What will a commercial print service do if the image I submit doesn't have enough pixels for the desired DPI?

Question

If I have a 4000px × 3000px image which I want to print at 200dpi, so its size will be printed at 20" × 15".

If I send the image to a commercial printers and ask it to be printed at 40" × 30" do they simply print at only 100dpi?

Or, does the printer machine still print at 200 dpi but guess every other dot based on the adjacent values?

Or, does the printer company resize the image so its 8000px × 6000px before printing

Would it be better if I resize the image myself. I use Lightroom... would I just do during export or is there a better way?

Answer 1

The first thing to do is understand that Pixels are not Dots. The best way to view it is that dots are physically printed, where pixels are virtual and what we massage in our respective image editors (PPI - Pixels Per Inch). Anyone who works with a lot of prints knows the difference but will be so used to knowing which form of "DPI" is actually meant that it's rarely noticed if you were to use the wrong one.

Without going too much into laborious detail, printing at this kind of size is likely to be CMYK or some variant where each dot will be one of those colours (Cyan, Yellow, Magenta, Key/Black.) You can change the 'dot' size in some processes but the actual colours remain the same and to get the illusion of continuous tone printers use a pattern of those dots at a much higher DPI level than the PPI of the image provided.

As part of the printing process it has to re-scale and convert the image from the DPI provided to the PPI fitting the printing mechanism (and sometimes other variables like the paper stock selected) in the same way it has to map the colour-space and sometimes colour model of the image received. Depending on the printer this might happen in hardware or in the driver, some printers have better tuned printer profiles than others and so the same model of printer may give you better results at one print-shop vs another.

From a technical perspective you don't need to do anything. You can upscale and use a tuned sharpening filter on your image but I've not seen evidence that they add enough to the image output to make it worthwhile (YMMV and the best thing to do may be to experiment.) Some print shops may apply something like this for you or it could also be implemented in the printer firmware.

The main thing is that you have to consider is actually 'viewing distance' - it's unlikely that you'd be viewing a 40x30" image from the same distance that you would a 20x15" for example, so you don't need such a high PPI value to get a decent output.

Answer 2

They usually leave the printer to its job because it has to do that anyway. Even when a printer prints a properly sized 300 DPI image, it must dither to its native resolution which works differently than the resolution a digital image.

Most printers need to use a pattern of dots to produce a single image pixel since a pixel can take on one of at least 16 million colors (For JPEG, more if converted from RAW) while a printer dot can take on between 3 and 12 colors usually with the possibility of varying the dot-size on some models.

The printer therefore uses whatever resolution it is given to fill the print area. In your example, then that becomes 100 DPI on input (where the D here really means pixel) and will be converted to the printer's resolution which is often 2400 to 9600 DPI (where the D here really means dot) and may not even be the same on both axis. In other words, there is a transformation from an image as an array of pixels to one which is a potentially non-uniform array of dots.

While you could resize yourself to a higher resolution, it won't necessarily help since there will now be two resampling passes which may introduce additional artifacts. Sharpening is sometimes done to offset from softness introduced by enlarging.

Answer 3

Simply print at only 100dpi.

This is the more feasible answer. They print your image at a given phisical size.

I must say. 100 ppi is not a bad resolution.

does the printer machine still print at 200 dpi but guesss every other dot based on the adjacent vales.

Not that exact values, but some plotter software make a simple smoothing so you do not see square pixels.

does the printer company resize the image so it's 8000px x 6000px before printing

I would say most likley not. No one should manipulate your images.

And what should I do, should I resize the image myself. I use Lightroom would I just do during export or is there a better way.

100 ppi is a good resolution. Yes, some resampling is also a good option. I recomend that you only resample at round numbers, in this case 200% with bicubic sharper or equivalent in lightroom.