magic number 72

"Paul Bullen" wrote in message

I was recently set straight about resolutions of 72 dpi or ppi being irrelevant to preparing images for the Web […] But I now notice that the first reference I was given […] has this: "If your image is intended for use on the Web, and not for printout, …set your resolution to 72 ppi…." I also notice that most of the books at Borders about how to use Photoshop Elements give the same advice. Am I missing something?

It matters. It doesn’t matter. It’s both at the same time.

Sorry, it had to be said. 🙂

One of the reasons it matters:

* Using "72" for the resolution when scanning images will usually produce an image of about the right quality for display on a computer monitor

One of the reasons it doesn’t matter:

* As has been mentioned here recently (I forget who said it, otherwise I would have credited him…I think it was a him), all that really matters to the web browser is pixels. I’m not aware of a single web browser that attempts to reconcile the video driver’s reported display resolution with an image’s encoded resolution.

The page on www.scantips.com is a bit misleading. It’s true that setting the resolution in an image doesn’t affect how that image is displayed on the screen (usually…very few programs bother to look at that). But using 72 dpi (or something similarly low) when you scan an image will avoid wasting time and disk space scanning at a higher resolution.

What I do when scanning an image that I want for video display is to decide before I scan (if I can) how many pixels I want it to be. Then I set the scanner resolution appropriately so that the image I’m scanning comes out to be the right number of pixels. For example, if I’m scanning a 4×6 photo print and I want to fill a 640×480 screen (note that the width and height are reversed when talking about prints version talking about display screens 🙂 ), then I’ll want a resolution of anywhere between 100 and 120 dpi (using 120 dpi will allow me to crop the scanned print so that it fits just right on a 640×480 screen; anything smaller and I’ll have to stretch the image at some point or leave a border on the top and bottom).

As far as web pages go, the best you can really do, using basic HTML, is to keep your page in proportion and let the user know what a good resolution to view the page would be. It’s theoretically possible to use some server side code to scale images and other page content to be appropriate to the size of the browser window but in practice, I’m not aware of any sites that do this. It’s a bit beyond what most people putting together a web site want to bother with.

However, even in the case where I’m setting the scanner resolution (two paragraphs up) and the case where the server side code feeds appropriately-sized images, dpi doesn’t really matter. What you really care about is how many pixels the image is versus how many pixels the screen is. You have no information regarding how large the screen is, so it would be pointless to try to factor in the screen’s resolution. You just don’t have that data.

Sorry…I think I’m kind of rambling. The bottom line is this:

When scanning the image, you might as well aim to get the pixels into the computer at the exact size you expect to use the image. This fits in with your other question about resampling: you are correct, if you want to change the number of pixels in an image, you have to resample. You should avoid resampling, which is why it’s better to scan at the correct resolution in the first place.

Once you’ve got the image in the computer, with the number of pixels you want, the dpi resolution of the image is irrelevant. None of the programs showing the image on the screen will bother to look at the dpi resolution and you can set it to whatever you want. The only place the dpi would matter is if the image gets printed, and even there, most people wind up scaling images to some other specific size for the purpose of printing, which winds up overriding the dpi set in the image. You can consider the dpi set in the image as a suggestion, and nothing more.

The most important thing, with respect to how good it looks when printed and with respect to whether it fits in your web site, is how many pixels are in the image. That’s the one thing that is invariable. It’s the one thing that is a fundamental characteristic of the data contained in the image. Everything else just relates the data in the image to the human concepts of physical manifestations.

Pete

p.s. One other reason to set an image not intended for printing to "72 dpi" is simply as a reminder to other humans that might have access to the image that it’s not intended for printing. An image that’s really only 72 dpi is going to look terrible on a printout. Again, this isn’t something the computer cares about…it’s just a way for one human to convey a message to another. 🙂

"Peter Duniho" wrote in message

What I do when scanning an image that I want for video display is to

decide

before I scan (if I can) how many pixels I want it to be. Then I set the scanner resolution appropriately so that the image I’m scanning comes out

to

be the right number of pixels.

