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Question about resolution and image sizes

Posted on 2004-09-21
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Last Modified: 2010-04-03
Can someone straighten me out on this?  I scanned a photo at the highest my scanner would go, 2400dpi.  I bring this image up in Photoshop and do an Image Size.  This is what I don't get:

Pixel Dimensions:
Width: 3120
Height: 2080

Document Size:
1.3"
0.867"
Resolution: 2400 dpi

Huh?  How can 2400 dots per inch = 1.3"?  I mean, I scanned this image to be massive for poster size print yet it only comes out 1.3" wide?  If I try to change the Document Width it won't let me set it any higher than 12.5"  Huh?  2400dpi and I can't print it any larger than 12.5" wide?

What am I missing here? :)
Thanks!
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Question by:MIKEV
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nicholassolutions earned 150 total points
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How big was the original photo you scanned?

Resolution is a RATIO, not a length. It tells you how many dots PER INCH your scanner will resolve, i.e. how 'close' can it look at what you are scanning.

It sounds to me like what happened is this:
You scanned a very small photo, about 1.3" x 0.867"
1.3" *2400dpi  = 3120 pixels
0.867" *2400dpi = 2080 pixels

So, assuming I'm right that your original photo was small, that is why the info you provided from Image Size is true. If the original oyu scan is small, you cannot expect to get out a crisp-looking poster size print. This is because, if you try to increase the size of the image after you've scanned, you have to DECREASE the resolution. Once you have scanned the image, you have a limited amount of information. In your case, you have 3120 x 2080 pixels. Each pixel holds information about the color at that point. If you increase the image size, you have to 'spread' the information out. Say you double the size of the image -- you can think about it like this:

1) photoshop 'spreads out' the pixels, placing the orignal pixel colors at every other pixel on the images.
2) Then it tries to 'guess' what the pixels inbetween should be, and maybe changes the shades of the 'original' pixels slightly to make things look right.

But now, every other pixel is a 'guess' and the image looks blurry as a result. Photoshop does a really great job of making the guesses the best that they can be so you can increase your image size with minimal loss in quality, but there is no way to avoid the loss, no matter what software you use.

That said, I don't think there should be a limit on the size that you can make your image -- but you will loose quality each time you increase the size. One word of advice here -- do not keep making your image bigger and bigger, or tinkering with the size, because each time you change image size, you loose quality. Try to do it in just one transformation.

The only reason I can think of that you might not be able to increase past 12" is that you will have a huge file if you do, and maybe your computer and/or photoshop cannot handle it.

If you want to make a poster size print of a small photo, the best thing to do is to find the negative, take it to a photo place, and either have them blow it up for you into a poster, or at least make a larger size photo for you to scan.

Cheers,
Matt
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by:weed
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Ok, first off, scanning at 2400 dpi is usually an interpolated rez, and isnt it's true optical rez.

Now, what happened when you scanned it? There are two settings when you scan. The DPI, and the actual dimensions. You set the DPI to 2400, but didnt change the dimensions. They were set to 1.3"x.86". So that's what you got. A 1.3"x.86" image, at 2400 dpi. What you COULD have done was set the DPI to 2400, and the actual dimensions to something like 24" across. That would have given you a truly huge image.

Now, since the optical rez of the scanner probably isnt 2400 dpi, and you dont really need it that high anyway, scan the document at say, 300 dpi, with a dimension larger than 1.3".

Just for definition's sake, the only thing DPI changes is how many dots are printed in an inch when you go to print it out. The DPI doesn't actually change the size of the image though when you print it, you only have so many dots and the printed image can only be so big. In your case, 1.3" at 2400 dpi, is 3120 pixels across. 3120 sound familiar?
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by:nicholassolutions
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Good point about the interpolated resolution -- in case you dont know, what weed means by this is that the scanner is 'guessing' from the getgo about the colors it should assign to many of the pixels, rather than truly 'looking' at the images (i.e. optically).

Cheers,
Matt
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by:webwoman
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What you've missed is that you don't need to scan at 2400 dpi. It's better to scan at 300-600 dpi but at 200-300% size (or more).

Then you get an image that is 300 dpi but 300% larger than your original. The quality may be crappy though. What you have is probably crappy too. Try resizing it, but turn off resampling. When you change the size the resolution will go down, which is fine. You don't print at 2400 dpi anyhow. You've got 3100 px -- you should be able to get a 10" x 300 dpi image with mimimal loss of quality from what you've got, but as I said, that may be crappy anyway, depending on the size of the original.

If you have a negative, get THAT enlarged, then scan the print. If you have a slide, get a print made from the slide. You'll get a MUCH better quality scan.

If you're scanning from a printed piece, shame on you. Contact the publication and get a real print or high res digital image. You'll probably have to pay for it.
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by:Lobo042399
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Hi Mike,

2400/300 = 8.

1.3*8 = 10.4.

You see where I'm going. Your original scan area was a rectangle of roughly 10.4*6.9 inches, right? The scanner was set to default at 300ppi.
What happened, as Weed explains, is that when changing the resolution to 2400ppi you forgot to change the image size. Nothing that can't be remedied by rescanning your image and making sure both size and resolution are set right. At 2400 ppi you should be able to get an image of 24960*16640 pixels.
However, following Weed's advice, I would scan at only the maximum optical resolution allowed by the scanner and do the interpolation in Photoshop. The image will look better that way.

Good Vibes!

Lobo
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by:weed
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You don't even need the max optical rez. Scan it at the DPI you'll be printing it at, and set the dimensions accordingly. The less interpolation you do, the better your final product will be.
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by:MIKEV
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Great replies, thanks.

