canon 50D ... what's up with 72 dpi?

[ptcollins]

I recently purchased a Canon 50D (previously owned a 10D) with the anticipation of stepping up my photographic skills and taking everything to the next level, but after taking numerous pictures in the Large Jpeg mode (not camera raw) I noticed that when opening the shots in Photoshop, all the images were 72 dpi, which does not work for the graphic print world which requires a minimum of 300 dpi for all images. My old 10D recorded images at 180 dpi in the same Large Jpeg mode. Can anyone tell me what to do to increase the dpi and/or explain why a better camera records smaller dpi resolution than its earlier predecessor?

Thanks

 

[Dwig]

You need to understand several things:

1. DPI stands for Dots Per Inch. It is a totally meaningless term in any environment where there aren't any inches.

2. DPI is an incorrect term for what you are referring to even though many otherwise good applications use it when they should be using PPI (Pixels Per Inch). Digital images have pixels not dots.

3. The PPI value associated with a digital image is mearly a note recorded in the file's header (actual file header, in the EXIF data space, or both) telling a page layout application (vector art tool like Corel DRAW, FreeHand, Illustrator, PageMaker, MS Word, ...) that the creator of this image desires that the pixels in the file be mapped onto that application's virtual page at a particular scale. It is not, in and of itself, a measure of anything concerning the file's quality.

4. The printing industry's desire for 300ppi (even though most incorrectly use the term DPI) is in reference to the effective PPI at the final reproduced size. It it not referring to whatever size and PPI data is stored in a digital file's header (see #3 above). Many publishers do prefer that files be tagged at 300ppi for their convience and it makes it easier for their archiving software to tell them the maximum reproduction size possible from any given image.

5. The resolution of a digital file is best determined by the file's size is pixels. When attempting to determine it from the PPI you must also take into account the associated linear dimensions in inches.

6. Applications don't always read the PPI recorded in a files header and not all software that creates image files bothers to write a PPI value. Also, there are several places in a file to store the information and not all applications are consistant in where they store it.

7. All applications that do attempt to use PPI (e.g. Photoshop, Illustrator, PageMaker, ...) have to have a built-in default to fall back on when they fail to read a PPI value in a file. Photoshop, like many applications, uses 72ppi as its default. When a file is opened in PS and PS doesn't find a defined ppi value it assigns 72ppi. This in no way alters the number of pixels in the image.

Thats a lot of info. Here's an example of two different files:

File A: a 300x300 pixel file that has been assigned 300ppi. It would be 1 inch x 1 inch when printed by an application (assuming no scaling when printing) that properly respected the PPI value.

File B: a 3600x3600 pixel file that has been assigned 72ppi. It would be 50 inches x 50 inches (over 4 foot square) when printed if it wasn't scaled in printing.

File B is substantially higher resolution than File A but it has a lower PPI value. If you simply changed the PPI setting on File B to 300ppi without any resampling, thus retaining exactly the same pixels, you would have a file that would print by default at 12" x 12".

From your description of Photoshop's reaction to the images from your two Canon cameras, either Canon has altered the camera's internal firmware to change the PPI it assigns or Photoshop is not finding a PPI value, either because there isn't one or its stored in some criptic place that your version of PS doesn't see.