I should probably have mentioned (if it weren’t for the fact that the post was already so long) that setting the resolution on the scanner often results in "resampling" too. The scanner has a "native" resolution and anything other than that gets resampled. The difference is that the scanner has a lot more to work with and can do a much better job with the resampling.

Pete

Pete,

First – thank you for the time and for the detailed information you have provided.

I, for one, am still confused. Here is my story:

I decided to scan-archive many of my older B&W photos.. They come in various sizes (up to 5 x 7).

Since this is a one time effort (took me almost 2 days… for some 700 or so B&W pictures), and in order to save them for good, I decided to scan them at 600dpi. This was a compromise resoulution which I thought would still serve my kids, and their kids therafter….

BTW – I scanned aprox. 5 at a time (on my Espon 2400)

Now, my old High School, has a web site on which we meet and to which many of us uplod their odl photos to share with others. It accepts JPEG formated pictures only (I belive that it takes GIF too, but am not sure…)

I selected 10 – 15 old B&W photos, which I jsut scanned at 600 dpi into PE-2, and saved them as JEPG, Quality of JPEG was set to 10, and uplodeed them into that HS Web site. I am not in control of, nor do I know what resoluitions are set on the viewer’s monoitors. Some wrote to me to tell me that the phoptos were too large, and that they could see only samll portions of my image, and would need to manipulate this etc… Not good !!

My question thus is;

Not bening able to tell what the Web size orgnizers and the viewers need in terms of resolution, what would you suggest that I do with my original 600 dpi image if I want to upload it to sich a Web, Should I JEPG it, or "Save to Web" it ? – (what is the difference?), If I JEPG it – what scale of compression should I set it to ? (Rememebr, all were scanned at 600 dpi, but they come in various sizes up to the 5 x 7).

I wouldn’t want to re-scan these and more which I still have. Prefer to use the scanned files I have alerady.

Thanks for any detailed reply.

ZR

On Thu, 4 Sep 2003 23:32:27 -0700, "Peter Duniho" wrote:

"Paul Bullen" wrote in message

I was recently set straight about resolutions of 72 dpi or ppi being irrelevant to preparing images for the Web […] But I now notice that the first reference I was given […] has this: "If your image is intended for use on the Web, and not for printout, …set your resolution to 72 ppi…." I also notice that most of the books at Borders about how to use Photoshop Elements give the same advice. Am I missing something?

It matters. It doesn’t matter. It’s both at the same time.
Sorry, it had to be said. 🙂

One of the reasons it matters:

* Using "72" for the resolution when scanning images will usually produce an image of about the right quality for display on a computer monitor

One of the reasons it doesn’t matter:

* As has been mentioned here recently (I forget who said it, otherwise I would have credited him…I think it was a him), all that really matters to the web browser is pixels. I’m not aware of a single web browser that attempts to reconcile the video driver’s reported display resolution with an image’s encoded resolution.

The page on www.scantips.com is a bit misleading. It’s true that setting the resolution in an image doesn’t affect how that image is displayed on the screen (usually…very few programs bother to look at that). But using 72 dpi (or something similarly low) when you scan an image will avoid wasting time and disk space scanning at a higher resolution.

What I do when scanning an image that I want for video display is to decide before I scan (if I can) how many pixels I want it to be. Then I set the scanner resolution appropriately so that the image I’m scanning comes out to be the right number of pixels. For example, if I’m scanning a 4×6 photo print and I want to fill a 640×480 screen (note that the width and height are reversed when talking about prints version talking about display screens 🙂 ), then I’ll want a resolution of anywhere between 100 and 120 dpi (using 120 dpi will allow me to crop the scanned print so that it fits just right on a 640×480 screen; anything smaller and I’ll have to stretch the image at some point or leave a border on the top and bottom).

