Frit-Happens !

Technical Forum => Photo help => Topic started by: Hazer on May 14, 2010, 01:58:21 PM

Title: DPI
Post by: Hazer on May 14, 2010, 01:58:21 PM
I've been through technical sites regarding dpi and resolution, but I can't find an answer that I understand helps me specifically.

I need photos that have a dpi of 300 - according to what I've read, using the highest quality settings and resolution on the camera will produce a high dpi, yet whichever settings I use, the computer insists the dpi is 72. Some sites suggested this is a moot point, and computer read the dpi as 72, even if it isn't. So now I'm confused.  ::) I can't alter the dpi on my camera, I can't find where to do it in Gimp, and I don't want to end up with photos that don't fit what I need.

So my question is...what's up with this? And also, help? :) :(

Title: Re: DPI
Post by: Dickie on May 14, 2010, 02:19:12 PM
DPI is just a "tag" in the file as an indication on how to display it. There is no difference in the file in terms of resolution.

Basically if a photo is 300px by 600px then if you send it to a printers who require 300dpi it will print at 1" x 2" (ie not very big, but it will look quite large on the screen)
so on a computer screen it will be approx 4.1" x 8.3" (assuming 72dpi)

At the end of the day, the 300dpi thing is all confused by many people. And printers often insist on 300dpi, normally because when they place an image, they don't want to have to shrink it down to the correct size, it's just them being lazy, having done the GBUK journal, I have received images in all different resolutions (dpi) and they all work, they just print at different sizes.
Also printers will tell you that images from the internet are not suitable for printing, but this is plain wrong... it all depends on the size of the image.
I can get an image from the internet that will fill an A4 page at 300dpi.

The point they are trying to make though is that a picture that looks big on your computer screen will not print at the same size in a book/leaflet etc. and that is correct.

The basic upshot is.

Ensure your image has the correct number of pixels. For example. if you need a picture to be printed at 6"x4" then assuming 300dpi you picture must be larger than (300x6) by (300x4) = 1800px x 1200px (which most cameras can manage)

There are programs that will allow you to change the dpi of an image without affecting the rest of the image, it's very easy to end up enlarging the image, and this must NOT be done.

What are you using to edit your photos?

(P.S. Hope I haven't confused you further)

Title: Re: DPI
Post by: Hazer on May 14, 2010, 03:06:22 PM
Thank you so much Dickie, for clearing some of that up. It does make sense how you've explained it, but now I'm not quite sure what settings I need to use for the photos - all they said was high-res and 300 dpi, and seeing as I don't know what size they want, I'm a bit confuzzled (presuming I have understood what you wrote). Would simply using the highest quality setting on my camera work?

I'm using Gimp 2 for editing, and my camera is a Olympus FE-5020, if that makes a difference.

Thanks again, I feel like I understand more ;D ;D :)

Title: Re: DPI
Post by: Zeldazog on May 14, 2010, 03:19:24 PM
I know Gimp is quite similar to Photoshop.... but I haven't got it in front of me (Gimp that is) so I can't see the menu.

But on PShop, along the top menu is IMAGE - and drop down in there is IMAGE SIZE - this will tell you what size your picture actually is.

ie. 3000px x 2000px - and, depending on what the dpi is set at, it will tell you the physical size of the image. 

If you alter the dpi, it will alter the image dimensions, but the file size will remain the same.  Or, if you tick re-sample, it will alter the file size (it does a complex maths calculation to resample pixels, but it doesn't always do it effectively, so I only resample if I really have to).

This of course applies to Photoshop, but when I last used Gimp, I did think there was a lot of similarities, so I would expect to find something similar somewhere.

What is the image file being used for - magazine entry, poster, postcard?

Title: Re: DPI
Post by: Hazer on May 14, 2010, 03:34:52 PM
I'm looking for magazine quality I guess. Hmm, well, I can find Scale Image, and from there I can alter resolution to 300x300 pixels/inch from 72x72...which results in 300x300 ppi. Is that anywhere near where I need to be? Am I just making this too hard?

(I think I naively expected there to be a giant button that said CHANGE DPI. ::) )

Anyway, thank you for the help - I feel like the fog is slowly lifting! :)

Title: Re: DPI
Post by: Zeldazog on May 14, 2010, 03:52:55 PM
dpi is ppi - tis the same (dots per inch, pixels per inch) - a pixel is a dot.

Title: Re: DPI
Post by: Hazer on May 14, 2010, 03:54:51 PM
That makes sense then - thank you ;D :D

Title: Re: DPI
Post by: mizgeorge on May 14, 2010, 04:36:39 PM
Just be careful when you go to 300dpi/ppi that you also adjust the image size downwards, so the file is no larger than it was to start with. You will lose quality if you enlarge the file too much.

Also remember that jpg is a very lossy format, so if possible do all your editing, adjusting resizing etc whilst saving in an uncompressed format (TIFF is pretty reliable). Only save as a jpg for the final file if possible.

Title: Re: DPI
Post by: Hazer on May 14, 2010, 05:18:36 PM
Thanks George - from what I can tell, the size has stayed constant, so I hope at least that's okay. Didn't think about TIFF, I'll have to use that in future.

Title: Re: DPI
Post by: nemeton on May 15, 2010, 03:27:57 AM
Dickie, thank you so much for finally coming up with an explanation I could understand - I have been wrestling with the same thing for ages (but was too embarrassed to admit it  :-[) - and AT LAST it makes sense!  ;D