I’m working on a project for work, for a billboard. Well, not a full sized billboard, but one that is 169"x96". It’s in an airport, so people will be walking by it. The printer says image needs to be minimum 100 DPI at final size. My image is high res at 300 DPI and is 5232x3488, but needs to be enlarged. When we have it the size we want, InDesign says it’s effective DPI is 31. My assumption is I need to take it into photoshop, resample and preserve details in changing the image size to size that will give me an effective DPI of 100? Or, is there another way to do this?
Of course that’s logical. BUT, there’s no singular line of logic that applies to every image and application. It could very well be that the image will print just as good, or better, from 31 ppi than it will if you use Photoshop to up-sample it enough to satisfy the 100 ppi technicality, which has the potential to ruin it at any printed size. It depends a lot on the nature of the image for one, but don’t forget about typical average viewing distance either. Even if the public has access to walk right up to it, no one expects to visually process a 169" x 96" composition from less than 10 feet away, if not farther. And, even if you already had the requisite 100 ppi already comfortably in hand, your printed wall graphic wouldn’t be clean and smooth at walk-up distance anyway.
Enlarging in Photoshop is worthless.
Your assumptions are incorrect. Here’s why.
First, PPI (or, as you termed it, DPI) is the number of pixels in a one-inch line of pixels at the size the image will be used. More pixels roughly equate to the ability of the image to contain more detail.
At ~250–300 ppi, a typical image contains enough pixels (and detail) to look sharp at normal hand-held reading distances of around 18 inches. As @HotButton pointed out, if the image will only be viewed from 10 feet, the PPI doesn’t need to be as high since it will still look sharp from that distance. The PPI can be correspondingly less for a highway billboard viewed from hundreds of feet.
Of course, upsampling the image to a higher PPI is doable. However, what you’re not considering is that doing so doesn’t create more detail or sharpness in the image. Photoshop can’t invent detail in an image out of thin air. All it can do is enlarge the photo to contain more pixels. In other words, you’ll have the same blurry image you had a 30 ppi, even though you stretched out that blurriness to contain three times as many pixels.
However, as Hotbutton also pointed out, a great deal depends on the sharpness and detail in the original images. If it’s very sharp or blurriness doesn’t matter, maybe you could get away with a 30 ppi image seen from 10 feet out.
However, that’s unlikely. I haven’t seen the images, of course, but chances are, nobody will be happy with it blown up to that size. For something meant to be viewed by people walking by at around ten feet, I’d be hesitant to use an image that didn’t already start out at 100 PPI. The only surefire solution is to use a larger image with the number of pixels you need already in it.
The image is 300 DPI licensed from Getty, just a stock image. It’s really crisp and clean. I’m just paranoid that the “effective PPI” in InDesign is throwing me off.
Effective ppi is what the ppi is at the scaled size in InDesign.
If it’s 300 ppi then that’s it. If it’s 30 ppi then that’s it.
It’s just a simple inverse proportion. If you scale the 300 ppi image to 2x its size, the effective resolution is 150 ppi. At 4x its original size, the effective resolution is 75 ppi.
What is the product and what is the viewing distance?
When we do roadside billboards, the rule is
scale = 1:10 with image at 300ppi.
so effectively that billboard is 30ppi when printed.
You are basially doing a 15’x8’ mural. You don’t look at a mural from arm’s length, usually (there are exceptions.) I’d normally expect to print a file at 75ppi for a mural that size. Maybe even as low as 50ppi depending on the content.
If it is a backlit mural printed on SEG fabric (silicone edge graphic) you can approach the lower end as there is a tiny bit of dot gain with fabrics that helps hide a slightly too low rez. But not much.
If it is mural vinyl stretched into a flex face frame, or if it is Self-adhesive vinyl (SAV) applied to a wall or board, that is why they are asking for 100ppi. You might ask them if they are printing at 600dpi (ink spew dots) or greater for that resolution…
Some do. Some don’t. If the imagery is good, you might get away with interpolating in Photoshop to 75ppi.
Photoshop has new interpolation engines. While it can’t make pixels out of thin air, it’s doing a pretty darn good job of guessing in the newer versions.
Read this: Photoshop's new AI feature quadruples the amount of pixels in your photos
The reason I asked what the product was, some things like high end merchandizing requires ultra sharp imagery to avoid appearing cheap. Usually such products are shot with a scan back so you have the super resolution to begin with. But where your image is from Getty, I’m guessing it is more background?
Don’t forget your bleeds. They may be larger than you think. 
I need to qualify the above that this assumes using a layout program where the image is placed and any vector art or text is going to be kept vector sharp. If you are flattening a photoshop file at 75ppi, everything will be 75ppi, including the text. If there is text meant to be read up close (one of those exceptions I mentioned) flattening is a bad idea.
Where you said this is in an airport, is it in an overhead display or a ground level walk-by? It’s really large for a ground level walk-by. Perhaps meant to be viewed from across the room/corridor?
We’ve been using Topaz Suite - Gigapixel AI, Sharpen AI and DeNoise AI - for enlargements that need more ‘oomph’ than even photoshop’s latest algorithm’s can provide.
We’ve gotten pretty good results out of them.
Perspective: We print on self-adhesive vinyl of various sorts, usually intended for vehicle fleets like tractor trailers. A lot of times the source imagery provided by customers is of a quality not terribly suitable to produce at 110 inches tall by 600-odd inches long. Effective PPI at output is usually somewhere in the 30-75 range by the time we’re finished processing / resampling it. Good enough for something you see from 30ft away moving at 60+ mph.
1 Like
I use Pixelmator Pro’s AI function called ‘Super Resolution’ which does a really good job in creating an image with around 2,5x the pixels.