As far as web pages go, the best you can really do, using basic HTML, is to keep your page in proportion and let the user know what a good resolution to view the page would be. It’s theoretically possible to use some server side code to scale images and other page content to be appropriate to the size of the browser window but in practice, I’m not aware of any sites that do this. It’s a bit beyond what most people putting together a web site want to bother with.

However, even in the case where I’m setting the scanner resolution (two paragraphs up) and the case where the server side code feeds appropriately-sized images, dpi doesn’t really matter. What you really care about is how many pixels the image is versus how many pixels the screen is. You have no information regarding how large the screen is, so it would be pointless to try to factor in the screen’s resolution. You just don’t have that data.

Sorry…I think I’m kind of rambling. The bottom line is this:
When scanning the image, you might as well aim to get the pixels into the computer at the exact size you expect to use the image. This fits in with your other question about resampling: you are correct, if you want to change the number of pixels in an image, you have to resample. You should avoid resampling, which is why it’s better to scan at the correct resolution in the first place.

Once you’ve got the image in the computer, with the number of pixels you want, the dpi resolution of the image is irrelevant. None of the programs showing the image on the screen will bother to look at the dpi resolution and you can set it to whatever you want. The only place the dpi would matter is if the image gets printed, and even there, most people wind up scaling images to some other specific size for the purpose of printing, which winds up overriding the dpi set in the image. You can consider the dpi set in the image as a suggestion, and nothing more.

The most important thing, with respect to how good it looks when printed and with respect to whether it fits in your web site, is how many pixels are in the image. That’s the one thing that is invariable. It’s the one thing that is a fundamental characteristic of the data contained in the image. Everything else just relates the data in the image to the human concepts of physical manifestations.

Pete

p.s. One other reason to set an image not intended for printing to "72 dpi" is simply as a reminder to other humans that might have access to the image that it’s not intended for printing. An image that’s really only 72 dpi is going to look terrible on a printout. Again, this isn’t something the computer cares about…it’s just a way for one human to convey a message to another. 🙂

Paul, a few more comments on this one.

There is nothing fundamentally wrong with the texts and tutorials that use 72 ppi as a benchmark; most people are more comfortable with thinking of their image as having linear (inch) dimensions rather than pixels. If I scan an 8 inch by 10 inch picture at a scanning resolution of 72 ppi, I’ll wind up with an image that’s 576 pixels by 720 pixels, which isn’t a bad size for web or e-mail use. The point is that it’s ultimately less confusing (in my opinion – others may see it differently) to think of your picture the way the computer sees it – as a matrix of pixels.

Where you can get fouled up is if you assume that because you’ve set the resolution to 72 ppi, you’ll get a certain inch-size picture on your monitor – not so. If I have a 13-inch monitor set at 1024×768 (pixel) resolution, a 72 ppi picture will appear smaller in inches than it would on a 19-inch monitor set at the same 1024×768 resolution. That’s because the pixels on the small screen are more compacted than those on the large screen. So what is 72 ppi all about? Some have opined that it was the original resolution of some early computer, perhaps the first Mac. It has become a convenient standard for creating approximately web-sized pictures, but there are few if any monitors out there that have exactly 72 ppi at a particular resolution.

That’s why I always go back to thinking of computer images in pixel dimensions. I know that an 800-by-600 pixel image will fit in landscape mode on my 1024×768 resolution monitor with a little border around it. If I want a portrait mode picture to fit, it needs to be something like 525×700.

And yes, if you start with an image that’s of greater pixel dimensions than the 800×600 web size, you do have to throw away pixels via resampling to achieve those dimensions. But if the image is going only be viewed full-screen, the pixels that are discarded wouldn’t display anyway. Under those circumstances, resampling isn’t a bad thing at all.

Enough for now…hope that helps

Chuck

Pete and Chuck. Thanks a lot. I think I need to get a technically dependable book in order to give myself a solid foundation.

