How to optimise DPI in PhotoShop for large format prints

The number of dots per inch (DPI) in your Photoshop file determines how crisp and clear a print made from it will be. Because the metric is normalised to inches, understanding how this figure works is key to getting great output at any scale and choosing the best format for large prints.
This becomes especially important when working with large format prints. In this article, we'll break down:

• What DPI really means for large prints
• How to use Photoshop tools to optimise resolution and select the best format for large prints
• How HP's printing technology ensures that your vision is realised with flawless detail

What Is DPI and Why Does It Matter for Large Format Prints?

DPI is the number of dots printed per linear inch of your output. You'll often see DPI and PPI (pixels per inch) used interchangeably in digital design. They have a 1:1 ratio, but DPI technically refers to the physical printing process, while PPI is used for screen displays. For large format prints, like posters, banners, blueprints, and more, DPI is a critical factor in determining the final print's clarity.

Large format prints have an advantage: they're designed to be viewed from several feet away. This means that a lower DPI can provide the same perceived effect as a larger one on a smaller sheet of paper. Understanding how the viewing distance impacts the resolution requirements of your image will help you optimise your workflows to get the best results.

The Relationship Between DPI, Image Size, and Print Quality

DPI, image size, and print quality are all directly related to one another. When you double the print size, each pixel is doubled in both the x and y directions. This means doubling the size makes each pixel four times larger. You've now got a much bigger size, but a quarter of the resolution. Because DPI is measured in one linear direction, that's halved when you double the size. This relationship between DPI, image sizes, and resolution forms the foundation of successful printing.

In practical terms, this means an image that has 300 DPI for an A4 print will only have 150 DPI when printed at A3 size. To understand how this change impacts your prints, you'll need to know how viewing distance enters the equation.

Exhibition graphics viewed from three to five feet away typically need 100-150 DPI to look good. Meanwhile, billboards are viewed from 20 feet or more away. From that distance, they can get by with 30-50 DPI and still be perceived as high-quality.

Best Practices for Different Print Applications

Let's take a more detailed look at common DPI requirements for the best format for large prints:

• Close viewing applications, those from zero to one meters, require 150-300 DPI for crisp detail when viewed up close.
• Medium viewing applications, from one to two and a half meters, can get by with 75-150 DPI to balance quality with file size.
• Larger distances can function effectively as low as 30-75 DPI, as the human eye can't distinguish the dots at that distance.

Setting the Correct DPI in Photoshop

How to Check and Adjust DPI Settings

You can check your DPI in Photoshop by navigating to Image > Image Size. The dialogue box will display your image's dimensions in pixels and its print size. Remember, DPI and PPI have a 1:1 ratio. So you'll find the DPI in the Resolution field, typically measured in "Pixels/Inch."
To adjust DPI, follow these steps:

1. Open your image in Photoshop
2. Go to Image > Image Size
3. Choose your resampling method (see below)
4. Enter your desired resolution value

Photoshop DPI Scaling Tips for Large Format Print

Simply doubling the size of the image will halve your effective DPI. However, there are several sampling methods that help retain crispness when the image is upscaled. Early, naive approaches like bicubic sampling are better than nothing, but still not great. Newer versions of Photoshop have an AI-enhanced method called "Preserve Details 2.0" that dramatically improves the results. You should use this whenever possible.

Another tip to zero in on the best format for large prints is to scale the image incrementally. The algorithms provide better results when they have to do less work. Increasing it by 10-20% increments helps them retain more quality at each step.

Finally, consider converting your image to a Smart Object before applying the scale. This will allow you to use a non-destructive workflow so your Photoshop scaling for large format print projects doesn't permanently degrade quality.

Using a Resolution Calculator to Plan Your Print

What Is a Large Format Print Resolution Calculator?

A print resolution calculator allows you to enter your desired DPI and print dimensions to get the exact resolution you'll need to hit that DPI target. They can be particularly helpful if you don't know your document's size in inches, as they'll do the conversion for you. However, calculating it yourself is easy. Just multiply the media size in inches by the desired DPI:

Example: Calculating DPI for an A0 Print

Let's say you want to print an A0 poster. In inches, that's 33.1 by 46.8. We'll calculate the desired resolution for two sizes:

For close viewing (3 feet), we'll choose 150 DPI:

33.1 x 150 = 4,965

46.8 x 150 = 7,020

So the desired resolution is 4,965 × 7,020 pixels. Depending on the format, this could result in a file size of around 100 MB.

Medium Distance

For medium viewing (6 feet), 100 DPI suffices:

33.1 x 100 = 3,310

46.8 x 100 = 4,680

This reduces the requirements to 3,310 × 4,680 pixels. That 100 MB file now might instead be around 45 MB. When the extra resolution isn't needed, this reduction in file size can be very beneficial.

Choosing the Best File Format for Large Prints

Best Format for Large Prints in Photoshop

Some formats are better than others. Lossy formats, like JPEG, can degrade the quality of the image. The file formats below will ensure accurate translation from screen to paper.

• TIFF: Supports layers, alpha channels, and high bit-depth images without compression artifacts
• PDF: Works for raster and vector elements, preserves fonts, and maintains colour accuracy
• PSD: Ideal for maintaining maximum Photoshop compatibility, but not supported directly by all printers

Preparing Print-Ready Files for HP Large Format Printers

HP large format prints, such as the DesignJet series, benefit from a specific file preparation process to achieve the best format for large prints.

• Colour management: HP provides downloadable profiles for various paper types and ink sets
• Bleed settings: Add 0.25-0.5 inches bleed around your artwork if needed to account for trimming
• Resolution optimisation: Align your file resolution with native DPI settings for optimal output
• Flattening layers: Flattening files can improve RIP processing speed and reduce potential errors

Turn Your Designs Into High-Impact Prints

When you master DPI optimisation in Photoshop, you'll transform good designs into good large format prints. Knowing the calculations we've discussed will help you get the best quality at the lowest file size. These techniques apply to prints of all sizes.

Remember that the most successful large format printing combines technical excellence with creative expertise. HP large format printers deliver technical excellence. Now, you can get a great deal on a new large format printer with the HP DesignJet Promo.