In response to the back and forth I've been having with Scott in one of his other threads, I figured I'd try to explain the image dimension terminology in plain English. The whole jumbling of terminology (dpi, ppi) and the meaning of all these things tend to lead many people astray and the ppi vs dpi thing has long been a pet peeve of mine. If anyone wants my rant/explanation on dpi vs ppi, just let me know.

There are basically three dimensioning methods Photoshop and other programs use, and I'll refer to them in one of three ways: pixel dimensions, physical dimensions and display scaling (what Photoshop calls resolution which is fine but confusing for many people).

Pixel dimensions:
Pixel dimensions are the overall x by y sizes for your image. For example an 800x600 pixel image has 800 pixels in one dimension and 600 in the other. Photoshop, in the Image Size window, lists these under Pixel Dimensions.

Physical dimensions and display scaling:
Below that you see the Document Size fields. These are more arbitrary, particularly when it comes to displaying on computer monitors where pixel densities between monitors vary anywhere from around 72ppi to around twice that amount just for monitors I've used and am aware of.

For example:
Take two monitors with the same physical dimentions (both are 20" 16:10 widescreen computer monitors), if one is 1280x800 it has a pixel density of roughly 75ppi. If the other is 1680x1050, the pixel density is approximately 100ppi. (For the record a 1600x1200 15" laptop screen has a pixel density of 133ppi). In other words a picture that is 1000 pixels wide is going to be 10 inches wide on a screen set at 1680x1050 and 13.25" wide on a 1280x800 monitor.

When you set the physical dimension of your image, it's a fairly arbitrary value in many cases, particularly for computer display as shown above. If your image says it's 10" wide, and your display scaling is 100ppi then by multiplying the two together you know it's a 1000 pixel image in that dimension. From that you could determine how large it will appear on varying computer screens.

Display scaling and physical dimensions have more application in printing, because the pixel dimensions don't tell the printer how big you want the image to appear on paper. The physical dimensions give the image the overall size on paper, and the display scaling again gets you to how big your overall pixel dimensions are. In some cases (and I'm out of the loop on this) particular display scalings (or resolutions) were best for certain printers. A lot of inkjet printers could run just fine on 180, 300 or other resolutions once you considered how it actually lays down the stated dpi in the printers specs.

By using the Image Size tool as a calculator, you could tell if your image is big enough to print on a 300dpi printer at 8 by 10 inches (you'd need a 2400x3000 pixel image for what it's worth). Outside of a printer though, it doesn't matter if you have that listed as an 8x10 at 300 or a 4x5 at 600. Both work out to a 2400x3000 pixel file.

jeebus thats a lot to read.



if this is in relation to submitting a pic for that contest then I suggest that you just shoot the pic as you normally would. put it into PS or whatever program you use. Just lower the DPI to 72 and resize your pic to nothing longer than 800 long.

for example... my pics that I take are in jpg format...not raw.

I put them into PS, resize them to 10inches wide at the same DPI (72)... the resolution is the same jus the size of the file is small, thus making the file size smaller. the pics that I've been posting are only about 300-400kb.. fairly small. technically I could INCREASE the dpi making it possible to take a compressed file and enlarge it.

if you shoot in raw format which IIRC is the largest filesize possible... uncompressed... you can change just about everything you want with it and make it just about any filesize imaginable(sp?)


I hope that helped

BTW.. forget about what size monitor you have or you are looking at the pic through... you can have a 60in screen and still only have the resolution set at 800X600. or visa versa... a smaller monitor with much higher resolution.

the resolution is gonna help to PRINT the picture... not so much how big it'll show in a monitor. thats gonna be the size of the picture itself..

You're confusing some of the terminology I think. You should be able to shoot at full camera resolution (pixel resolution) in JPEG or RAW. You can choose and dpi you want, but your physical dimensions will have to change otherwise you're upscaling or downscaling the image (interpolation) which is fine if you want that.

Like I said, the physical dimensions (inches) are meaningless for web. I just resize to a particular size, usually 800 pixels in the long dimension for web, sometimes 1000. Changing the DPI and adjusting the physical dimensions accordingly to match, so that your pixel dimensions remain the same, has no effect on the actual pixel size of the image (and thus its file size) or how big it is on the computer display.

I mentioned this in another thread here, but it's not necessary to print at 300dpi; 150dpi or thereabouts will do just fine.

Originally Posted by emorphien

You're confusing some of the terminology I think. You should be able to shoot at full camera resolution (pixel resolution) in JPEG or RAW. You can choose and dpi you want, but your physical dimensions will have to change otherwise you're upscaling or downscaling the image (interpolation) which is fine if you want that.

Like I said, the physical dimensions (inches) are meaningless for web. I just resize to a particular size, usually 800 pixels in the long dimension for web, sometimes 1000. Changing the DPI and adjusting the physical dimensions accordingly to match, so that your pixel dimensions remain the same, has no effect on the actual pixel size of the image (and thus its file size) or how big it is on the computer display.

Uhhhhhh...as long as you are resizing your pictures proportionally then you should even have to worry about matching DPI to physical size... I think.

if you take a file... open it up... change the DPI to something lower... the image size doesn't change. if you change the image size only then your DPI doesn't change. changes of either will make a smaller file size but of you change BOTH, then you can get the desired FILESIZE easier.

are we confusing each other?

Originally Posted by mkfotos

I mentioned this in another thread here, but it's not necessary to print at 300dpi; 150dpi or thereabouts will do just fine.

basically

Originally Posted by beezwax

Uhhhhhh...as long as you are resizing your pictures proportionally then you should even have to worry about matching DPI to physical size... I think.

I'm sorry?

if you take a file... open it up... change the DPI to something lower... the image size doesn't change. if you change the image size only then your DPI doesn't change. changes of either will make a smaller file size but of you change BOTH, then you can get the desired FILESIZE easier.

Depends on what you have photoshop set for. For what it's worth, you can change the dpi or the physical dimensions and still get a larger file size as long as you increase either of them. But that's sort of the crux of the matter, people will change both all over the place and get confused about what each is doing. I've seen tons of people trying to use physical dimensions and ppi to size their images and complicating things for themselves when all they're trying to do is put it online. Those numbers only matter when you're going to print something. The whole point of what I'm saying here is: don't use that if you're not printing if you find it at all confusing because you don't need it.

If you have "Resample Image" turned on, then when you change the ppi, it will change your overall pixel dimensions. If you turn it off then changing the ppi will result in a corresponding change to the physical dimensions that ensures your overall pixel dimensions (and thus file size) do not change.

are we confusing each other?

Beats me, I don't understand your first sentence

Originally Posted by mkfotos

I mentioned this in another thread here, but it's not necessary to print at 300dpi; 150dpi or thereabouts will do just fine.

Generally speaking, 150ppi is close but I usually used 180ppi iirc.

Basically, the point of this was to explain to Scott or anyone else why the ppi isn't very useful when working on images for web display. You can take a guess, like 72ppi and then if someone happens to have a screen that's 72ppi it will display on their screen at the physical size you wanted but on any other screen the displayed size will vary.

I probably could have made more sense had I not written this last night, I had insomnia and was tired, so it probably seemed like a good idea at the time.

I don't know why I wrote this at nearly 2AM. I'm going to have to re-write it when I'm in a more lucid state so it makes more sense.