevity) than the new pigment ink printers. I see Canon is coming out with the new 4000, it is a pigment ink printer. the only disapointing thing is it's only 1200 x 1200. Epson has already eclipsed that with 2880 x 1440 in the epson 2200 but the Canon printer produces 24 inch wide prints. Now, after having the previous discussion, is anything higher than 1200 x 1200 even needed. Would 2880 x 1440 look better than 1200 x 1200 for close viewing. I guess this is the new area I have to learn, scanning, printing for longevity, color correctness between monitor and printer, and now I am learning that some third party software may be better to download raw images to than the canon software.

OK one more question, can I do all this with a cheap 19 inch samsung 955df monitor?

Thanks
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Old February 27th, 2004, 09:43 AM   #15
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Fuji Frontiers use a laser and traditional color paper processing (RA-4) to produce prints in the manner you describe. Frontiers use Fuji Crystal Archive paper and it is independently rated to last 80 to 100 years if stored properly.

Epson pigment based inks and their enhanced matte paper or heavy weight paper are independently tested to last the same 80 to 100 years. Some test suggest the pigment based prints may last as long as 200 years. Dye based inks are usually rated around 25 years. Kodak is releasing a new inkjet paper that they say will last 100 years with dye based inks. These are Kodak claims are not independently verified.

I use an Epson 2200 and routinely use the 1440 dpi setting for better speed. Only on rare occasions can a difference be seen between 2880 and 1440 dpi. The Canon at 1200 should produce respectable prints up to 8x10 and maybe 11x14.

Yes, issue regarding color space and color management are probably your next concern.
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