Pictures are made up of pixels, but knowing the number of pixels doesn't necessarily give us the dimensions of each pixel. For instance, a headshot that is 3000px by 2000px doesn't reveal the size of the headshot itself.
To clarify, each picture has a resolution, representing the number of pixels per inch (PPI). A higher resolution means smaller pixels, and a lower resolution means larger pixels, which can become visible when the resolution is small enough.
Factor #1 Viewing Distance
The detail you can see in a graphic is influenced by the distance from which you view it. Close up, you need a higher resolution to see more details. From further away, you can get away with a lower resolution without losing perceived sharpness.
Shorter Viewing Distance
At closer viewing distances, higher resolutions are necessary to maintain a sharp appearance in images. Lowering resolution at close range can noticeably degrade the perceived sharpness of an image.
Further Viewing Distance
High resolution can increase printing time and costs. For images viewed from a distance, a lower resolution can yield the same perceived sharpness, due to the way distance blends light rays, resulting in a seemingly sharp image even if the resolution is lower.
The resolution of a billboard-sized photograph is often only 10 to 20 ppi.
The resolution doesn't need to be higher because the viewer is so far away from the photograph.
As you move away from your monitor, the lower-resolution versions of the above photograph, below, gradually become as sharp as the original. The photograph has been cropped to make it easier to fit on your screen.
Uncropped Print Size = 55" x 83"
Typical Viewing Distance = 12.4'/~3.8m
Uncropped Print Size = 28" x 42"
Typical Viewing Distance = 6.3'/~1.2m
Uncropped Print Size = 13" x 20"
Typical Viewing Distance = 3'/~0.9m
Determining Print Viewing Distance
A formula for determining the ideal viewing distance is to multiply the print's diagonal measurement by 1.5. This distance is where the print's quality appears optimal.
Determining PPI needed
To calculate the necessary PPI for acceptable print quality, divide 3438 by the viewing distance. This number comes from known facts about human vision and the visual acuity angle.
Factor #2: Viewing Condition
The conditions under which a print is viewed can significantly impact the perceived quality. Clear weather allows for higher resolution viewing, while adverse conditions like snow can obscure details, requiring less resolution for an acceptable quality.
Good lighting conditions necessitate higher resolution prints to accommodate the viewer's ability to see finer details. Conversely, in dim lighting, the resolution can be reduced without compromising perceived quality.
Temperature affects air humidity and light refraction, impacting the perceived sharpness of a print. Extreme temperatures might necessitate a reduction in resolution due to the already compromised viewing quality.
If a print is to be viewed for an extended period, higher resolution may be necessary as viewers will have more time to notice finer details and any imperfections.
Factor #3: The Subject
The subject of the print, such as a headshot or a full portrait, influences the required resolution. Close-up images like headshots need higher resolution to capture detailed facial expressions compared to full portraits.
Factor #4: Noise
Noise refers to any unwanted visual artifacts in a print or graphic. Factors contributing to noise include paper and ink quality, image compression, and the inherent resolution. Adjusting the resolution can help mitigate noise, but the relationship is complex as resolution can also contribute to noise appearance.
Factors Attributing to Noise
Various factors contribute to noise in prints, including the texture of the paper, the quality of the ink used, the level of image compression, and the resolution. Each of these can either introduce or amplify noise, affecting the overall clarity of the print.
Factor #5: Brightness
The brightness of colors in a print can influence perception more than color itself. Bright colors can sometimes overshadow the need for high resolution, as they capture the viewer's attention and make other details less noticeable.
Considering print size and viewing distance is crucial in determining the necessary resolution. All the factors discussed contribute to the perceived quality and sharpness of the print and must be taken into account during the planning stage.