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For wall sized graphics and large banners (e.g 3m x 5m), what is an acceptable PPI for print.

As i understand it 300 PPI is the typical for 'small' artworks (esp. for clean text resolution), however for small artworks the audience usually engages at a much closer distance. Therefore in the interests of keeping a bitmap file down to a resonable size can you get away with a much reduced PPI for a large format/banner print?

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9 Answers

up vote 19 down vote accepted

In general you should use vector graphics in the artwork wherever practical, and deliver final artwork to the printer in PDF or other vector format. Your finished print will then be limited only by the output resolution of the print device.

This is particularly important with text and line art -- visible rasterization in the finished print will be very obvious and look amateurish.

While a PDF can contain photographic images, they will have a fixed resolution, and while 300 ppi is a good rule of thumb for publications and small posters viewed at close range, there's no way you'll achieve anything close to that at the dimensions you're talking about. Certainly you should aim for as high as possible, but I don't think there's a hard-and-fast lower limit. I've certainly ended up with around 75ppi in photographic elements of 2m high event banners, which looked fine at relative distance. I never felt the need to do any interpolated upscaling of the raster images.

So, again in general, your workflow would be:

  • produce raster elements in Photoshop and output as TIFF files
  • complex vector elements like logos or illustration in Illustrator and output as EPS or PDF files
  • compose the final artwork in InDesign, linking in the raster and vector files, adding text and simple vector elements like colour blocks.
  • output composite PDF from InDesign with mix of raster and vector elements, ensuring that raster elements are output at their native resolution, i.e. not downsampled to a particular output resolution.
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I like the accepted answer, it has good advice, but I thought I'd expand on it a bit.

For wall sized graphics and large banners (e.g 3m x 5m), what is an acceptable PPI/DPI for print.

Here's definitions, so we know what we're talking about.

  • DPI = Dots per inch = units used to measure the resolution of a printer
  • LPI = Lines per inch = The offset printing 'lines' or dots per inch in a halftone or line screen.
  • PPI = Pixels per inch = the number of pixels per inch in screen/scanner file terms.

Now you said "As i understand it 300 DPI is the typical for 'small' artworks (esp. for clean text resolution)" -- here you're confusing DPI with PPI (often done.) Raster artwork for print is generally scanned at 300 PPI. Why? Because most raster artwork is printed with CYMK processes at a maximum (generally) of 150 LPI. The rule of thumb is that we need 1.5 to 2 times the LPI in PPI to get acceptable results.

Why are most things printed at 133 or 150 LPI? Because at reading distance for CMYK printing the dots aren't generally discernible. Because of high-speed printing and cheaper paper/printing of newspapers, they are often as low as 85 LPI, so you can see the individual dots easily on the funny pages.

So your question can be distilled down to: what is the minimum LPI halftone screen I need so that it's not distracting at the distance the poster will be viewed? I did a little searching, and actually found a research paper on this subject. The subject was black and white printing, but since colour halftone dot patterns should even be less noticeable, I think the advice can be extrapolated.

Here's the chart:

Distance   Present Study     
20 feet    greater than 10 LPI   
18 feet    18.75 LPI or greater  
16 feet    18.75 LPI or greater  
14 feet    37.5 LPI or greater   
12 feet    37.5 LPI or greater   
10 feet    50 LPI or greater     
8 feet     65 LPI or greater     
6 feet     85 LPI or greater     
4 feet     100 LPI or greater    
2 feet     133 LPI or greater    
1 foot     150 LPI or greater    
6 inches   150 LPI or greater

Presumably, for a banner 3m x 5m, you'd be standing at a minimum of, say, 10 feet. (Just eyeballing the wall here.) So, by this table, you'll need 50 LPI minimum. That would mean your raster graphics should be about 100 PPI, or 75 PPI at 12-14 feet. Considering that and the fact that 2x LPI is pretty conservative for reproducing fidelity (often 1.5xLPI is "enough"), this agrees with @e100's advice of 75 PPI being acceptable.

