I don't think i understand ppi...

Andreal · Jul 21, 2007

Ok so im not 100% sure that its ppi causing my problem, but I think it is. Now I understand that ppi is the number of pixels that are in a inch of your image. But how do you work with it?

For example, today I decided I would try and border one of my photos to see how it looks and what effect it had on the image, and then if I thought the image good enough I planned to print it and show off my new found hobby

.

I though to myself that a nice A3 image would be nice, so I went to photoshop, created a new document with the preset of A3 (which sets ppi to 300). Now I work on a 24" screen which SHOULD be slightly bigger than a A3 piece of paper. Only the newly created document was far bigger than my screen, I had it a 1/3 of its size just to fit it on the screen

. The picture that I thought I would have to scale down to fit on the A3 piece of paper turned out to just be a small sqaure in the mass of white

So I guess my question is, how do you go about sizing things?

On a related note, how does ppi relate to dpi? It seems odd that you can print like 1400dpi when your image is only 300ppi, wouldn't this mean that the printer would just end up printing alot of dots of the exact same colour to make up for the lower number of pixels?

Garbz · Jul 21, 2007

DPI is literally dots per inch. They are normally dots of Cyan Magenta Yellow and Black, a few more colours for some fancier photo printers. To get smooth gradients and be able to generate all possible colours the DPI a printer must lay down should be many thousand tiny dots. When printing or scanning always keep DPI as high as it will go for maximum quality.

PPI on the other hand is pixels per inch. A single pixel can contain any possible colour value. The key here is to have enough pixels to keep the eye from seeing them. Computer screens PPI depends on the screen resolution. A 19" screen at 1024x768 you can clearly see the number of pixels used to make up a letter from about half a metre away. The quoted standard of 300PPI for high quality photos I believe is the human eye's resolving threshold at arms length. This means that you hold a photo at arms length that it 300PPI you should not be able to see any individual pixel.

To work with this you must separate your workflow. Ignore the size of the final print, and work with the original size of the images you have. You say you want to make an image A3 or for simplicity sake we'll say 24x16 since that is in the 3:2 ratio that you get from your camera. Open the image, adjust what you need to adjust, add your borders however you wish to do it, and the last step before printing is to pick the final size. Under Image Resize set the desired dimensions and pixels per inch. 24x16 would result in an image 7200x4800, larger than your camera can most likely produce and thus bicubic resampling is often employed to help make this upscaling more visually appealing. Finally print.

Alex_B · Jul 21, 2007

well, if your screen has less than 300 pixels per inch (which i guess

), then an image, which is 300 ppi with reference to A3, will appear larger than your screen if you display it in 100%.

e.g.:
the file is saved as 300 ppi for A3, but your screen has 100 ppi, then if you display on the screen at 100%, then the image will appear 3 times wider and 3 times higher than A3. Hence it does not fit onb your screen anymore.

ppi and dpi:
ppi is usually used when scanning, as in how many pixels are scanned per inch of the original. or when you talk about your screen resolution, also then ppi would be justified.

dpi however, is used for printing, and refers to the dots per inch.

Anyway, as my personal advice: I would always think in absolute number of pixels, since that is the only thing which counts for the digital image. only when it comes to actual printing, then you can think about dpi to get the right resolution for your printer. it does not make too much sense to talk about ppi or dpi when talking about a jpg or tiff, since dpi or ppi only make sense when referring to an actual size of the printed image.

Andreal said:
Ok so im not 100% sure that its ppi causing my problem, but I think it is. Now I understand that ppi is the number of pixels that are in a inch of your image. But how do you work with it?

For example, today I decided I would try and border one of my photos to see how it looks and what effect it had on the image, and then if I thought the image good enough I planned to print it and show off my new found hobby .

I though to myself that a nice A3 image would be nice, so I went to photoshop, created a new document with the preset of A3 (which sets ppi to 300). Now I work on a 24" screen which SHOULD be slightly bigger than a A3 piece of paper. Only the newly created document was far bigger than my screen, I had it a 1/3 of its size just to fit it on the screen . The picture that I thought I would have to scale down to fit on the A3 piece of paper turned out to just be a small sqaure in the mass of white

So I guess my question is, how do you go about sizing things?

On a related note, how does ppi relate to dpi? It seems odd that you can print like 1400dpi when your image is only 300ppi, wouldn't this mean that the printer would just end up printing alot of dots of the exact same colour to make up for the lower number of pixels?

Alex_B · Jul 21, 2007

Garbz said:
The quoted standard of 300PPI for high quality photos I believe is the human eye's resolving threshold at arms length.

But some people have short arms and others have long arms!

Andreal · Jul 21, 2007

Ok I think im starting to understand

I guess my next question just out of curiosity then what would be the max size I could print a image at 300ppi for good quality if I took it on my D80. Im gonna take a guess here but please feel free to correct me. Now my D80 is 10.2 megapixle. If I say I want to print a square just for ease of math sake then when I square root 10,200,000 I get 3193x3193...would that mean I could keep an image at 3193x3193 (or any variation) and keep a 1:1 scale? I don't like the idea of scaleing up, it seems to me like your adding stuff that isn't really part of the image, although at the same time I think A3 would be a nice size to print photos to hang around my room (my walls are extremely boring at the moment

). So I guess my only choice would be to upsize them then? Or to lower the quality which I am even less fond of

Oh the compromises

Thanks for the replies, always enjoy learning more

Alex_B · Jul 21, 2007

I think the D80 is [SIZE=-1]3872x2592 pixels.

That means at 300 dpi you could print a maximum of 12.9 x 8.6 inches.

[/SIZE]A3 is 16,535 x 11,693 inches however. That means you have to print with slightly less dpi to get to A3 .. however, you would not really see the difference. just go for it

Even 150 dpi are still ok-ish ...

Andreal · Jul 21, 2007

You mean ppi right?

hah maybe im just getting myself confused, but I definatly was not too fond of enlarging the image, that water looked so grainy afterwards, so I think I will stick with dropping the ppi a bit

thanks for the help in understanding!

Garbz · Jul 21, 2007

Wrong idea. To make it less grainy you need to raise the PPI and in the process apply some resize filtering. If I take an image that is 450x300 (6x4@75)and send it directly to the printer I will end up with ugly blocks. But suppose you now resize it to 1800x1200 (6x4@300 ok extreme example but proves my point) you will end up with a picture that looks horrid on screen but rather than blocks you will have somewhat smoother image when you print it at 6x4.
The printer will print it the way it gets it so this is the difference between resizing using a resampler, or just printing big square pixels.

And yes alex did mean ppi