Scanning quality and compression rate

[legolasz]

Hi all,

A couple of weeks ago I bought my first film scanner, a Minolta dImage Dual Scan 3.

I intend to scan all my slides at the highest quality possible with this scanner (2820 dpi) but was wondering.. does that make sense? Is it worth the HD/CD/DVD space? My purpose of the scans is 1. to have a digital archive of my images (using Photoshop Album 2.0) and 2. to show my pictures on the television screen and 3. to be able to order additional prints without the risk of loosing my original slide.
Maybe it's better to scan at a lower quality to save the space? Any ideas?

Furthermore, I have some questions about compression rate.

Which compression rate is best for display on a TV screen (not being HDTV, 786 x 540 or something like that)?

And which compression is best for ordering copies using a digital photo print service?

Thanks for any help/ideas, I am a newby in the art of scanning.

Regards,
Hans

[ugot2bkdng]

I believe you should scan at the highest resolution if for no other reason than to have the best quality possible archived. The 2820dpi scans will just barely give you "good" (depending on the original source quality) 8x10 prints.

Compression rate and image pixel size are not synonymous. Compression rate usually refers to the amount of compression used to create the size of the file that contains the image. Simple example:
800 pixels by 600 pixels full color image is 480,000 total pixels. For full 8 bit/pixel/color the original file size would be 480,000 x 8 x 3 = 11,520,000. That's a lot of bits! So we have techniques for reducing the file size. One of the more common techniques is jpeg compression which has selectable "rates" that produce different degrees of quality in the compressed/decompressed image.

Your scanner probably outputs TIF files. Some scanners have a quality selection, others don't. My 35 mm scanner will output jpp or TIF files. I usually scan at full resolution and create maximum quality TIFs. A full frame 35mm slide produces about a 25MB TIF file. I use those files for my archive. I import them into Photoshop where I crop and make other adjustments and then output them as jpg files at various sizes and qualities depending on what I am going to use them for (eg, for pbase I usually use 72 bit/pixel @ 640x480 jpgs compressed to about 150KB).

Back to your question on TV display. For NTSC, the picture occupies approximately 480 of the 525 scan lines. For broadcasts the portion of a scan line that is visible can hold up to about 440 dots so a grid 480 high by 440 wide represents the maximum amount of picture detail possible.

[bobtrips]

I scanned in ~ 12,000 slides. I hate to think how many CDs it would have taken if I had made max rex TIFFs.

How about scanning for viewing? Say ~3 meg scans and medium jpeg compression. That's going to give you more than you need for HDTV. You'll be able to print an excellent 5"x7", nice 8"x10".

Then when you encounter any 'very special' shots, images that you might want to print large some day, rescan them at full bore....

[ugot2bkdng]

Bob,

Well, I can certainly see your point there. Fortunately (or unfortunately depending on ones point of view), I only have a couple hundred scans. I only scan those that I know I want to keep.

I do shoot a few digitals with my 10D. In that case I always shoot RAW (pardon the expression ). On images that I decide to keep, I convert them to TIF files. Everything is archived on DVDs. I don't have a lot of photos yet (only about 21 disks worth to date). They are all indexed on line via IMatch. When I get into this photography thing big time, as you apparently are, I will probably have to rethink my current strategy.

Thanks for the info.

[pstewart]

I've been scanning most pics at only 600dpi and have printed great 8by10's from them. That's for prints though. I have a ton of slides to scan...would you guys recommend that I increase the dpi setting for slides? I know I don't want to store the max size...what minimum size would still print well, do you think? Is 1200 okay?

[bobtrips]

Phyllis, that doesn't sound right to me. A slide is only about an inch wide. If you're scanning at 600 dpi then you are taking around 600 dots and printing them across 10" of paper.

That's only 60 dpi, well under what most people would consider a minimum (125-150 dpi).

You sure about those numbers?

[ugot2bkdng]

Hi Bob,

I think she has been scanning prints. 600 dpi for say 5 by 7 and even 8 by 10 prints is probably okay. For slides, that is a different matter. Actually it isn't the slides, it is the size of the scan. A 35mm frame is only about .95 by 1.4 inches. So, if you want to print 8 by 10s and you consider 300 dpi the right res for your printer, then you need roughly 3000 dpi scans (I have rounded the math here but hope you get the idea Phyllis).

[graphicsmanip]

I know this is petty so please ignore this post if you desire...

Printers output is measured in dpi,(dots per inch).

Scanners output is measured in ppi,(pixels per inch).

Just a little fetish I have...

[pstewart]

Well, just in time...I am about to put the first tray of slides into the scanner...will use that setting, Chuck...thanks!

[legolasz]

Thanks Chuck, Bob et al for the replies.

I have already scanned about 150 slides at maximum quality: these are TIFs of approx 29MB big. I am tending of moving towards a lower quality based on Bob's comment, to save CDs/DVDs. But then again: my printer outputs at 600 dpi, that means that (if I understand you correctly Chuck) even 2820 dpi is in fact not enough for good quality 8x10s. So I will probably stick to scanning at max resolution and then burn them to DVD.

