EXIF "resolution" values

[daveleo]

I apparently don't understand the significance of the x- / y- resolution values shown in image EXIF data. I understand what "pixels / inch" is and that pixels vary in size, but . . .

The EXIF resolution values for JPG images coming out of my cameras varies from 72 (Fuji X100), 180 (Panasonic LX3), 300 (Nikon DSLR and also my netbook's camera). The Nikon and Fuji have the same sensor size but different # pixels; ratio-ing the Nikon 300px/in, the Fuji should show about 234 px/in (not 72). Note: all my numbers here are at the "Fine" JPG level from the camera.

Are these numbers directly comparable? Meaningfully?

The Fujix100 is making fabulous images even viewed at 1X. My netbook camera is a joke! The 72 px/in for the Fuji just cannot be an indication of the resolution (can it?) .

Internet explanations of EXIF numbers hasn't help my understanding.

 

[d__b]

I think those numbers are just an arbitrary value with no meaning unless you print those files and the printer takes these numbers to determine the print resolution instead of using its own settings.

 

[Dwig]

The numbers are not arbitrary, but also don't have anything to do with the resolution of the image file itself.

These numbers are notes placed in the file by the creater software (camera's firmware when "shooting" JPEG) to inform printing and page layout software how the "creator" intended that they be printed. One should also keep in mind that there is no requirement for the "creator" to put any value in this field and that when this field is empty it is the convention (not an absolute rule) that 72ppi be used by default. Some advanced image editing apps also use this field when compositing images.

When you place an image into a page layout or printing application it must size the image relative to its output "page". This PPI value is what it uses to decide how many of the image's pixels are mapped to each "inch" in the page by default. Almost all such apps allow you to alter this while retaining all of the original pixels and simply changing the effective PPI.

Also, while it's rarely done, some image formats allow for non-square pixels. As a result, they have independent PPI values for width (X-axis) and height (Y-axis).

 

[daveleo]

Thanks for those replies.
Amazing that I never looked at that data before, and when I did the other day, I thought that I set my (new) X100 to the wrong resolution values? Got all confused.

Thanks.