How does enlargement affect resolution in the digital negative process?

Question

I was wondering recently about the digital negative process and was surprised to see very little on here relating to it. As I understand it, most people use it to create contact prints of monochrome negatives, created by printing a digital image capture onto transparent material (such as overhead projector transparency) with an ink jet printer. Any wet printing process from cyanotype to silver halide can be used.

My question is this, would there be any difference in result between a large negative contact printed and a smaller one printed with an enlarger.For instance a negative printed at A4 then contact printed and a negative printed at A5 then blown up to A4 with an enlarger? How would this relationship continue as the amount of enlargement increased?
This enlarger answer suggests that the larger contact print will always be better, presumably due to enlarger lens defects. Even without the lens, diffraction and the fact that the light source is not a point source will create soft edges that become softer the further the negative is moved from the print ie. the greater the enlargement.

I realise you can't create information that isn't in the original file, but eventually a digital file will pixelate when printed too large showing the jaggies "staircase" pattern of aliasing on diagonal and curved edges. Would the wet print from the enlarger just become more and more blurry and indistinct?

Answer 1

This may depend on your original negative size (what you have to work with), but 35 mm film ought to print A4 OK

Printers have resolution limit that they can print. The modes are:
1-bit line-art might be 2400 dpi (optimistic for inkjet ink drops)
8-bit grayscale might be 600 dpi (I'm kinda guessing on grayscale).
24-bit color is commonly about 300 dpi.

This is because grayscale must print several dots per pixel to simulate 256 gray tones with only black ink.
Color must print several dots per pixel to simulate 16.7 million colors with 4 colors of CYMK ink. All of these ink drops must approximately fit in a pixel space.
1-bit line-art prints only one black ink dot per pixel.
Dots may of course be omitted to be white paper color.

Ink jet carriage and paper motor step spacing specifications are NOT the same thing thing as ink drop size, nor as pixel dpi.

So resolution of your larger A4 case depends on the size and resolution of your original negative, and on the resolution capability of the printer. Probably OK.

Your smaller A5 case has those same requirements, but neither size can exceed resolution of what the printer can print. Then enlarging that to be 1.4x larger only suffers, due to no additional detail from the original source. It simply just interpolates, which is a blurring process. We might quibble about what the final resolution of A4 case is, but whatever it is, the A5 case then enlarged 1.4x will be that much less.

Answer 2

I think short answer is that inkjet printer has resolution: pixels per inch (PPI). So whether you print on A4 or A5, you are limited by the same PPI resolution.

Inkjet printers have typically 300-700 PPI, whether digital cameras (say, nikon d600 with 24 megapixels) have about 6000x4000 pixels.

This means that your final print resolution is limited by your printer and there will be difference between printing negative on A4 or A5, because in latter case you compress your original high-res image into less dots than with A4.

In order to answer your second question, you can calculate how many dots going to be in one line on A4 and A5 prints. The difference (210mm vs 148mm) is about 30% and will stay that way no matter how large prints you make from these negatives.

Answer 3

How much difference you'll see between using A4 and A5 will depend on a couple of variables you haven't revealed to us:

• The maximum resolution of your image files in pixels
• The maximum resolution of your printer in PPI (pixels per inch)

If your printer can outresolve your image files at A5, printing larger at A4 won't have a tremendous effect. But if your image files are large enough that they need to be downsampled to print at A5 or even A4, then the larger negative will give you an advantage.

Assuming you are using a 6000x4000 pixel digital camera, your image file would scale to about 512 pixels per inch on A4 and 725 PPI on A5. If your printer can print at 725 PPI¹ or higher resolution then using A4 or A5 would make very little difference. But most printers can only print at around 600 PPI or less. Sometimes a lot less. In which case you would get higher resolution results printing any enlargements from an A4 negative than an A5 negative.

Alternately, let's say you are using the 8688x5792 pixel Canon EOS 5Ds. Now your digital file scales to 742 PPI on A4 and 1,050 PPI on A5. Unless you have a printer that can print more than 1,050 PPI, the difference will be even greater between A4 and A5. On the other hand, if you are shooting with a 12.2MP 4272x2848 camera, the printer only has to be able to reach 517 PPI to make results from an A5 negative about the same as from an A4 negative.

_{¹ NOT DPI, which is a measure of each drop of ink. Each pixel requires multiple droplets of ink. Please see How do I generate high quality prints with an ink jet printer?}

Answer 4

I want to add -- The image resolution is far greater than the printed image. This is also true for images viewed on computer screen. What do you think happens to the pixels that are not printed or displayed? Answer, they are discarded by the software and you will have little or no control over which pixels are used vs. those that are discarded.