Photograph Resolution - increasing?

Skint_Catt

How do I actually find out what dpi my photos are - maybe I'm doing it wrong?

Alfie_E

Skint_Catt wrote: »

how do I work out what DPI my photos are?

You don’t. DPI only has meaning for a surface. It makes sense when you have a scanner. The scanner is producing an image from a flat piece of paper. A camera takes in light from a volume. Your camera has an angle of view. As your camera has eight million pixels evenly covering that angle of view, each pixel will have an angle of view an eight millionth of the whole.

Imagine you take two photos. One is of a flower on the table, the other is of a tree on the horizon. The flower appears in its image as 300 pixels high by 300 pixels wide. The tree appears in its image as 300 pixels high by 300 pixels wide. Even though the two objects are of very different sizes, the different distances mean they cover the same ‘two-dimensional’ angle.

It’s only when you make prints of your photos can you think about DPI. If your prints are at 300 DPI, your flower will appear one inch by one inch. Similarly, the tree will appear as one inch by one inch.

Skint_Catt

So all these comps asking for a min of 300dpi can't work it out either? So why ask then?

weegie.geek

Your digital photos ARE NOT any particular DPI, and it's been mentioned several times.

DPI means Dots Per Inch, and is a printing term. DPI is meaningless in terms of digital photos.

Take a 10 megapixel photo. 4032*2480, give or take, and depending on the aspect ratio.

You want to print it at 300dpi? Divide 4032 by 300, and you've got 13.4". Divide 2480 by 300 and you've got 8.26".

A 10 megapixel image, printed at 300dpi, will be 13.4" by 8.26 ".

basically, unless you're going to print the photo, DPI means nothing whatsoever. I've no idea why the photo competition asks for 300dpi photos, because there's literally no such thing.
Perhaps they want photos of a sufficiently high resolution that printing them at such and such a size would enable them to be printed at 300dpi without needing to upscale the photo?


If they've literally asked for digital photos of a certain DPI, it's time to look for a new competition, because that one's run by idiots.

Flickr has lots of nice groups, if you don't have an account yet you should join up and submit your photos to some competitions on there.

tomstickland

Maybe they mean 300DPI when printed with no scaling factor.
Resampling to give higher resolution is only a nominal exercise. You cannot create more information than was already there.

Marty_J

This guide will explain how to change the dpi with Irfanview:

http://www.rideau-info.com/photos/changedpi.html

I have no idea why they want images that are 300 dpi, but this should enable you to satisfy their request.

TimBuckTeeth

I would just send the full 8MP image. I'm sure that is good enough resolution.
There is no point resizing it larger as it doesn't add any detail and will make it quite a large file size.

M4RKM

i'm going to but in here, and say, why not just resample the image to 300dpi, which is easy enough to do, and you then have a 300dpi picture..

i use photoshop, and it is easy enough to do in there... it doesn't create any more pixels, all it does, is when being printed, cram more pixels into that inch square.. i resample all the time before i print, out of habit really... especially when i'm designing..

it is odd though especially if the competition is an online only one...

M

Alfie_E

Skint_Catt wrote: »

So all these comps asking for a min of 300dpi can't work it out either? So why ask then?

If they specify a physical size, it makes sense. Looking at, say, the 2008 SPAA Photo Competition, they give both a DPI and a size. 10”x8” at 250 DPI would be met by a normal digital camera with 5.33 megapixels or higher. To get 10”x8” at 300 DPI would need 7.7 megapixels.

If they don’t specify a size, there may be an implicit assumption among photographic competition entrants that the size is 6”x4”. But, not being that keen a photographer, I wouldn’t know.

unrich

One comp I want to enter has a requirement for A4 printing and 300 d.p.i ruling out most of the snapping nation. I might send a link for the dpi article with my entry.