Trouble understanding how resolution effects document size

[g._snow]

<p>OK. I've never been able to wrap my brain around the (mathmatical?) relationship between dpi, document size, and pixel dimensions. I need help understanding, or correcting, a problem a recently ran into. I only use PS for post production.<br>

I shot my first wedding last month and went to burn her images to disc yesterday. When trying to decide which action to record to use batch processing (I wanted to give her an additional folder on the CD she requested that contained smaller size images for email, upload to facebook, etc.) My first thought was to simply change resolution of original files to 72dpi and be done with it. But I was shocked by some of the image sizes of my final, edited, original size prints. I finally realized that the images I cropped using crop tool set to 2:3 or 3:2 ratio and resolution left emply (I wanted to crop some images but maintain original aspect ratio, because I did not know what size print she would be making) where the ones that resulted in image sizes I don't know how to deal with.<br>

For example, an image that originally was 240dpi and aprox 3008 x 2000, once cropped, resulted in these dimensions: pixel dimensions, 2930x1953; document size, 3x2 in; resolution, 976.667.<br>

I'm not particularly concerned about print quality. I think I understand that I haven't lost any pixel information (besides what I cropped out.) But how do I change the resolution of these quarky images back to 240 and maintain original print size and quality? Is there some math I need to sort out?<br>

If anyone could help me solve this headache I would greatly appreciate it. There's got to be a simple solution, right? Please tell me there's a simple solution!</p>

[johnw63]

<p>So, you're saying that by cropping slightly, Photoshop changed the resolution from 240dpi to about 975 dpi ? That doesn't make sense. You could be able to alter the size without affecting the resolution. I'll have to wait for someone to explain this too.</p>

<p> </p>

[rjjackson]

<p>h'm, let's see. think of the resolution as a relationship between the real size of pixels and a printing device. so it works like this: an image has a certain number of pixels measured base times height. the base and height determine the real size of the image. now, the resolution tells a printing device how many of these pixels get put into a unit of measurement. for example, 300 pixels per inch is very common for small prints, or 200 pixels per inch for very large. resolution does not directly determine the size of the image. </p>

<p>to make a smaller image, you need to change the real size of the image, that is, the number of pixels. if you change the resolution when "resample image" is selected, this will keep the same relative resolution, and therefore necessarily change the real image size. this is working backwards. you really only need to worry about pixel dimensions and a suitalbe resolution for printing. you can look at an image on the web that is 400px by 600 px and a 900 dpi. it will be a very small jpg, despite this high resolution. if printed, the printer will print the picture with 900 pixels within one inch (so 2/3 of inch wide).</p>

<p>also, the crop tool can be useful but i tend to only use this when i'm finalizing an image for printing, because i can set the pixel dimensions to the real size of the paper and the resolution to the optimum for that printer.</p>

<p>it might help to sort out a workflow where you export raw files at 300 dpi, for example, into whatever working size you prefer. then you can create an action to resize for smaller images (web viewing, etc). the action should change the size by selecting image > image size > height OR width. resolution will remain the same. for webviewing, 500 px to 800 px wide is plenty. </p>

<p>when i'm giving someone a CD of images, i probe a little and try to find out what image sizes they are likely to print at, should the occasion arise. then i export a few variations for them with appropriate resolution and adjustments for that print size. for a batch of 10 to 20 images this isn't much extra work.</p>

<p> </p>

[dbcooper]

<p>A good article: http://www.rideau-info.com/photos/mythdpi.html</p>

<p>PPI relates to the image size like this: an 8" x 10" photo at 300 ppi needs to have a resolution (size) of 2400 x 3000 pixels. (8 inches x 300 pixels per inch = 2400 pixels; 10 inches x 300 pixels per inch = 3000 pixels). A 2400 x 3000 pixel image at 200 ppi would measure 12" x 15" (2400 pixels / 200 ppi = 12"; 3000 pixels / 200 ppi = 15"), and so on.</p>

[rjjackson]

<p>hey, mr cooper, where'd you land after all? :)</p>

<p> </p>

[kari v]

<blockquote>

<p>But how do I change the resolution of these quarky images back to 240 and maintain original print size and quality?</p>

</blockquote>

<p>DPI setting doesn't matter here, you can enter whatever you want in the dpi field when you have resample box *unchecked*. If you want to change 976 to 240 just do it but it makes no difference.</p>

<p>You got 976 dpi because you did 3:2 crop and crop tool had inches selected. Now Photoshop thinks you wanted to print 3x2" image. Pixel dimensions (actual file resolution) are still the same but for 3x2 print you have 976 dpi.</p>

