Now here is where things can get confusing!
When we talk about image files we are really talking about Pixels Per Inch (ppi) not Dots per Inch (dpi) which is used to describe printers and print resolution.
When you begin scanning images at home or using files from your digital camera you will often read or hear people say thing like "This image (or scan) is 2400 dpi, so it must be big enough to make huge prints" or "that scan is way too small, it's only 72dpi". Sounds reasonable.
Well, maybe or maybe not. In accurately describing an image, the only real numbers that mean anything are the width in pixels and the height in pixels. The reason for this is simple mathematics - An image that is 8x10 at 300ppi (dpi) is the SAME as an image that is 33x42 at 72ppi (dpi). So, if you say that an image is 1200 dpi but don't specify a corresponding size the 1200 number is meaningless:
Notice how the original file contains 1200 pixels vertically and 1800 pixels horizontally, and that at 4x6 that works out to 300ppi, and at 8x12 it is only 150ppi. The file size has not changed and the resolution has not changed either - a 72ppi image can indeed be way bigger, with better resolution, than a 2400ppi image.
When you scan film the same holds true - when
you scan a film frame that is 1 inch tall and 1.5 inch wide (35mm) at 1200
dpi you get a file that is 1200x1800. That is the same as our file above.
So you can see that saying your film scan is 1200 dpi is not any better than
our example image is at 300ppi.
So what can you do with this knowledge when making prints?
Simple - pick your largest print size based on the pixels dimension divided
by the size of the desired print, and keep that number above 300 for smaller
prints, 240 for medium prints and 180 for larger size prints.
Say for example our file is 2400x3600 and we want to make some prints, but how big can we go without losing quality or resampling?
At 4x6 the print resolution will be 600dpi
- very good
At 8x12 the print resolution will be 300 dpi - good
At 16x24 the print resolution will be 150 dpi - decent, but consider
resampling to 240
At 24x36 the print resolution will be 100 dpi - consider resampling to 180
Resampling is when you artificially increase the pixel dimensions and
size, and your image editing software is forced to interpolate, or make up
pixels to fill in the missing data. This process is not as bad as some
people might think, just don't go overboard and, for example, force a
1200x1800 file to be interpolated way up to 3600x5400 - you won't like the
resulting digital artifacts and softness.
In Photoshop we recommend using 'Bicubic Smoother' for upsampling images, or
you can get a dedicated program like Genuine Fractals to do the work for
you.
Did this help? If you still have questions feel free to contact us and we will be happy to help!
