Imagine yourself waking in a strange land. In the distance you can hear some voices speaking in a strange language, the words are unintelligible.

Reaching into your pocket, you find some foreign looking coins and paper 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 world. In a sense, digital imaging providers like BowHaus are traders and sellers of pixels. We operate as "Currency Exchanges", using scanners to convert pictures into pixels and digital printers to convert pixels back into pictures.

To carry the currency analogy a step further, resolution is the "exchange rate". During scanning, input resolution determines the pixel dimensions of the resulting image file. Scanning at higher resolutions will increase the pixel dimensions, and also the amount of detail captured.

Within the computer, your pixels are stored as bits and bytes, which are 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 have to go through another "exchange rate": output resolution, which determines the 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 different.

If the input and output resolution are equal, then the digital output will be the same size as the original. If the output resolution is lower than the 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 resolution to precisely control the dimensions of your final digital output.

During scanning, input resolution determines the pixel dimensions of the resulting image file.

This 35mm negative was scanned at 1780 PPI, which is also approximately 70.08 PPM. The actual area scanned measured 37mm x 25mm.

To calculate the resulting pixel dimensions, multiply the millimeters by the PPM, or multiply inches by PPI.

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 to calculate the resulting file size in bytes, kilobytes or megabytes. The file size allows us to verify that enough storage media has been supplied with the scanning job.

Multiply the width and height for the total number of pixels: 2593 x 1752 = 4542936.

Multiply the number of pixels by the number of bytes per pixel (bit-depth) 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

Divide the bytes by 1024 to convert to kilobytes: 4542936 / 1024 = 4,436 Kb. One megabyte is 1024 kilobytes. Divide the Kb by 1024 to convert Kb to Mb. 4436 / 1024 = 4.3 Mb. You can also skip the bytes to Kb conversion and go directly from bytes to Mb by dividing bytes by 1048576 (1024 x 1024). 4542936 / 1048576 = 4.3 Mb

Output resolution determines the final size of your output.

In this example, our scanned image file was 2593 x 1752 pixels. Once in the computer, we can edit and modify the image in a raster image-editing program like Photoshop.

The image was cropped slightly to 2442 x 1584 pixels. The original was grayscale, but to increase the visual impact we converted the color mode (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. Before 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 (IMAGE>IMAGE SIZE) reveals the current size and output resolution settings.

The box titled: "Resolution:" is the output resolution. The original was scanned at 1780 PPI and the output resolution was set to 304.8 dpi when the 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 8.012" x 5.197" at 304.8 dpi. Since we already knew the pixel dimensions of this image, we could have calculate the final print size at any output resolution without Photoshop using this simple formula.

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 output resolution.

Fixed resolution devices, like the LVT and Fujix Pictrography, restrict you to a set of resolution options. On these devices, you may only print at one of the fixed resolution options. Interpolation is often needed for precise control of the final size.

In this case we are not using a fixed resolution device. So to calculate the output resolution setting that will make this image 6.50" wide divide the 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".