<p>When you select dpi for intended print size and want to change file resolution you have to work in inches and resample box checked.<br /> If you want high quality 4x6, select inches, enter 4x6 and 300dpi. That gives you 1800x1200 file.<br /> I suggest you use bicubic sharper resample method unless you want to sharpen every image separately.</p>

<p>For web use forget dpi totally, monitors don't work with concept of dots and inches. Only pixel dimensions matter, 800x533 is ok. DPI field may read something very silly but leave it as is, monitors show pixels.</p>

[dbcooper]

<p>@robert: sshhhh! ;)</p>

[James G. Dainis]

DPI is a print command. It doesn't have anything to do with resolution. Resolution means image size. A 3000 x 2000 image has a resolution of

 

3000 times 2000 = 6,000,000 pixels or 6 megapixels (MP). I doesn't matter what the DPI says.

 

If you were to print it at 300 DPI you would get a 10 x 6.67 inch print.

 

3000/300 = 10, 2000/300 = 6.67

 

If you were to print it at 200 DPI you would get a 15 x 10 inch print

 

3000/200 = 15, 2000/200 = 10

 

 

You size the image by how you want it to appear on the monitor screen. If the monitor is 1280 pixels wide, you would size the image to 640 pixels wide to fill 1/2 the screen width. It doesn't matter what the DPI is. It could be 300, 200, 100...etc and the image would still be 640 pixels wide.

[James G. Dainis]

I think where most people go wrong is they go to the image editor and change things in the Document section of editing. "Document" means the print that one is making. If the print (document) size is 10x8 and the print Resolution is 300 PPI/DPI the image pixel dimensions will be 3000 x 2400.

 

If the print resolution is changed to 200 PPI/DPI the image pixel dimensions will change to 2000 x 1600 to maintain getting a 10 x 8 inch print at that setting. That isn't the way to change image size for display on the web. You directly change the pixel dimensions for web use or monitor display.

[g._snow]

<p>Thanks everyone for your comments. I have learned alot, but now I have more questions.<br>

Robert, yes, I am trying to set up a workflow where I can run actions with batch processsing to use for burning CDs. I thought I would change resolution to 70dpi for smaller images I don't want clients to reproduce, because I thought that would be too low res for print output. So if I record an action for batch processing that changes pixel dimensions to 500px by 800px for all images, will this distort any images? How does ps know if an image is 2:3 aspect ratio or 3:2?<br>

DB, thanks for the article. These kinds of equations is what I was looking for. If the pixel width of image divided by the dpi = the width of print, I was about to give this client an image that would only be suitable for a 2.9x1.9 print!!<br>

Kari, yes, I used crop tool set to 3:2 inches and left resolution box blank because I wanted to maintain an aspect ratio similiar to standard size print because I had no idea what the final print size would be (she asked for all images on CD and will print them herself). Thanks for explaining to me why the crazy dpi/ppi change, I wasn't expecting that! Do you suggest a better way of cropping to maintain an aspect ratio when you don't know final print size that doesn't involve the extra step of having to correct the dpi in image size menu.<br>

James, there is still one thing I'm confused about. I thought it was best not to print anything at less than 300 dpi. Are you saying that I can print at lower dpi as long as the pixel dimensions of image (resolution?) meet the final print size (3000/200 = 15, 2000/200 = 10 for a 15x10 print)?</p>

 

[rjjackson]

<p>i think the tool you need is the rectangular marquee selection tool. this lets you make selections in a fixed aspect ratio. once the tool is selected, thenn in the top of the photoshop window you'll see an option for "style". in this pull down menu, select "fixed aspect ratio" then type in the ratio you desire. this will leave the dpi alone -- really you don't have to bother with dpi until you print. (if you're not printing, export raw at 300 dpi and forget about it.) once you've used the marquee tool to select something in the same aspect ratio as you want, just select image > crop, and you'll have cropped the image.</p>

<p>the second issue for changing the pixel dimensions is that for horizontal and vertical shots you'll have to run two separate batches or rotate one of them so they are all the same direction. then you run a batch to change image size to the specified width, say, 900px wide.</p>

<p>feel free to email me if you need more help.</p>

<p>rj</p>

 

[James G. Dainis]

Most pros print at 300 DPI/PPI That means each printed pixel is 1/300th of an inch in size. I think one hour print places print at 200 DPI/ PPI. That means that each printed pixel is 1/200th of an inch in size. That is still small enough so you can't discern the square pixels that make up the print. If you start printing at 100 or 50 DPI/PPI then you will see the tiny 1/50 inch square pixels, like a mosaic made of tiny tiles.

