DPI Help - DPChallenge Forums

04/11/2003 09:35:35 AM · #1

I went back about 13 pages and didn't find what I need. I'd really like to wrap my brain around this DPI concept today.

I'm shooting with a 6 mp camera. When I open an image in photoshop it shows up as 72 dpi. In the past with my 3 mp camera, I've never adjusted dpi and got 8 x 10 prints from ofoto that looked fine. Okay, so I now understand that I should up the dpi to 300. This will make the dimensions of my image much smaller. Since the dimensions are smaller, will I still have the capability to print upwards of 8 X 10? My understanding is that the dimensions of the image correlate directly to printable size.

Thanks in advance,

Elizabeth Nelson

joshua

Canon EOS-500D Rebel T1i

Canon EF 50mm f/1.8 II

04/11/2003 09:37:13 AM · #2

//www.dpchallenge.com/tutorial.php?TUTORIAL_ID=10

Message edited by author 2003-04-11 09:37:27.

04/11/2003 09:45:35 AM · #3

Perfect! It also answered my questions about cropping to a specific ratio.

Thanks!

Elizabeth Nelson

04/11/2003 10:14:51 AM · #4

Okay, I completely understand until I get the the heading: "Getting More Resolution"

"Change the image size to the proper inch dimension for printing"
and it shows 1920 pixels by 2560 pixels.

In the previous section we used the 'constrained aspect ratio' to get the image size to the propers ratio. So when I read, "Change the image size to the proper inch dimension for printing", I wonder, 1. Didn't I just do this? and 2. How in the heck do I know what the proper inch dimension for printing is??

Thanks.

04/11/2003 10:38:30 AM · #5

Does this mean change the image size to 8 X 10 or whatever size I'm printing? I guess I'm confused because the highlighted number is shown in pixels.

04/11/2003 10:44:20 AM · #6

Never mind, I think I got it.

severin

Sony DSC-R1

04/11/2003 08:35:00 PM · #7

just incase you still have a few questions, I just went through the same process of trying to understand DPI. What I can tell you is the following. You can increase your print quality a few ways. The 72 that you see when you first open is automatically set by the program which means your image size is pretty big something like 16x24" since you have a 6 MP camera. So if you take bicubic off (uncheck it) and then choose a size like 5x8 or whatever you cropped it too you can adjust it so you have a size which is pretty close to 300 DPI. This is what I would consider your base picture. You don't want your smallest picture to be more than twice this value and you don't want your largest to be less than half this value. If you stay in those guidelines you will end up with some really nice prints.

I hope this answered or confirmed your ideas as to what you needed to do.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

04/11/2003 11:33:33 PM · #8

The critical thing to remember is that DPI (or really PPI) is pixels PER inch. It doesn't matter that the "native" resolutions of (most) cameras is 72PPI, the maximum print size is determined by the total number of pixels captured. It's just arithmetic.

720 pixels:

= 10 inches at 72 PPI (720/72=10)
= 7.2 inches at 100 PPI (720/100=7.2)
= 4.8 inches at 150 PPI (720/150=4.8)
= 2.4 inches at 300 PPI (720/300=2.4)

04/14/2003 04:49:21 PM · #9

Fantastic, thanks Severin!! That was very helpful. : )

Originally posted by severin:

04/14/2003 04:50:02 PM · #10

That simplifies it a lot. Thanks GeneralE!

Originally posted by GeneralE:

04/14/2003 10:42:11 PM · #11

Originally posted by lizardbeth:

That simplifies it a lot. Thanks GeneralE!

Originally posted by GeneralE:
The critical thing to remember is that DPI (or really PPI) is pixels PER inch. It doesn't matter that the "native" resolutions of (most) cameras is 72PPI, the maximum print size is determined by the total number of pixels captured. It's just arithmetic.

720 pixels:

= 10 inches at 72 PPI (720/72=10)
= 7.2 inches at 100 PPI (720/100=7.2)
= 4.8 inches at 150 PPI (720/150=4.8)
= 2.4 inches at 300 PPI (720/300=2.4)

I thought the DPI was basically for printers. Everywhere I read on this site says that the more DPI, the smaller picture i can get. I thought that if you used more pixels, you would be able to make a better print on your printer. what i am reading here is that DPI has nothing to do with that. If I scan in a picture at 600 DPI, will I not get a better picture for printing than at 50 DPI? Please help me on this as I am still confused.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

04/14/2003 11:11:00 PM · #12

Scanning is a different story, because the scanner captures more total pixels at higher resolutions. A digital camera captures a fixed number of pixels.