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+1: this covers the rationale behind the rule –  horatio Mar 29 '11 at 21:07
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Grande format resolutions, as various folks have pointed out, depend on viewing distance. Several answers refer to line screens, but technology has moved on. Very few, if any, grande format jobs are printed that way, and in any case, none of the answers indicated how to translate from PPI (dots) resolution in Photoshop to LPI (lines of dots at different angles) resolution in offset. They are NOT the same thing. The related question e100 refers to has answers that also don't really cover the necessary bases.

There are two answers, depending on whether you have a images (raster) or shapes (vector, including text) or a mixture of both. As e100 pointed out, vectors are a Good Thing when you need to scale stuff up for grande format, but clients are clients, jobs are what they are, and we don't often get to choose.

RASTER (IMAGE) DATA

The overall, practical rule is: don't go much above the minimum resolution you need, to keep file sizes manageable and avoid problems with the printer's RIP (Raster Image Processor -- the software that translates you artwork to the physical dots that will hit the paper). Beyond a certain fairly definite point, which I'll cover, increasing the file size does nothing to increase output quality.

Let's start with offset, since most of us are familiar with it, then we'll look at big format stuff. Traditional offset printing uses four inks laid down in a definite order, "screened" using variable-sized dots arranged in lines spaced a certain distance apart. This is the origin of the term "line screen". The lines of dots are at a different angle for each color, carefully arranged to minimize the visual interference patterns called the "Moire effect." How far apart these lines are spaced gives us "lines per inch" (lpi). Newspapers typically use a coarse screen of 75 lpi, magazines 133 lpi, fine art magazines and books up to 200 lpi or more. This is NOT "dots per inch" (dpi).

You can figure out what minimum ppi you need for offset work by multiplying the lpi (given to you by the publication or print provider) by two to give you the ppi you should maintain in your image at full size. (There's good, sound math behind this calculation, involving esoteric stuff like Nyquist limits. -- I didn't invent it. I just work with it.) Thus for magazines, brochures and the like, 266 ppi (or dpi, depending on the program you're working with) is a good number if the final output is regular offset.

There is another, increasingly common type of "screening" for offset printing, that you'll see referred to as "stochastic" (which means "random") screening. This uses an irregular placement of the dots, giving more even gradations and finer detail for the same nominal "dot screen". For stochastic screen printing you can go as low as 200 ppi/dpi for handheld pieces, in practical terms. I've not seen any formal study on this, so this is partly empirical data from my own experience, proving out advice from printing industry sources.

300 ppi, which is often touted as "print resolution" is a generally-regarded-as-safe number that will work up to 150 lpi screen, but will not be enough for a high-quality 180 or 200 lpi job and is unnecessarily large for newsprint. It works well for that inkjet printer sitting beside your desk, however.

If you use a higher resolution than is necessary, the RIP will simply throw away the excess pixels and make its own decision about which ones to keep, so you don't gain by sending in artwork at 600 ppi for a 75 lpi job that only requires 150 ppi of image data.

Inkjet processes are very different from offset. They don't put pigment on the paper in 4 angled lines of dots, and the dots usually don't vary in size, so there's no such thing as "lpi" with inkjet. Because the dots are sprayed and overlapped, you can go as low as 150 ppi for about the same visual effect as 300 ppi/150 lpi in an offset printed piece. You can also push resolution as high as the native resolution of the specific printer (typically 600 to 1200 dpi, higher for professional desktop photo printers), especially if your image contains lots of very fine detail, but you don't gain anything by going above it. In practical terms, you can translate ppi in the artwork straight across to dpi on the substrate for all inkjet-based manufacturing methods.

For the sake of completeness, I'll throw in dye-sublimation printing, used in some photo printers. Dye-sub is about 300 dpi, far less than a typical inkjet. High-end dye-subs go as high as 325 dpi.

Grande format is almost invariably printed using an inkjet type technology, so these "straight across" ppi = dpi ratios are adequate and correct.