[James G. Dainis]

"Do you suggest a better way of cropping to maintain an aspect ratio when you don't know final print size..."

 

Even with film frames that was always a problem. A film frame is 24 x 36mm or 2:3 ratio. That is fine for a 4x6 print but to make an 8x10 print you get 8x12 inches and have to crop off one inch from each side (or two inches from one side, etc). I would just keep a 2:3 ratio, the actual original image ratio and let the person printing decide what to crop off.

 

I don't know why print sizes are given in length by width (4x6, 5x7, 8x10, etc) and image sizes are given in width by length (640x480, 1280x960, 3000x2000, etc.) but it is all the same thing. To avoid confusion I try to keep the same width length nomenclature. Thus:

 

An 1800 x1200 image printed at 300 PPI will give a 6 x 4 inch print. It still is what we call a 4 x 6 inch print but it seems a little clearer stated that way.

[g._snow]

<p>Robert, that makes sense. In the future I will save my final images in two seperate folders, one for horizontal and one for vertical, and run seperate batches. My goal here is to give the client who asks for images on CD a folder with original large files for printing and a folder with smaller files in case they are going to email images to friends and family or upload to facebook, etc. I want the smaller images to be too small to print. I'd rather drive people to my website and purchase prints from there. 900px seems a little big. If that is so, can you recommend dimensions that would be inappropriate for print but still OK for web use?</p>

[rjjackson]

<p>i think between 500 and 700 px (on the longest side) is ok. someone with determination could still make at least a small print from these, though. i mentioned above that i give clients images ready for printing. well, my reasoning for this is that the print they make ultimately reflects upon me so i want what they show people to be the best possible. i give only images that are edited properly and finished and for printing. (usually i provide a set of prints free, since it's a small part of the cost.) this just seems to make everyone happy and saves me from worrying too much. others of course may disagree.</p>

<p> </p>

[silvermeat]

For resizing mixed orientation images, the last N photoshops have had a "Fit Image" command under File>Automate.

You give this a width and a height, and it will resize the image to fit, at its original aspect ratio, within the rectangle

specified. Used as part of a batch process, it will resize both portrait and landscape images correctly.

(The only thing you miss out on with this is the newer option to specify the bicubic-smoother/-sharper interpolation for the

scaling.)

 

If you are outputting for your client's use on the web etc, using Save for Web will automatically set the image to 72 px/i

without affecting the actual pixel dimensions. (This strips out all EXIF data. To preserve that, use Save As>JPG)

 

As for the target pixel dimensions, I would recommend 640 on the long edge (so feed 640x640 to Fit Image) - it's a common size, yields small filesizes,

divides well, etc. If you know a little bit about the specific channels along which your client will distribute these, at least

initially, you can make a more targeted decision, but it's likely to fall near there anyway.

[andylynn]

<p>Here's what I do:</p>

<p>NEVER, under any circumstances, make any change to the "pixels/inch" setting of a file (AKA PPI - this is the setting that people mistakenly refer to as DPI which is actually a property of the printer) in Photoshop. It is meaningless. Most software ignores it.</p>

<p>What is meaningful is the pixel dimensions of the image in pixels wide by pixels high. This is how much information your image contains, in pixels. Also meaningful is the size the image ends up getting printed at, which you choose when you print. The PPI you end up printing at is the pixel dimension divided by the print size. So say you have an 1800x1200 JPG file and print it at 6"x4". It printed at 300 PPI.</p>

<p>The reason I never touch the setting in Photoshop is that it can lead me to make a mistake and resample the image (convert it to some other number of pixels) inadvertently, and resampling a photo is usually not something I woud do. But when sending small size files to people you may need to resample, in which case you set the pixel dimension, not the DPI.</p>

<p>BTW when you record the image resize in an action it records the number you typed in, if you only type in one dimension the other one will be determined based on the original image proportions.</p>

[James G. Dainis]

DPI, dots per inch, is supposed to mean how many ink dots per inch a printer sprays. That is a fixed characteristic of the printer. PPI, pixels per inch, is how many pixels the printer prints per inch. If you could set the printer to 1 DPI, it would send out an ink dot one inch in diameter. If you set it to 1 PPI it will print one pixel per inch but that one pixel will be made up of 1200 ink dpts if you have a 1200 DPI printer.

 

Since a small pixel looks like a tiny dot many people, and software call it a dot and use, incorrectly, dots per inch or DPI. That is why I often have to use "DPI/PPI " when explaining something.

