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Which Resolution? > Long Answer >

1 - Print Size & Viewing Distance

The pixel dimensions of your photograph, such as 3000 x 2000, don't give it a size.

The pixel dimensions describe only how many pixels there are in your photograph.

If we only know the pixel dimensions, we don't know how large each pixel is.

Resolution is the number of pixels per inch (ppi).

When you enter a resolution figure, you're giving a size to the pixels of your photograph.

For example, a resolution of 300 pixels per inch produces small pixels.

A resolution of 72 pixels per inch produces larger pixels.

If you enter a resolution that's too low, the pixels will become visible.

If you enter a resolution that's unnecessarily high, your eyes will not see any improvement in the print, and it will take longer to print.

What's the best resolution for your prints?

Go to the Printing & PPI Flow Chart for the quick answer.

For the detailed answer, read on.

Six Factors

The best resolution for a print depends largely on six factors.

The first factor is the most important.

Factor #1 - Print Size & Viewing Distance

The best resolution is determined by considering the print size and print viewing distance.

Smaller prints are viewed from closer distances.

Therefore, higher resolutions are needed for smaller prints.

Larger prints are viewed from further away.

Therefore, high resolutions are not needed larger prints.

So, as print size increases, less resolution is needed.

Example #1

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.

Example #2

q

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.

q

36 ppi

Uncropped Print Size = 55" x 83"

Typical Viewing Distance = 12.4'

When you're 12.4 feet from your monitor, the bicycle in the above photograph will appear to be sharp.

q

72 ppi

Uncropped Print Size = 28" x 42"

Typical Viewing Distance = 6.3'

q

150 ppi

Uncropped Print Size = 13" x 20"

Typical Viewing Distance = 3'

Determining Print Viewing Distance

Harald Johnson, in Mastering Digital Printing, cites a formula by Joe Butts for determining viewing distance.

Viewing Distance = 1.5 x Diagonal of the Print

Some photographers use the less stringent multiplier of 2 instead of 1.5.

Here are the viewing distances for common print sizes, with normal lighting, with the resolution needed for a print with acceptable quality.

If the viewing distance is expected to be closer than the figures below, or if the viewing conditions are optimal, then you may need higher resolutions.

Viewing Distance Chart
Print Size Diagonal

Viewing Distance

(1.5 x Diagonal)

PPI

Needed

4 x 6" 7.21" 11" 313
8 x 10" 12.81" 19" 181
8 x 12" 14.42" 22" 156
 11 x 14" 17.80" 27" 156
16 x 20" 25.61" 38" (3.17') 89
16 x 24" 28.84" 43" 80
20 x 30" 36.06" 54" (4.5') 64
40 x 60" 72.11" 108" (9') 32

Determining the PPI Needed

The ppi needed for a print with acceptable quality is determined by dividing 3438 by the viewing distance.

3438 ÷ Viewing Distance

3438 is derived from the following formulas.

1 ÷ ppi = 2 x Viewing Distance x tan(.000290888 ÷ 2)

1 ÷ ppi = Viewing Distance x tan(.000290888)

ppi = 3438 ÷ Viewing Distance

.000290888 is the visual acuity angle, a measure of how much resolution the human visual system can perceive.

The above explanation is based on Resolution by Andrew Gregory.

Printing & PPI

The chart below compares the ppi needed for a print with acceptable quality, with the actual ppi.

As you can see, you can use a camera with few megapixels to make quality prints, when viewing distance is also considered.

The figures for PPI Needed, below, are based on the viewing distances in the above chart.

Printing & PPI Flow Chart
Megapixels

Pixel

Dimensions

JPEG

Quality

Print

Size

PPI

Needed

Actual

PPI

Print

Quality

2 1600 x 1200 Highest 4" x 6" 313 267 Fair
8" x 12" 156 133 Fair
16" x 24" 80 67 Fair
3 2048 x 1536 Highest 4" x 6" 313 341 Excellent
8" x 12" 156 171 Excellent
16" x 24" 80 85 Good
4 2272 x 1704 Highest 4" x 6" 313 379 Excellent
8" x 12" 156 189 Excellent
16" x 24" 80 95 Good
5 2592 x 1944 Highest 4" x 6" 313 432 Excellent
8" x 12" 156 216 Excellent
16" x 24" 80 108 Excellent
6 3008 x 2000 Highest 4" x 6" 313 501 Excellent
8" x 12" 156 251 Excellent
16" x 24" 80 125 Excellent
7 3072 x 2304 Highest 4" x 6" 313 512 Excellent
8" x 12" 156 256 Excellent
16" x 24" 80 128 Excellent
8 3264 x 2448 Highest 4" x 6" 313 544 Excellent
8" x 12" 156 272 Excellent
16" x 24" 80 136 Excellent
10 3888 x 2592 Highest 4" x 6" 313 648 Excellent
8" x 12" 156 324 Excellent
16" x 24" 80 162 Excellent

Print size, in tandem with viewing distance, are the main determinants of the best resolution needed for a print.

You may want to use the Pixels, PPI, & Print Size Calculators.

Also, consider the following factors.