reducing image resolution

[weasel_bar]

What happens if the image resolution exceeds the printer's dots per inch? For example, if an image intended to be 4x6 inches is 3600x5400 pixels, then the digital image resolution is 900 pixels per inch. If a printer can print 300 dpi, then each one inch square of image contains 900x900 pixels but must be represented by 300x300 dots of ink. 

 

The documentation of my editing software says that to represent multiple pixels by one, down-sampling is performed by taking a weighted average of neighboring pixels. Different methods give different amount of weight to the original pixel vs neighbors.

 

-There is obviously a limit to UP-sampling, as you are adding "fake" data. Is there such a thing as too much DOWN-sampling? 

-There are a bunch of different algorithms in the options, like bilinear, bicubic etc. Is there a "best" method/setting to choose for reducing image size? Do these algorithms only apply to interpolation methods for INCREASING size, or do they also apply to reducing size?

-if I DON'T resize the image, and just send the large file to the printer and ask it to print 4x6, does the printer have its own algorithm to downsize? 

 

[kmac]

1 hour ago, weasel_bar said:

-if I DON'T resize the image, and just send the large file to the printer and ask it to print 4x6, does the printer have its own algorithm to downsize?

As far as I know, the image data is adjusted automatically when a smaller paper size is selected. Other members know far more about this than I do, but I'm sure the data must be adequate for the size of the printed image. If there is enough information data for an A3 size print for example, then you'll be right for print sizes smaller than A3.

Edited January 4, 2023 by kmac

[digitaldog]

One of the best pieces on the subject: https://www.digitalphotopro.com/technique/photography-workflow/the-right-resolution/

[glen_h]

There is no "too much", but there is doing it wrong.

 

It has to be done, at least close to, right to stop aliasing.

Consider down-sampling from 901 to 300 (in whatever unit).

Most of the new pixels will have 9 of the original, but some will

have more, 12 or 16. If not done right, a line will be visible

where it changes.

[paddler4]

I've done some informal testing of Schewe's advice, using a Canon Prograf printer (native resolution 300/600 dpi). Schewe's advice is to avoid substantial downsizing, so to use the higher native printer resolution with a print that roughly matches or exceeds the number of dpi at the selected print size. Printing on coated paper, I was able to see slight improvement in the large images printed at 600 dpi when using a magnifying glass, but in actual practice, at least with eyes as old as mine, there was not an appreciable practical difference. As a practical matter, printing at 300 dpi is fine.

One caveat: I print from Lightroom, which Schewe recommends but that many people disdain. It handles resizing very well and very simply You just make sure that the print resolution is set to one of the printer's native resolutions and let the software handle resizing.

[digitaldog]

1 hour ago, paddler4 said:

IOne caveat: I print from Lightroom, which Schewe recommends but that many people disdain. 

"If fifty million people say a foolish thing, it is still a foolish thing."-Bertrand Russell