For the output to my television screen I will compress my images to JPEG.

Thanks,
Hans

[ugot2bkdng]

I think 600 dpi printing is a bit overkill. 300dpi is probably more than enough and 150 is usually quite satisfactory.

[castledude]

graphicsmanip Posted: 12 Jan 2004 01:08 am Post subject:

--------------------------------------------------------------------------------

I know this is petty so please ignore this post if you desire...

Printers output is measured in dpi,(dots per inch).

Scanners output is measured in ppi,(pixels per inch).

Just a little fetish I have...

This is not petty... DPI is not the same as PPI. Dots are only one colors worth of information. Pixels are multiple colors of information.

I have this definition in my clips so I am going to quote it here:
(It is not from PBASE but from another board)

Andy Piper , oct 28, 2003; 05:07 p.m.
There is some confusion here: Pixel resolution (in the image) and printer dot resolution are completely different things.

Seth: you are basically right - a 2400x1200 printer prints 1200 dots (droplets of ink) per inch in the direction the print head travels back and forth. It also prints 1200 dots per inch in the other dimension (in terms of the holes in the inkjet printer head), but moves the paper only a half-dot at a time, so it can cram in 2400 overlapping circular dots in the same inch in that direction. The 'true' resolution is always the smaller number.


You need at least TWO dots to resolve a line or other fine detail point. One dot for the line/detail, and one for the white background. (Think about printing black type - to make a letter "a" you need to be able to see the hole in the "a".) So the detail resolution will never be more than half the 'dot' resolution. And that's for pure black/white images (no grays/no colors).


To get a range of colors or grays, you need multiple printer dots to provide tonality. If you want, say, 36 possible grays in an image, each pixel in the scanned file must be represented on paper by a 6x6 matrix of printer dots - with some or all of the dots 'missing' to make the matrix contain any number of ink dots from 0 to 36. (I'm using a single ink for example purposes - multiple inks will improve things).


So the detail resolution of the printer will be 1/4 or 1/6th or 1/8th the advertised 'dot' resolution. Thus a 1440 x 1440 Epson printer can resolve smoothly-toned photographic detail only out to about 1440 divided by 6 (or whatever the matrix size is) per pixel. Which works out to 240 pixels per inch. That's what someone meant by saying the image is resampled on the fly by the printer software to 240 or 360 or so. It is. The 360 is made possible by the multiple inks, which extend the tonal range to 'photographic' even with only 16 dots per ink per pixel.


The point is: don't confuse pixels-per-inch in your image with dots-per-inch in the printer. They are not the same thing at all, and there isn't even much of a relationship between the two - any more that the size of the sand grains on a beach (printer dots) determines how many cars can be parked on it (image pixels).

A minor note: the above is for inkjet type printers. Die Sublimination printers are different animals that print in layers and actually print in dpi.


With all that said I would still scan at the maximum that you can. If nothing you may find out later you want to crop the picture down.

You might want to try some other formats to get a little extra space. TIFF is standard but other formats are newer and may give better results.

[bobtrips]

I'm with graphicsmanip and castledude on this one. And I'm guilty of getting it wrong. It would be a very good idea if we would correctly use ppi and dpi. It will possibly prevent future misunderstandings.

So, here's my take. Think about why you are scanning.

Scanning for prints? Get as many ppi from your negative or transparency as your scanner and media allow. Remember that while your scanner might detect one of those mythical zillion angles on the head of a pin, you do reach a point where you are no longer scanning detail but grain. There's no reason that I can see to spend scanning time and use up storage space by scanning a clumpy film at extremely high ppi.

Scanning for viewing? Why bother with extremely high ppi scan rates when you only need a couple megs of info at the most? Scan at a high enough ppi rate to give yourself some room to straighten/crop and still end up with a decent sized file. If you're like me you've got lots of good memories that you will never want to print on 8"x10" paper. (This suggestion is for those of us with thousands of slides/negatives and a limited lifespan.)

Printing? Learn about your printer. Some inkjets have 'sweet spots'. While the 'ideal' might be a 300 dpi print, your printer might actually do better at 280 dpi or 320 dpi.

(OK, did I get it right this times? )

[graphicsmanip]

Bob my system actually does a great job of printing with an image resolution of 200 ppi. I'm using Photoshop 7.01 and a new Canon i960 printer. My old Epson 1270 did a great job at that resolution too(when I wan't cleaning the print heads)..

Most print shops want a 300 ppi image to work with.

You aren't the first and won't be the last person to confuse dpi and ppi..

[legolasz]

Does anyone has experience with sending scanned images to a professional photo printing service? Are the results worth the money?

And how is the quality of prints on photo paper compared to prints of a professional photo printing service? Which of the two last longer?

Regards,
Hans