Grande format jobs are often scaled in Photoshop. A Lamar (national billboard advertising company) 48 foot by 18 foot billboard lays out in a 17.64 inch by 6.84 inch Photoshop file at 300 ppi according to their published spec, which includes generous bleed on all four edges. When you do the math, that's 9 dpi "in the air", which is pretty typical for that market. 6 to 12 dpi is the normal range. You have to scale billboards, obviously, because you can't input 48 feet as an image width. But you would also have to scale a 3m x 5m image if you want to output as a PDF (which you do -- I'll get to that under "Output").

Illuminated advertising, such as the pedestals you see in your local mall, are at a high resolution of 150 ppi because they might be looked at from 3 feet/1 meter, but they can easily go as low as 75 ppi, especially if they don't have a lot of fine detail, without problems.

Vinyl banners of 10ft/3m or greater never need to be more than 50 ppi. (Finally! The answer to the original question!) The intended viewing distance is the deciding factor, and when a piece gets to be 10 ft wide, 10 feet away is about as close as you can get for comfortable viewing.

Without getting unnecessarily mathematical, anything that will be see from 10 feet away (across a small room) should be at most 75 ppi at full scale; at 20 feet or more, 30 ppi is plenty.

All that said, there is a BIG caveat about images: don't up-rez an image in Photoshop and think you have a "higher resolution image." You don't. What you have is a bigger fuzzier approximation of your original image with the same amount of image information padded out by what the software guessed the extra pixels ought to look like. An original, unscaled image printed at 50 ppi will look at least as good as, and usually better than the same image pseudo-enlarged in Photoshop to make it "150 ppi". And never, never, never take a super-compressed jpeg that your client "got from the website" and try to scale it up for print. Those jpeg compression artifacts get uglier and uglier the more they're scaled up.

VECTORS and TEXT

Shapes and text should be created and output as vectors. Because a vector image is composed of mathematical expressions, not pixels, it scales to any size. The trick is to keep that vector information in your output file and not rasterize it too soon, especially if the job is going to end up on an offset press.

When text or vectors pass through the printer's RIP to go to a printing plate, they are automatically turned into a raster format, typically at 2400 to 2800 dpi. Examine a printed picture and its caption in any issue of a magazine, and you'll see at once that the edges of the text are much more precise than the edges of objects in the image. For this reason, advice to output text at 300, 600 or even 1200 dpi is misplaced. You will only degrade the quality of the final output by rasterizing text or vectors before you send them to press.

When you send work to an inkjet press, vectors end up at whatever the output resolution is of the particular machine. For grande format, that's a top end of 1200 dpi, more often 600 or less, depending on the size of the output, so preserving vector information doesn't gain you quite as much, but it's still better to put that conversion off to the last possible moment rather than doing it yourself.

OUTPUT

A jpeg, tiff or png is a raster image. That's fine if it's all a photographic composition. But if your file includes text or vectors (such as a company logo), the fine resolution of the vector information is lost. Avoid this if possible. "Well," you say. "I'll just keep it as a native Photoshop file." Unfortunately, that often doesn't work out, because what most programs (except Photoshop itself) extract from a PSD is the flattened tiff copy of the composite image that is saved within the PSD, not the native Photoshop information. (Worse, unless you included the actual fonts that you used in the project, the printer's prepress department may use your native Photoshop file, but substitute a similar-but-not-quite-the-same font that will alter your design. The chances of that coming out better than you intended are... well, images come to mind of snowballs trying to survive in furnaces.)

The answer is to output a PDF from Photoshop, which preserves all the vector information as vectors, while leaving the raster data intact.

For best results (as they say on the label) TALK TO THE PRINT PROVIDER to find out what flavor of PDF they prefer, and if possible have them send you their PDF spec (often in a file with an extension of .joboptions) for you to follow. If the provider can't handle a PDF, it's a good sign you need to shop around for a different print shop or some serious lucky charms.