Since you mention scanning, my father, who also has Photoshop Elements (as a result of my recommendation) is planning to get a scanner with which to put old family photos (including those of my grandfather in a Royal Navy jazz band in the 1920s and some earlier) on to CDs for the various children. In order to approximate what could be done by paying somene else to do it (a photolab), how much would he need to spend on a scanner? Based on what you two and others have said, I have already told him that it seems like it would be a good idea to make two scans of every picture: one at maximum pixels and one that would be of a size that would be immediately usable for looking at the picture on a computer screen. Is that right?

Also, do you need a special scanner for scanning negatives and slides?

Thanks.

Paul

"Paul Bullen" wrote in message

[…] In order to approximate what could be done by
paying somene else to do it (a photolab), how much
would he need to spend on a scanner?

IMHO, for home use — i.e. to "approximate" what a photolab might produce — pretty much any of the name-brand scanners available today ought to do a good job. For basic scanning, I’d get something with 1200 or 2400 dpi resolution (just because that’s commonly available — beware the difference between "optical" and "interpolated" resolution…"optical" is the specification you care about) and that has a transparency adapter for scanning slides and negatives. You can get as decent scanner with those features for $100-200. In the consumer market, all you get for more money is additional features that don’t really have much to do with the quality of the scan.

One thing someone else is going to have to answer is whether a scanner that can do better than 24-bit color matters. I’ve never run into software that deals with anything greater than 8 bits per color channel, but then I haven’t used any of the professional-grade photo editing packages. For sure, the best you’re going to see on the computer screen is 24-bits, but a) more color resolution might give a nice printer more to work with, and b) it would for sure give you more "overhead" for doing contrast/brightness/etc adjustments after the scan (would be useful for cleaning up over- or under-exposed photos, for example). You would need software that understands the extra color information though…as far as I know, Elements doesn’t.

Of course, even the low-end scanners these days claim up to 48-bit color support, so I guess this is kind of a moot point.

Personally, I’d buy a Canon or Epson, but that’s just because I have had great luck with their products. I don’t really know anything specific about them that would lead me to believe that they are better than the other brands (like HP, Minolta, Umax, etc.).

Keeping in mind, of course, that at a good photolab, you’re not just paying for the equipment. They have the expertise to produce the best quality scan, as well as fixing up any color issues after the fact. Which you can do also, but I know it would be a lot harder for a novice like me to produce exactly the same results.

Based on what you two and others have said, I have already told him that it seems like it would be a good idea to make two scans of every picture: one at maximum pixels and one that would be of a size that would be immediately usable for looking at the picture on a computer screen. Is that right?

Not really. There’s no reason to bother scanning at a high resolution if all you’re doing is screen stuff. But if you want to print the scan and display it on the screen, you can scan it once at a high resolution suitable for printing and then use something like Elements to create a smaller version of the scanned image for display on a web site or as an email attachment.

You might try scanning two versions with a few pictures, just to see if the scanner does a noticeably better job resampling than Elements does. But my experience is that for the purpose of showing the image on a screen, it’s not necessary to get really picky.

All IMHO, of course.

Also, do you need a special scanner for scanning negatives and slides?

I hope I answered that question above. 🙂 One thing I’ve been thinking about is whether for the highest quality scan, it would be better to scan a high quality enlargement of the photo (giving you more pixels to work with) or if the fact that scanning from a negative is basically first-generation would compensate for the reduction in effective resolution. Again, this is an area where I lack expertise, and hopefully someone else can answer. That said, I would get the transparency support just for the convenience factor, even if the highest quality scans actually came from enlargements. (When I say "enlargement", I’m including even the normal print sizes, like 4×6)

Pete

Pete, Photoshop 7 handles 16/48 bit color, albeit with a limited set of program features available. Will be interesting to see if the next generation of editing products have enhanced capabilities in this area.

Chuck

Paul, I’d like to step in with my .02 worth, since I’ve been working on the kind of project your dad is starting for almost three years now. I can’t say I’ve learned all there is to know, but I’ve definitely had some experience and picked up some pointers.

