How resolution affects the outcome during input and output.
Imagine yourself waking in a strange land. In the distance you can hear
voices speaking in a strange language, the words are unintelligible.
Reaching into your pocket, you find some foreign looking coins and
bills. You could be looking at fifty dollars or fifty cents.
Waking up in the digital age can be like waking up in a foreign land.
Learning the language and understanding the currency will keep you from
becoming isolated and confused.
Bits, bytes, and pixels are the "currency" of the Digital Imaging
a sense, digital imaging providers like BowHaus are traders and sellers
pixels. We operate as "Currency Exchanges", using scanners to convert
pictures into pixels and digital printers to convert pixels back into
To carry the currency analogy a step further, resolution is the
rate". During scanning, input resolution determines the pixel
the resulting image file. Scanning at higher resolutions will increase
pixel dimensions, and also the amount of detail captured.
Within the computer, your pixels are stored as bits and bytes, which
just numbers. In the computer, or on your storage media, your original
picture exists as bits, bearing no resemblance to the original.
In order to view bits and bytes in a meaningful "image-like" form we
go through another "exchange rate": output resolution, which determines
final size of the digital output.
It's important to understand that input and output resolution are two
separate "exchange rates". They may be the same, or they may be
If the input and output resolution are equal, then the digital output
be the same size as the original. If the output resolution is lower
input resolution, the digital output will be larger than the original.
Confusing? Not to worry. The following primer will show you how to
calculate: pixel dimensions, file size, and how to use output
precisely control the dimensions of your final digital output.
During scanning, input resolution determines the pixel dimensions of
resulting image file.
This 35mm negative was scanned at 1780 PPI, which is
approximately 70.08 PPM. The actual area scanned measured 37mm x 25mm.
To calculate the resulting pixel dimensions, multiply the millimeters
PPM, or multiply inches by PPI.
| Input Resolution
| Inches x PPI = Pixels
| Millimeters x PPM = Pixels
In this example:
37 x 70.08 = 2593
25 x 70.08 = 1752
the result is: 2593 x 1752 pixels.
Kilobytes and Megabytes
Once you know the image file's pixel dimensions, it's a simple matter
calculate the resulting file size in bytes, kilobytes or megabytes.
file size allows us to verify that enough storage media has been
with the scanning job.
Multiply the width and height for the total number of pixels: 2593 x
Pixels(X) x Pixels(Y) = Total Pixels
Multiply the number of pixels by the number of bytes per pixel
to arrive at the total number of bytes.
This image is Grayscale, so the
number of bytes is one (28). 4542936 X 1 = 4542936 bytes
Total Pixels x Bit-depth = Bytes
Divide the bytes by 1024 to convert to kilobytes: 4542936 / 1024 =
One megabyte is 1024 kilobytes. Divide the Kb by 1024 to convert Kb to
4436 / 1024 = 4.3 Mb. You can also skip the bytes to Kb conversion and
directly from bytes to Mb by dividing bytes by 1048576 (1024 x 1024).
4542936 / 1048576 = 4.3 Mb
Bytes, Kilobytes & Megabytes
Bytes divided by 1024 = Kilobytes
Kilobytes divided by 1024 = Megabytes
Bytes divided by 1048576 = Megabytes
Output resolution determines the final size of your output.
In this example, our scanned image file was 2593 x 1752 pixels. Once in
computer, we can edit and modify the image in a raster image-editing
The image was cropped slightly to 2442 x 1584 pixels. The original was
grayscale, but to increase the visual impact we converted the color
(bit-depth) of the image to RGB (IMAGE>MODE>RGB COLOR).
In the RGB Color mode, we have lots of control over the image. Using
Photoshop's powerful color editing tools we added color to the image.
printing, this image will be converted to CMYK.
In Photoshop, the Image Size command gives us complete control over the
final size and output resolution. Opening the Image Size Window
SIZE) reveals the current size and output resolution settings.
The box titled: "Resolution:" is the output resolution. The original
scanned at 1780 PPI and the output resolution was set to 304.8 dpi when
file was saved. At BowHaus, all drum scans are set to 304.8 dpi before
saving to disk, unless otherwise instructed.
As shown in the Image Size Dialog Window above, this image will be
5.197" at 304.8 dpi. Since we already knew the pixel dimensions of
image, we could have calculate the final print size at any output
without Photoshop using this simple formula.
Output resolution (A)
Pixels divided by DPI = Inches
Pixels divided by PPM = Millimeters
In this example:
2442 / 304.8 = 8.012, 1584 / 304.8 = 5.197
Voila! The same results without Photoshop!
We want this image to print 6.50" wide in a 4-color brochure, not
8.012". On most output devices, we can precisely control the final size
without interpolating (resampling) the image by simply modifying the
Fixed resolution devices, like the LVT and Fujix Pictrography, restrict you to
of resolution options. On these devices, you may only print at one of
fixed resolution options. Interpolation is often needed for precise
of the final size.
In this case we are not using a fixed resolution device. So to
output resolution setting that will make this image 6.50" wide divide
pixel width by 6.50.
2442 / 6.50 = 375.69
With the "Resample Image:" box unchecked, enter "375.69" into the
"Resolution:" box. The image will now print 6.50" x 4.216".
Output resolution (B)
Pixels divided by Desired Inches = DPI
Pixels divided by Desired Millimeters = PPM