One of the things we have to wrestle with when we want to make a print is how big can I print this image and get good results? And how large should I print it? There is a lot of advice out there. Some of it is good.
Film vs. Digital
Virtually all images have to be scaled up for printing. The print you want to hang on your wall is many times larger than the sensor or piece of film you start from. Hardly any of us are shooting 8×10 negatives these days. Even if we are, we still usually want to make larger prints.
The technology has changed completely from the film days. Enlargement used to be optical. By adjusting the enlarger lens and the distance from the film carrier to the print surface, the image was blown up to the desired size. If the lens is good, it faithfully magnifies everything, including grain and defects. If the lens is cheap, it enlarges and introduces distortion and blurring.
Digital enlarging is a totally different process. A digital image is an array of pixels. My little printer at my studio likes to have 300 pixels/inch for optimum quality. So if I want to make an 8×10 print and I have at least 2400×3000 pixels, it will print at its best quality without changing a thing. Digital enlarging is a matter of changing the number of pixels.
Digital enlarging
But usually I want to print a larger size than the number of pixels I have. Here the digital technology gets interesting. And wonderful. Going back to my example, if I want to make a 16×20 print and maintain best quality, I would have to double the pixels in each dimension. It would have to go to 4800×6000 pixels.
Photoshop has the ability to scale the number of pixels in your image. There are several algorithms, but the default, just called “Automatic”, usually does a great job. Here is the difference from film: software algorithms are used to intelligently “stretch” the pixels, preserving detail as much as possible and keeping smooth transitions looking good. Lightroom Classic has similar scaling for making a print, but it is automatically applied behind the scenes. Smoke and mirrors.
The result is the ability to scale the image larger with good quality.
Print technology
In a recent article I discussed a little of how an inkjet printer makes great looking prints using discrete dots of ink. There are other technologies, such as dye sublimation or laser writing on photosensitive paper, but they are far less used these days.
It should be obvious, but to make a really big print, you need a really big printer, at least in the short dimension of the print. Really big printers are really expensive and tricky to set up and use. That is why most of us send large prints out to a business that does this professionally.
Why do I say the printer has to be big in the short dimension of the print? Past a certain size, most prints are done on roll feed printers. They have a large roll of paper in them. Say you have a printer that prints 44″ wide. The roll of paper is 44 inches wide and many feet long.
We want to take our same 8×10 aspect ratio image and make a 44×55 inch print. If it was film, we would require an enlarger with at least a 44×55 inch bed and a cut sheet of paper that is 44×55 inch. But an inkjet printer prints a narrow strip at a time across the paper. The heads move across and print a narrow 44 inch long strip of the image, the printer moves the paper a little bit, and it prints another narrow strip. Continuing until it has printed the entire 55 inch length. Then the printer automatically cuts off the print.
But if we naively follow the recommendations for optimum quality, we have to scale our poor little 2400×3000 pixel image up to 13200×16500 pixels. Even the best software algorithms may introduce objectionable artifacts at that magnification.
Viewing distance
Maybe we don’t have to blindly scale everything to 300 (or 360) pixels/inch.
A key question is: at what distance will the image be viewed? Years of studies and observation led to the conclusion that people are most comfortable viewing an image at about 1.5 to 2 times the image diagonal length. This lets the natural angle of the human eye take in the whole image easily. For the example we have been using of the very large print, people would naturally choose to view it from about 105 to 140 inches.
Along with the natural viewing distance there is the acuity of the human eye. I won’t get into detail, but the eye can resolve detail at about 1 arc minute of resolution (0.000290888 radians for the nerds). Simply, the further away something is, the less detail we can see.
Going through the calculations, if our audience is viewing the large print from 1.5 times the diagonal, it only has to be printed at 33 ppi! Finer detail than that cannot be seen from that viewing distance.
I have heard photographers who have images printed for billboards or the sides of a large building talk about inches/pixel. It would look like Lego blocks up close, but it looks sharp from where the viewer is.
Nature of the image
This is true unless the audience is photographers. They are going to get right up to the print, as close as their nose will allow, to see every blemish and defect. 🙂 But normal humans will view it from a distance.
There are modifications to the pixels vs. viewing distance calculations depending on the nature of the image. If the image contains highly detailed structure it will encourage viewers to come closer to examine it. If the image is very low contrast, smooth gradations, it could be even lower resolution.
Printing at the highest possible resolution that you can for the data you have is always a good idea.
Your mileage may vary
How big of a print can you make? It depends – don’t you get tired of hearing that? It is true, though. The real world is messy and simplistic “hacks” often don’t work well. It is better to understand things and know how to make a decision.
When it comes down to it, these are great times for making prints, even large ones. My normal print service lists prints as large as 54×108 inches on their price list. I know even larger ones are possible.
How big should you print? How big can you print?
Conventional wisdom is that scaling the pixels 2x each dimension should usually be safe. My camera’s native size is 8256×5504 pixels. Scaling an image 2x would be 16512×11008 pixels. This would be a “perfect” quality print of 55×36 inches on a Canon printer. I have yet to need to print larger than that.
Given the perceptive effects of visual acuity, I am confident I could create much larger prints. Larger than is even possible by current printers. And they would look good at a reasonable distance.
A key question is who are you printing for? A photographer or engineer will be right up to the print with a magnifying glass looking at each pixel. Most reasonable people will want to stand back at a comfortable distance and appreciate the image as a whole. Who is your audience?
Learn how to scale your image without artifacts and how to use print shapening to correct for problems. Know the perceptual effects of human visual acuity. This is part of the craftsmanship we have to learn in our trade. Given those tools, the rest is artistic judgment. With today’s equipment and careful technique and craftsmanship we can create wonderful results.
Your mileage may vary.
The image with this article is very small – 3 MPix. I would not have a problem making a 13×19 print of it. I doubt you could see the pixels.
Have you tried to make large prints? How did it go? Let me know!