I’ll start by saying that I checked into having the old slides professionally done. Not only was it an expensive proposition, there are few people who even do it, at least around here. I’m glad I made that decision, because – using Elements – I’ve been amazed how how much life could be brought back into old faded slides that I probably wouldn’t have even taken to anyone for scanning. It would have been a shame to lose out on some of those pictures.

Have him count up the number of slides he wants to scan. There are a number of consumer flatbed scanners with adapters, but if he has many to do, he might want to consider a dedicated scanner. I believe a Minolta Dimage is what several people on the forum have chosen. Mine is an old HP that’s no longer available. Scanning slides for the purpose of printing can be tedious, because they do have to be scanned at a high resolution since they’re so small. A dedicated scanner wouldn’t necessarily be faster on a per slide basis, but handling the slides is very awkward, and if he has thousands to do – like some of us – he’s going to get tired of fiddling with them fast. The dedicated scanners like the Minolta do four at a time I believe. They also scan at a higher resolution than many flatbeds.

Also, I see no reason to do two scans. (He’ll undoubtedly find this project takes longer than he thought anyway – no point in doubling the work!) One high quality scan, archived, can then be used for either prints or monitor display. Resampling downward, using Elements Save for Web feature for example, is fast and easy. Scanning isn’t the fun part – that comes with the editing and sharing.

Choosing bit depth of scanners will probably be a moot point. I have a cheap little Canon flatbed USB that’s 48 bit. It doesn’t produce a scan any better than my ancient Umax that’s 16 bit. As a matter of fact, I haven’t analyzed why, but the old scanner seems to produce a sharper image than the new one. You might, again depending on how much scanning he thinks he’ll be doing, want to steer him toward one that has either a firewire or USB 2 connection. My USB Canon is slow as molasses in January compared to the old (obsolete) SCSI scanner.

And invite him to start hanging out on the forum. There are several of us here who are working on projects similar to his. It’s a great place to get help.

Thank you all very much. I will encourage my Dad to join the forum when he gets down to work. In fact, I have forwarded your comments to him.– Paul

but the old scanner seems to produce a sharper image than the new one.

Hi, Beth. Just saw this. I think it’s because canons tend to be a little soft overall. It’s the one thing I’m not crazy about about my n1240U and I noticed when I was reading the reviews of the S400 camera that several reviewers noted that canon temds to "conservative" about these things and so I expect the camera images may tend that way a bit as well.

Oh, this is so interesting, as I have many hundreds of old slides, some taken in the Middle East and world wide, back in the ’50s by my father-in-law, my old underwater ones, and many others that are really neat.
So far, I have been using my Nikon digicam with an aftermarket gadget that actually takes a picture of the slide in Macro mode. It really does a great job, but it is very very time consuming. If I could do this with a group of slides, I would be happier than a,,,,,,,really happy.
I will be watching how you all do this, as I am still a beginner at all this. Thanks,, Jane

I’ll just add to Beth’s comments on film scanners. You will get MUCH better quality scans of negatives and slides from even a low-end film scanner…as compared to a film adapter on a flatbed.
There are two reasons for this: First, the resolution is higher. My Minolta scans at 2820 dpi. The max resolution of most flatbeds is 1200. Some claim 2400, but that’s interpolated, not native.
The second reason is that film scanners have greater dynamic range. That means they can capture a blacker black. If you blow up a dark area of a color slide scanned with a flatbed scanner, it looks like somebody scattered colored confetti on it. That’s electrical noise in the scanner electronics. Most film scanners have lower noise levels.
So, if you have many slides or negs to scan, a dedicated film scanner is a good investment. As an example, you can buy a Minolta Dimage Dual Scan III for around $300, maybe a little less.
Bert

I might as well add my two cents on the "72" issue. I am a disciple of Wayne Fulton and his book, "A Few Tips on Scanning" which I have a copy of. He says the only possible reason to set resolution to 72 dpi is to get an approximate idea of the size of the image on a monitor. As others have pointed out, though, the size will depend greatly on both the physical size of the monitor and the resolution setting.
FWIW
Bert