[kelly_flanigan1]

<p>What matters is the number of pixels in an image; or the amount of dollars you pay for your cellphone bill; or the number of pounds loaded on an airplane at takeoff.<br>

Worrying just about ppi or dpi without the number of pixels is wonky; worrying about whether our cell phone bill was paid with pennies; dollars; or twentys is wonky too if one ignores the actual total bill's cost.<br /> <br /> Tell a friend your got a new job and are paid in twentys and be real excited about the job; when they ask how much you make; just say I get paid in twentys! Folks who just mention ppi or dpi WITHOUT the print size or total number of pixels appear just as goofy; dwelling on a variable that doesnt really matter.<br /> <br /> rant on:) The useage of dpi with scanning pitch of scanners is older than Photo.net or Photoshop; older than most folks that write photoshop books; its used in fax machines for many decades; it is in US Patents 4 decades ago. Here the term *dpi* is used with scanners; since thats what my scan wond used back in 1984; or our 1988 digital 35mm slide scanner for a Mac; or many flatbeds and film scanners.<br /> <br /> If a customer of mine says they scanned their 35mm negative in a Nikon film scanner at 4000 ppi; I know what they mean. A farm store in Middle of Nowwhere Iowa probably knows many slang terms for seeds, manure, bailing wire, corn too. What matters is understanding the other person; few if any folks will actually change the terms they use in daily life. Thus a person who got started in digital 5 years ago really is not going to change my usage of dpi with scanners; or impress me by saying I am wrong. Where were these folks in 1988 when I scanned 35mm stuff with software that had dpi settings; they were off shooting film. They probably were not also writing Basic programs that turned pixels/dots on and off on a computers screen; saving the program on a cassette either. rant off :)<br /> <br /> If one tells a client; customer; friend, print shop that one has a VGA image (480x640 pixels) the image's size is clear. It is like saying one paid 20 bucks for gasoline to fill up the car. Saying one has an image that is 4x6 inches at 300 ppi/dpi is clear too; one has an image that is 1200 by 1800 pixels. Saying one has an image at 72 ppi/dpi; 300 dpi/ppi has no real info; it just has the *tag*. It is like saying one bought gasoline with 1 dollar bills; WITHOUT mentioning the number of bills.<br /> <br /> As a print shop I get daily questions by folks who say they have a 72 dpi image and what to make a GOOD giant poster ; or XYZ size print many times a day. Often the cannot say what camera th image is from ; or what the number of real pixels the image has. It seems many folks *focus* on what the image is tagged at; and cannot bound even withing a factor of ten what the number of pixels is. AS a trend I would say the issue is way worse than long ago.</p>

<p>In printing I have received proof CD's from clients that are suppose to be low res images for dinky prints; but are the full bore full res images. There are many folks who dwell on dpi and ppi and ignore the mount of pixels; even many pros cannot understand this.<br /> <br /> *****All the images below have the SAME number of pixels; ie what really matters.<br /> <br /> Like paying a plumber; what matters is the total bucks; the total dollars. The plumber doesnt care if you pay him 10 Twentys; 4 fiftys; or 2 hundreds, or 20 tens; all four combos are spendable at Walmart .<br /> The image below has the same number of pixels; first one is at 1 pixel per inch;</p>

<p><img src="http://i4.photobucket.com/albums/y148/ektar/desktop/OLDGOAT1.jpg?t=1238246171" alt="" width="539" height="532" /></p>

<p>This is at 72 pixels per inch:</p>

<p><img src="http://i4.photobucket.com/albums/y148/ektar/desktop/OLDGOAT72.jpg?t=1238246516" alt="" width="539" height="532" /></p>

<p>This is at 300 pixels per inch:</p>

<p><img src="http://i4.photobucket.com/albums/y148/ektar/desktop/OLDGOAT300.jpg?t=1238246649" alt="" width="539" height="532" /></p>

<p>This image is at TEN THOUSAND PIXELS PER INCH:</p>

<p><img src="http://i4.photobucket.com/albums/y148/ektar/desktop/OLDGOAT10000.jpg?t=1238246747" alt="" width="539" height="532" /></p>

[kelly_flanigan1]

<p>Not everybody's brain is wired to understand analogys.<br>

<br /> IF the original image is say 2000x3000 pixels out of your camera; if you give a CD that has images at 2000x3000 pixels tagged at 72 dpi/ppi; 300 dpi/ppi; 1 ppi/dpi; 10,000 dpi/ppi the client still gets the WHOLE image; no matter how the images dpi/ppi is tagged.<br>