This was scanned from a 35mm negative with an HP S20 negative scanner.  My end result is to be as large a final image in print as possible.  Poster size or larger.  There are no options within the scanning software for actual dimensions, just dpi.  I figured that if I scanned it at 300 or 600dpi, I'm going to get a grainy image when printed to say soemthing like 2'x3'.  So I scan the image at as high a resolution as possible to give me a larger and clearer end result.  But I end up with an image the size of the negative.  Hmm.

I did get a quote from a print shop based on the dpi of the image.  I can't remember the dimensions, something around 38" wide but they said anything larger would degrade the image quality.  Does this make sense?  How would they/I go about printing this larger than 1.3"?
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by:weed
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300-600 dpi is considered about average for printed material. High quality gloss magazines are probably only 300. It's not going to look grainy.

Your scanner software MUST provide some place to adjust the dimensions of the scanned image. They all do. It's got to be there somewhere. Perhaps in advanced options?
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by:Lobo042399
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Aha!!!

So the original WAS 1.3" * 0.867" after all.  In that case what you need is to have your negative scanned with a better scanner. The Epson 4180, for example, offers an optical resolution of 4800x9600 with an interpolated res of 12800ppi.
Other higher end scanners like the Creo iQsmart3 offer 5500ppi optical (10,000ppi interpolated) resolution but have technology that allows these images to be blown up to 600% while maintaining image quality. I would make a few phone calls to service bureaus in your area and check what kind of resolution they offer.

Also, check with your printer again. Tell them what size you need the image to be, and ask them what resolution they need the image set at. Really large format printers (like the ones used for billboards or bus stop posters) do not require a high resolution because these images are not meant to be viewed at close range. Then you do the math:

Output size / required resolution = required image size in pixels.

Knowing the required image size in pixels, you can do some math again:

Required image width in pixels / 1.3 (your negative's actual width) = optimum scanning resolution.

Remember that this optimum scanning resolution is only a guideline. That is the resolution the image should be scanned at ideally so that no resizing is required. If that scanning resolution is too high or is not available in your area you can resize your image in Photoshop but only so much depending on the quality of the scan. That's where the Creo scanner I mentioned comes into play by allowing an image to be resized more without considerable quality loss.

Good Vibes!

Lobo
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by:AndrewGW
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I would suggest looking for a company that scans slides professionally...  I pay $5.00 a slide (okay I live in a 3rd world country so it's cheap) to have image scanned on a super high res scanner and I have taken a 35mm slide up to a billboard and the side of a large bus with no problem.   These scanners are optically scanning at 2400 or greater "ppi" so the result is incredible.
Your printer may know of a place that handles such scanning - quite often it's a shop that makes color separations as well.

You need to determine what type of machine your final output will be printed on, what resolution it prints at and from that you can do the math as described above.   Don't confuse the final printer output  "dpi" (dots per inch) with the resolution of the image  "ppi" (pixels per inch) they aren't the same thing.

I used to print on a 36" wide plotter at 600 dpi (we told clients it printed at 1200dpi because that's what they wanted to hear and they never knew the difference) and our rule of thumb was that the original artwork should be at 100% final output size with a resolution of between 100 and 200 ppi.  If the image was smaller than the final output size (quite often the case)  then the resolution would need to increase proportionately.
This does make for quite large files - 100 to 250 MB.
The printer's RIP (Raster Imaging Processor - tells the printer what color dot to place where) also makes a huge difference and the more expensive and complete the RIP is the better output you will get - check with your printer - this may be where the limitations come from as some RIP's can't handle really big files - they grind to a halt - hence the limitation in size or that's the max width of their printer.

Just some other ideas to go along with the good stuff from the others....
Cheers
Andrew
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by:MIKEV
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Oh I get it....  If the printer is going to print the image at 300dpi, then a 2400dpi image will be 8" wide in crystal quality.  Printing at a lower resolution will increase the printed size of the image....  That's what I was missing.

The original print shop mentioned above asked what the size of the image was and then gave me a maximum print size without losing image quality.  It was somewhere around 38" wide.  That all makes loads of sense now.

I'll be having this done by the print shop.  They'll scan, print and posterboard the entire thing in photographic glossy for about $180Cdn.  The guy that maintains our photocopiers has taken the images to take a whack at it with some large printers they carry, for free.  I was just concerned with sending him a 1.3" picture for a >24" result.  I spose it all depends on what resolution it's being printed at.

Thanks, thanks, thanks!
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by:MIKEV
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Hmm...  Did I get that right? :)
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by:Lobo042399
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Hi Mike,

I think you got the idea now. Canada, eh? If you're anywhere in the GTA area I'd highly recommend Ink City. They do excellent work not only in digital and large format but also in offset printing, specializing in short run jobs.

Good Vibes!

Lobo
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by:AndrewGW
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What size will the final image be?  Trying to guage if the $180 CDN is a good price or not...  or whether I should have moved my operation up there for better prices than I could charge here in Central America - that's why I'm not printing anymore - couldn't make any $$$

Always had fun with clients asking us to print a banner or poster using the images off their website - they couldn't understand why if it looks so clear on their screen why we couldn't make a billboard out of it...  

But I think you have the hang of how image size and resolution works now...

Cheers,
Andrew
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by:MIKEV
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Cool, thanks!  I tried to spread the points around, all were excellent contributions.  

Andrew:  Somewhere around 38" wide.  Mounted and all in the quality I'm looking for I think it's worth it.
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by:AndrewGW
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Yes it probably is, considering they will do it all...
Thanks for the points and glad we could help
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