Message edited by author 2003-04-14 23:11:52.

Lustre

Nikon D7000

Nikon AF-S DX Zoom-Nikkor 17-55mm f/2.8G IF-ED

04/14/2003 11:12:07 PM · #13

Originally posted by tecent642:

Let me try to help...

DPI is "Dots Per Inch". Therefore if you image is (say) 2100 * 1400 pixels then at 72 DPI you image would print to ~30 inches x ~20 inches. At 300 DPI it would print at 7 inches x ~4.5 inches.

I aim to print at around 300 DPI, although you can print lower or higher. If you go much higher though you are just wasting your time since many printers can't print that fine. If you go too low then the picture will look chunky and grainy. Try to picture a space of 1 inch. If you try to draw 72 lines side by side in that space then the lines can be fairly thick, trying to fit 300 lines in the same space requires a much thinner line.

In a nutshell - look at how many pixels your image has. Divide this by how large you want to print your image. The result is the DPI you would be printing at... Is this figure fine enough for you? To achieve something photorealistic you'll want at least 200 DPI.

Hopefully I've helped more than confused you.

cykhansen

Canon PowerShot G1

04/14/2003 11:53:38 PM · #14

Originally posted by sparky_mark:

DPI is "Dots Per Inch". Therefore if you image is (say) 2100 * 1400 pixels then at 72 DPI you image would print to ~30 inches x ~20 inches. At 300 DPI it would print at 7 inches x ~4.5 inches.
etc.
In a nutshell - look at how many pixels your image has. Divide this by how large you want to print your image. The result is the DPI you would be printing at... Is this figure fine enough for you? To achieve something photorealistic you'll want at least 200 DPI.

That's REALLY HELPFUL, sparky_mark.

04/15/2003 12:00:42 AM · #15

sparky, thanks for that, but I must be hard headed. I understand the DPI as far as the lines per inch like a tape measure, but if most cameras put out at 72 dpi, then why do we need printers that can do more than that? so each line is 1/72 of an inch. why would I want to print any higher than this? 1/200 of an inch does what for me? so if i want to print an 8X10, i should use 200dpi and not 600 dpi?

I am still lost.

thanks

jimmythefish

Canon EF 17-40mm f/4L USM

04/15/2003 12:31:53 AM · #16

Originally posted by tecent642:

As General pointed out above, it's irrelevant what the camera puts out as far as DPI. What matters is the total number of pixels. The properties of a computer monitor are such that you don't need nearly as high a DPI on-screen as you do on a hard-copy print. On-screen, 72 DPI produces a sharp image, whereas on paper you need a minimum of 150 for OK quality, and 300 for a professional quality. The # of pixels available dictates how big the photograph can be at a given resolution.

04/15/2003 12:43:51 AM · #17

Originally posted by jimmythefish:

Originally posted by tecent642:
sparky, thanks for that, but I must be hard headed. I understand the DPI as far as the lines per inch like a tape measure, but if most cameras put out at 72 dpi, then why do we need printers that can do more than that? so each line is 1/72 of an inch. why would I want to print any higher than this? 1/200 of an inch does what for me? so if i want to print an 8X10, i should use 200dpi and not 600 dpi?

I am still lost.

thanks

As General pointed out above, it's irrelevant what the camera puts out as far as DPI. What matters is the total number of pixels. The properties of a computer monitor are such that you don't need nearly as high a DPI on-screen as you do on a hard-copy print. On-screen, 72 DPI produces a sharp image, whereas on paper you need a minimum of 150 for OK quality, and 300 for a professional quality. The # of pixels available dictates how big the photograph can be at a given resolution.