<br /> In the paying the plumbers case if you pay him 200 bucks; it really doenst matter what combo of paper bills he gets paid in. He still has 200 dollars if you pay him 200 as 10 twentys; or 200 as 4 fifties. He might not like Fifties if in the Deep South; or maybe some folks might no like images tagged at 13 or 666 dpi/ppi either.<br /> Focus on the size of the image in pixels you give a client; the number of actual dollars you pay a plumber or for your cellphone bill. Changing the dpi/ppi tag; or number of paper bills doesnt matter; thus avoid dwelling on this useless stuff.<br>

<br /> If one goes the the store to buy 144 eggs for a breakfast event the Cook doesnt care if you have to buy twelve cartons of a dozen eggs; or eight cartons of 18 eggs. The cook wants 144 eggs; ie like wanting 144 pixels for a logo on a webpage. *worrying* about the dpi/ppi ONLY is like worrying about the number of eggs per carton; hotdogs per container without worrying about the goal at hand.<br>

<br /> Many folks are *fixated* on just the dpi/ppi of an image; thus they might miss the cooks task of buying 144 eggs; and *fixate* on the number of eggs per carton; Lbs coffee per container; hotdog Buns per package; liters cola per package.<br>

<br /> In the end what matter is that there are enough buns per hotdogs; maybe the cook wants 144 total buns and hotdogs. One buys 144 of each.<br>

<br /> If one gives a client a 2000x3000 pixel image it really doesnt matter if one retags in a zillion ways and still gives him a 2000x3000 pixel image.</p>

[andylynn]

<p>Are photo/print shop customers usually really dense? I ask because I keep having these experiences, like the other day when I had got some film back from one shop, processing and scans, and I wanted high resolution copies (they gave me 2MP JPG files at under 1MB) but the woman at the one shop had no idea what any technical term involving "mega" meant so I went to another one where the people know what they're doing.</p>

<p>I could not, for the life of me, get the guy to tell me what resolution and file size he could give me. We went back and forth for a long time with him saying things like "I don't usually talk about resolution because it confuses people" and "We can do regular, medium or high quality" and me saying things like "Really, I know what I'm doing and I just want to know what resolution you can scan my negatives at!" Finally got him to say that medium is 3000 DPI and max JPG settings.</p>

<p>It's like they're so used to dealing with clueless people, they can't answer a straightforward technical question.</p>

[silvermeat]

I don't know that it's a function of density as much as just really not caring. And what you and I can see as the simple

mobile of bits of information abut an image, and how they are all interrelated, requires a huge field of background knowledge

which people can't be bothered with. Just like there are fields like plastics or dressmaking or auto mechanics where I am

equally uninformed and only able to learn rote "rules" without trying to get the feel of the entire field.

 

Which can be a really bad thing, if someone drilled into this hypothetical dense customer's head "you want 300 dpi" -- now

your job in getting across just how irrelevant "dots per inch" might be to their application got WAY harder -- people don't like

to be wrong, even if it is about rules of thumb they internalized from a less than ideal source 10 years ago.

 

The bad part is when people whose job it is to handle these files have decided that "but this is what the spec says" with

NO concern or interest in deciphering why the spec might have been written that way. Try trafficking a video ad online. Not

pretty.

[James G. Dainis]

I think a lot of people who should know better are also clueless when it comes to digital photo files. One person asked a question on a forum. He wanted to submit some images somewhere and the only submission guideline was that the images had to be 300 DPI. The only thing I could figure was, the submission editor was used to 8x10 photos and wanted photos that when printed at 300 DPI/PPI would get an 8x10 photo. I suggested he submit 3000 x 2400 pixel images. If it were up to me, I would have submitted 64 x 48 pixel images at 300 DPI and said, "You wanted 300 DPI images, you got 300 DPI images."

 

Note: I noticed in my previous post I was using the word "drop" when I should have been using "dot". That happens when I don't have my morning coffee. I have edited it to "dot"s.

[kelly_flanigan1]

<p>Many editors do not like to be lied to. Thus an digital image that is up-sized to 300 pixels per inch at a 8x10" size might tick them off; if it is from a Barbie Cam's VGA 480 by 640 pixel image. They quickly see thru that the supplied 2400 by 3000 pixel image is full of fluff; BS; hot air and then equate these adjectives to the Photographers true way he/she operates. Thus they might ponder what also is being passed off as real; is the image legal; or stolen?</p>

[Spearhead]

<blockquote>

<p>Finally got him to say that medium is 3000 DPI and max JPG settings.</p>

</blockquote>

<p><br /> This sounds like scanning resolution, not what is being talked about here.</p>