# Enlarging small photo for a large format print (banner) [duplicate]

I don’t have a lot of experience with large-format prints and was assigned a task of creating a wall vinyl banner 2,80 m × 2,92 m. I’m working on a smaller artboard in InDesign (25 % of given measurements). There’s a picture that I need to use and it’s quite small for this job: 15 cm × 17 cm, 300 dpi. How much could I enlarge it so that it would fill the good amount of the canvas but it wouldn’t be blurred or pixelated?

I would also appreciate the help with the text size on this project. What is the usual size for heading, body text, dates?

2,80x2,92 meters is quite a big size. At first you must decide what is the shortest distance where it should be sharp enough. Think, how far away the nearest serious watchers are. If they are so far away that they seem your full banner as big as you see your image onscreen and the image is ok, the nothing special is needed for the sharpness.

You must learn how to calculate the apparent size,. It means: how big your banner must be on the screen to look out the same as in reality.

Let the real watching distance be 6 meters. Let's abbreviate it R = 6 m. Let your onscreen watching distance be 0,5 meters; S = 0,5 m. Let your banner have width 2,8 be meters: BW = 2,8 m. Then the apparent width (=onscreen width) OSW can be calculated following way:

OSW = BW * (S/R). In the example OSW = 2,8 * ( 0,5/6) meters = 0,23 meters = 23 cm. That's probably quite near a half of your screen width.

When watched from 3 meters, its apparent width is 46 cm

You must make all text and important details to be readable in the apparent size. Place your photo artistically properly into the layout and find, does it look good in the apparent size without anything else processing. If ok, then nothing special is needed. If it's blurry, then find a new image or use it in smaller size with other images or make it sharper.

Making sharper is really possible by resizing the image to bigger DPI by special image resizer program that retains the sharpness. Of course it can't know the missing details, but it can quess passably. ON1 Perfect Resize is a good one, but there are many more.

Another method to make it sharp enough is to enlargen the DPI and using some sharp edge producing artistic filter. Of course it's thatafter not a photo, but a fake illustration. But it's a sharp and high contrast fake illustration.

One thing can surprise you if you do not check it beforehand. Namely "how sharply the printhouse outputs". Ask them, what is their output DPI in a banner that big. Calculate how many pixels wide the banner will be when printed. Normal CMYK printing should look out good, if your apparent sized banner looks out good and the final banner has a width in pixels at least 2x, merely 4x as much as your apparent banner has screen pixels.

More thinking

• consult with the printhouse in early phase and see their actual printed samples that have samelike contents and have printed by using THE SAME process, inks and base material which you have thought. Do not accept substitutes unless you have seen samples of them, too.
• spot colors (=one coloured separate inks) have the best contrast
• the contrast in a photo must be extreme if it must be the same as spot color texts or drawings.
• is your color management ON and the color profiles right including the printhouse's. If not, you should check the result in the printhouse being personally present.
• do they glue narrower slices together - beware shifted seams. The seam must not be in important place.

If a shifted seam slices a human face, it looks out awful. Have it written how much shift is allowed.

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