PPI/DPI?

Ricky91t

Hi all,
well recently browsing through the inpho site i noticed all their images were around 20mb,i know their cameras are top of the range but even the 1dmkIII don't produce 20mb jpegs,And the only difference was the resolution which are 70ppi on my images and 300ppi on theirs

Now a few questions about it:
Whats the advantage of saving images to a higher PPI?
Will i see differences when viewing them on a pc?
Will i see differences when printing?
How many of you save at 300 ppi and why?

Thanks all!

DaireQuinlan

Ricky91t wrote: »

Hi all,
well recently browsing through the inpho site i noticed all their images were around 20mb,i know their cameras are top of the range but even the 1dmkIII don't produce 20mb jpegs,And the only difference was the resolution which are 70ppi on my images and 300ppi on theirs

Now a few questions about it:
Whats the advantage of saving images to a higher PPI?
Will i see differences when viewing them on a pc?
Will i see differences when printing?
How many of you save at 300 ppi and why?

Thanks all!

No, It makes no difference whatsoever to any of the above. The DPI value is just a number in the exif or the image header. It really has no significance whatsoever in and of itself.

Paddy@CIRL

There is a difference between PPI and DPI, Pixels per inch and Dots per inch, one refers to on screen resolution, the other to print. The best man here to ask would be stcstc.

DPI is hugely important for printing, with 300dpi being pretty much the standard. It's a bit tricky but lets say you've taken a photograph and its A4 in size and its 125dpi. The only true way of getting that image to 300dpi would be to reduce its dimensions to A5. Digital images is basically just information. You can't increase (enlarge) the information if its not there but you can compress it.

At 300dpi:

There won't be too much difference on screen.

There will be a significant difference in print, although this is dependant on the printer.

I only process to 300dpi when I'm outputting for print. It often requires enlarging the image to get it to 300dpi which can be a PITA.

Actually if stcstc is on, he might explain to us a proper way of enlarging an image for print?

DaireQuinlan

Paddy@CIRL wrote: »

There is a difference between PPI and DPI, Pixels per inch and Dots per inch, one refers to on screen resolution, the other to print. The best man here to ask would be stcstc.

DPI is hugely important for printing, with 300dpi being pretty much the standard. It's a bit tricky but lets say you've taken a photograph and its A4 in size and its 125dpi.

I think there's a lot of misunderstandings about the significance of DPI/PPI. I mean, no-one takes a photograph at "A4 and 125dpi". They take a photo that's a particular resolution. Given that resolution, and knowing that I want to print at 300 DPI then I can work out what my maximum print size is. So If I take a photo thats sized 3000x2400 and I want to print at 300 dpi then I know I can print to at least 10x8 inches.

The question ricky91t originally asked is if there's a difference saving the photo at a "different PPI". So long as the resolution isn't changed then, no, there won't be any difference. The PPI/DPI value in the header will be changed and thats it. I know the default behaviour in PS is to resample the image when you change the DPI though, something which always caused me no end of irritation.

Paulw

Have to agree with Paddy.

Anything I do for web/screen is set at 72 ppi.

Anything for print is set at 300 dpi.

It really depends on the end use of the image. If you set an image to 72 ppi, and then try printing large, you will see a difference from the image at 300 dpi. The larger the print size, the more quality (and dpi) you will need.

DaireQuinlan

Paulw wrote: »

Have to agree with Paddy.

Anything I do for web/screen is set at 72 ppi.

Anything for print is set at 300 dpi.

It really depends on the end use of the image. If you set an image to 72 ppi, and then try printing large, you will see a difference from the image at 300 dpi. The larger the print size, the more quality (and dpi) you will need.

Is the image being resampled when you change the DPI ? Then yes of course you'll see a difference. Maybe I'm missing something here, but I thought Rickys question was to do with the figure in isolation. IE he wants to know if changing the DPI will make any difference to the image. In and of itself, no, because its just a field in the header/exif. If by changing it you also cause the image to be resampled then yes of course there'll be a difference, but thats a different matter.

smelltheglove

Steve always requests me to have a min 240ppi for my prints I order. I often need to resize the print size up when I want a large image but the ppi does need to be high for a good quality print.

stcstc

Daire is correct

the real important part though is that the resolution doesnt change!!


if you have a 4000 pixel wide image, as long as this doesnt change saving it at diff res makes no difference cos all the information is still there.

BUT

when it comes to printing it makes a massive difference

if your trying to produce images like traditional photos, you need a high ppi and depending on the printing system a high DPI

my printer uses a native resolution of 360 ppi, then i print at 1440 dpi

if i give the printer a lower ppi it will mean the printer does maths on the fly to up the res, where as when i give it 360 ppi images its a derivative of that which is the dpi. if that makes sense, meaning the printer does have to do as much maths and means slightly better quality.

BUT the difference in quality is small unless your talking about large prints and side by side comparission you might not even notice.

most people are scaling images for the web, ie throwing away information, so the ppi you shoulh use is NOT 72 as most people think but actually 96ppi. in this instance the ppi does make a difference, as your maybe throwing away more than 3/4 of the pixels captured with your camera.

there are some really good tutorials on the cambridge in colour website

http://www.cambridgeincolour.com/tutorials.htm

Paddy@CIRL

Whats the difference between PPI & DPI ?

Great thread BTW !

stcstc

DPI is how many dots the printer prints per inch, so for example my print i print at 1440 dpi

pixels per inch ppi are the ammount of the pixels from your camera that your computer is giving the printer per inch

if that makes sense

Paddy@CIRL

Is there a method of conversion for figuring out how PPI translates to DPI ? i.e. What would a 300PPI image be in DPI as I'd imagine there's a difference between the two on paper ?

stcstc

the DPI figure is device specific, ie epson printers use multiples of 360

i belive canon & HP use 300

but the two figures are not directly connected, you could give me an image at 96ppi and i could still print it at 1440 dpi,

the main trick is use a printer to do your stuff who knows what they are talking about, and then stick with their suggested settings.

i know thats a little vague but there isnt a direct way to convert, its from experience of the printer.

Cameraman

stcstc wrote: »

most people are scaling images for the web, ie throwing away information, so the ppi you shoulh use is NOT 72 as most people think but actually 96ppi. in this instance the ppi does make a difference, as your maybe throwing away more than 3/4 of the pixels captured with your camera.
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Can you explain this a bit more. My understanding was that dpi/ppi made no difference on the web (i.e. for display on a screen). The reason for that being that a screen displays uses its own pixels, and the resolutions is fixed by that, so the ppi value set for the image makes no difference.

stcstc

what i ment was to optimise images for the web, ie make them small file sizes etc

you should resize to something like 800 pixels on longest side. ie your throwing away over 3000 pixels from an average dslr

the ppi of 96 is he optimum, buy setting it to this, your browser etc isnt having to rework the pixels if that make sense

in practical terms you may never notice the difference, but it does under the hood, and so why not just make it right from start to finish, if that makes sense

Cameraman

stcstc wrote: »

the ppi of 96 is he optimum, buy setting it to this, your browser etc isnt having to rework the pixels if that make sense

OK - that's what confused me. If your image is 800 pixels wide, then the browser displays exactly 800 pixels on your screen. I don't see where any reworking would come in.

(Print of course is a whole different story, I know).