Actually, DPI is everything in the printing process.
An image at 4000x2000@72DPI has an actual print size that will be tiny.
DPI is about
resizing to a destination dot or line raster screen (which has a given number of dots or lines per inch). It has nothing to do with the number of pixels in the file, only the destination rasterizing parameters.
As I said, an antiquated workflow, like for traditional press, is stuck in old concepts. Modern pre-press workflows do understand the difference, because they start with pixels, then define the required output size, divide number of pixels by output size in e.g. inches (or centimeters) and the result is the resampling factor for a given destination (without the destination parameters the DPI is useless and superfluous info in an original data image, that's why often just 72 PPI is put in the tag field placeholder). When the output size is not defined as inches or centimeters, it is just a conversion to the same dimension in output dots/lines/pica's whatever one is comfortable with.
An image at 4000x2000@300dpi will print at actual size.
No it won't. A 4000x2000 pixel image is dimensionless until the physical output dimensions are assigned, and after resampling is done. Try printing a 4000x2000 pixel image as a newspaper image, and try printing it as a high quality glossy. Both use different output screening settings, requiring different resampling to achieve the same physical output size.
My Canon 6D's produce an image size of 5089x3393@300dpi natively. That equals a print size of 11.31x16.963inches@300dpi.
It doesn't produce an image at any physical size, it is dimensionless until one specifies how large a pixel is to become in output. 5089x3393 is dimensionless, the 300
PPI(!) is a tag which can be set to anything (it's a preference in Canon's DPP converter). Changing the PPI tag, doesn't resize the image, it remains 5089x3393 pixels. All that the tag does is say that
IF the image is printed at an output size of 300
pixels per inch, it will measure 5089 / 300 PPI = 16.9633 inch by 3393 / 300 PPI = 11.31 inch.
Now imagine that same image printed at a print size and dpi of 11.31x16.963@72dpi. The print resolution would be horrible at 11.31x16.963inches@72dpi but would look fine printed at its actual print size of 2.713x4.07inches. But I need a printing resolution much larger than that. So you can see that dpi is everything when it comes to an actual physical print.
Sorry, that's not what happens. It might if you
resample the image to fewer pixels, but 5089 pixels / 4.07 inches = 1250 pixels / inch (PPI, the acronym says it all,
pixels
per
inch). Dots have nothing to do with it, unless the output screen has dots. If it has raster lines then it would be LPI. Likewise, 3393 pixels / 2.713 inch = 1251 pixels / inch PPI.
Photoshop's resize function allows to change the PPI tag,
and either resample the image, or not. In fact, it also may change the value that the user puts in the PPI field without telling, and that value will get saved. It can only be detected if the image is reloaded in the resize dialog, and it may now be different, caused by imprecise rounding.
Lightroom typically resizes for an output dimension in a physical dimension, like inches or centimeters. That's why that dialog is called Resolution and it is in PPI, not DPI because it can't know what raster screen resolution (in DPI or LPI, or LPC) it will be printed on.
Cheers,
Bart