Not wanting to hi-jack the recent thread about *how* to up-res befor printing, I would like to know *why*.
My thinking is this. I shot an image with a certain actual resolution, cropped it and edited the remaining 'camera' pixels. If I send those to the printer, the printer driver is going to map those pixels to actual nozzle-squirts as effectively as it knows how.
What is to be gained by increasing the resolution of the image in Photoshop (or similar) rather than letting the printer driver have maximum control over the mapping? Can Photoshop create a better interpolated pixel than the printer driver?
The reason for increasing resolution is so that print sharpening can be performed. We print-sharpen to restore the detail that is lost through the printing process.
Before I dive into it let me give an example. Here are images of two prints made from the same image file. The first was printed after some light and typical processing (contrast, sharpening, etc.) That processed images was then print sharpened to give the second print. The different is dramatic...
I can tell you from having the image and actual memory board that the second print is much closer to the actual object. So whatís going on? Well, printers are not monitors. They donít create images in the same way. As such, the same pattern of pixels will give different results.
On a monitor, every pixel element is square and has a black border around it (take a look with a magnifying glass.) This provides separation and definition (many computer icons will have black borders for the same reason.) On a printer, however, every image pixel ends up as a matrix of dots, and those dots overlap one another...both within the matrix and along the border with other pixel matrices. Dots must overlap because they are round, and round dots placed next to each other will leave white space between the dots. This overlapping of dots has the effect of reducing the sharpness of the image.
The solution to this degradation of image quality is to create tiny halos around detail. When printed, these halos will blend, reducing the sharpness, and providing an image that contains all the detail that can be seen on the monitor.
Before print sharpening, an image should be upsized to the native resolution of the printer (600 PPI for Canon, and 720 PPI for Epson.) Why? Because you want to create halos as small as possible. At 600 PPI, a 1-pixel wide halo is only 1/600 of an inch wide. Thatís what you want. Youíre not going to see these halos...youíre only going to see the combined effect of the halos. If your image is already going to print at 600 PPI or slightly greater without upsizing, then just leave it.
So back to the examples. The calculated PPI on these prints was 712 PPI, so I didnít have to resize the image for print sharpening. As I said, the second print was processed for printing. In this case I simply applied unsharp-mask at values of 500, 0.8, 2 (threshold should be adjusted if there are smooth gradients, like sky.) This created small-scale halos around detail. When the image was printed, the halos disappeared due to overlapping ink dots between pixels, leaving a much improved image. The improvement is clearly visible at a viewing distance of 12 inches.
There is software that will do this for you. Qimage does a great job of restoring detail, Nik Sharpener Pro provides an output sharpening function, as does other software. So if you donít want to do it yourself then you can have software do it for you. But it really should be done. I encourage people to download the evaluation version of Qimage just to get a print and see the difference. I was quite surprised and a bit perplexed at how big the difference was. But now that I know why, it makes perfect sense.