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300 dpi is the norm for most printers, but it all depends on the way you'll be printing. High-end offset printers will need 300 dpi. Kinkos? Not so much.

You can get away with 120 dpi, but I wouldn't go any lower than that. You will definitely see some loss in quality up close, but if you're not going with a high end print and you're planning on doing this large format, you shouldn't worry about people getting too close to the art.

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First, PPI and DPI are not interchangeable. One is a measurement of screen pixel density, the other is printer dot density.

Then, for large-scale printing, LPI comes into effect as well (measurement of the line screen).

For wall graphics, you'll likely want something between 72dpi and 260dpi depending on what you are printing, the substrate, and the eyeball distance of the viewer.

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Okay, what you really need to know is the angular resolution of the human eye, but most graphic arts folks failed trigonometry. :-)

The so-called "300 PPI Rule" is for a viewing distance of about 9", which is about the closest the average under-40 eye can focus. So what does that correspond to? Don't worry about trig tables, [there are lots of places on the web that will calculate that for you].1 One of them told me that is an angle of 0.02 degrees.

So, scaling that upward, you can determine what dot pitch frequency you need, given any viewing distance. As others have mentioned, the viewing distance is absolutely critical! If you reall expect those under 40 years old to look at it from 9" and not see pixels, you're going to have to do it at 300 ppi!

But let's say you choose a more reasonable viewing distance for you 3m x 5m poster, say 2 feet. Plugging that back into the trig calc of your choice, and you get a more reasonable 125 PPI.

I think the table @ghoppe gave is a great resource, but wanted to explain the math to anyone who cared...

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A good way I find to test if your image will come out at a decent enough resolution or not is to actually print out a small section of it with whatever printer you have easily accessible.

In my case, it's a pretty standard office laser printer. I blow it up to the correct size (illustrator is good for mock ups as it tells you the exact size and dpi it will print at on the fly) then print out an important section of the artwork onto an A4 sheet. You can blue tack/sticky tape it onto the wall, stand back and view it from whatever distance is relevant to your project and see how legible/readable/viewable it is at that size. I found that some banners which have been around the 70 dpi mark are perfectly viewable from a few meters back which is as close as the audience will come to the banners.

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I think Jan Steinman was close with his angular explanation. The DPI table is good as well but in the end it all comes down to pixels not DPI for photographic images.

Forget DPI, a good rule of thumb is that across your field of view your eye can not see more than 8,000 pixels. Therefore you should not create a bitmap image of more than 8,000 pixels across. If those 8,000 pixels are across 100 inches then your net DPI will be about 80 DPI (8,000 / 100).

If those same 8,000 pixels are across 20 inches you will get 400 dpi (8,000 / 20). An image which is only 20 inches wide will be viewed from closer up so it needs to be a higher DPI. If you were to stretch those 8,000 pixels to an 8,000 inch wide image you would only get 1 DPI but to view that image you would be standing a long way away and you would still not be able to see the pixels.

Use the 8000 pixel max rule and you will not go wrong. The only case where this rule breaks down is if the image is intended to be viewed more than one field of view wide like a long poster going up an escalator.

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This is an interesting concept, though tends to break down at certain sizes. For instance, you'd never use 8000 pixels on a business card. –  DA01 May 21 '13 at 22:50
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For upper bounds, 1200 dpi for black&white is common for mid-range ink jets. You'll see 9600 dpi with photo printers.

It really depends on how closely your readers will examine your work. For large graphics on a billboard or a drape, you can probably get away with less than 120 dpi. If there's fine work or small writing (up to 27 pt) you'll need higher than 300 dpi.

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You seem to be comparing apples and oranges here -- print device resolution in the first para, and image resolution in the second? –  e100 Jan 18 '11 at 9:41
    
@e100: The first paragraph isn't an answer by itself, it's just giving numbers as a reference point. I guess that isn't clear; I should come back and rewrite this answer. –  Charles Stewart Jan 18 '11 at 9:56
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protected by Gramps Dec 8 '13 at 7:26

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