Dear Abbey,
A hardcopy needs more dpi to keep looking grainy, right? kind of like expanding 100 speed or 400 speed negatives? screen could basically care less. I guess i still don't understand why if you shoot a pic at large scale appx 2500 x 1800 (what ever those numbers are actually) then why would you actually get a smaller picture than if you have less dpi. I read through Jim's tutorial and have been reading this thread. if i want a better 8x10, then i need to shoot at full scale in fine mode, right? if i shoot 640x480 and print at 300 dpi, i only a 2 x 1.5 inch pic. at 300 dpi, why cant i transform that to an 8x10? I know i am wrong, but i would think this gives you a better enlargement.

signed,
troubled in St. Louis

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

04/15/2003 10:48:19 AM · #18

Think of a pixel as a mosaic tile. If you can only put 72 tiles in one inch of picture, you have a limited amount of detail you can render. If you have 300 smaller tiles to fit in the same inch of picture, you can depict finer detail.

Once the picture is captured, it contains a fixed number of pixels. If you want to print the picture larger, you must either make each pixel/tile larger (visible) by reSIZING, or spread the pixels apart and create new pixels to fill in the spaces by reSAMPLING. Carefully executed resampling can yield a larger image of acceptable quality, but not as good as the original. Resizing should only be used for special effects where you'd want to see the individual pixels.

If you are using Photoshop, use the Bi-Cubic setting, and read the tutorial on Resampling (under the "Learn" menu).

Gordon

RIM BlackBerry Curve

Canon EF 85mm f/1.8 USM

04/15/2003 11:03:33 AM · #19

You take a picture at a particular resolution. This the actual number of pixels (picture elements) in the image. So the resolution is the basic number of dots that you have, say 2000x1000 pixels.

Now, dots per inch (DPI) is an arbitary number. Typically for your monitor there are 72 little dots on the screen per inch of glass. So the cameras usually just set this arbitary number to 72. This means that if you look at your picture 'actual size' on the screen, you'll have a 27.7"x13.8" representation on your monitor that you'll probably have to scroll around to see.

Printers on the other hand typically lay down 300 little dots per inch of paper, as the dots are smaller. Look real close on your monitor and you can probably see the dots, but your printer can create much finer dots on your prints. So, without resampling or resizing or doing any changes at all, to the original 2000 pixels x 1000 pixel image, if you just change the arbitary DPI value to 300, you get a6.6"x3.3" printout.

You can also set the arbitary DPI value to 150DPI and get a 13.3"x6.6" printout. However, in this case, your printer driver will do the resampling for you and internally convert your picture to 300DPI before printing - you may or may not like the results.

If you don't like your printer driver doing these interpolations for you, you can resample in software like QImage, or Genuine Fractals or Photoshop to get your picture to the native DPI for your printer driver (the native value is the one it doesn't do any resampling for - this is usally 300DPI or 360DPI depending on the printer)

Printers also arrive rated at things like 2880 DPI or 1440 DPI. These are related but actually a different meaning of Dots Per Inch. All of the previous values in the computer are actually more correctly called Pixels Per Inch (PPI). In the printer world several dots get laid down per pixel, hence the higher numbers for printer resolution. For ink-jet prints you can probably tell the difference between a 1400DPI and a 720DPI print, but would be hard pressed to notice the difference between a 1440DPI and 2880DPI print, even with a loupe. The main difference is you'll use up ink faster...

I hope this hasn't confused things even more...

04/15/2003 09:24:26 PM · #20

thanks Gordon and General. I am getting the "picture" now.

This brings me to a different question that is based. if i have this picture that is way too big to fit on my screen, like you said a 27x13 screen shot, then i print it, i don't have to do anything to it. I just hit print and my HP printer will take over and convert it to a 6x3? if this is the case, then how do I acurately print out an 8X10 print of a picture? The camera that I am supposed to be getting (the f717) says that it will take 2560x1920, which when divided by 300DPI gives me an 8.53"x6.4". what happens if i "stretch" this from the Windows Picture printer to a full 8x10, do I lose quality, or not enough to worry about.

So far, I have only scanned in picutres, so this whole thing has me messed up. I have pretty good luck with computers, but this is driving me insane.

thanks again,

chris

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

04/15/2003 09:34:49 PM · #21

Hi Chris,

Don't know what your printer will do -- it depends on the printer driver and the program you are printing from, to some extent on the resolution of the printer. Many programs have a "Print Preview" function. Otherwise, just print page 1-1 (so you don't print a bunch if it breaks the image into tiles) and see what you get. If it prints a tiny bit blown up, you'll have to change the resolution of the file.

If you're printing from Photoshop, click the cursor on the tiny window at the bottom which shows the file size and it will pop up a preview of the page setup.