Image Resolution and Printing

Bit & Bytes

A bit is the smallest unit of data in the computer world. It can be 1 or 0, on or off, black or white.

A byte comprises of 8 bits. A byte can therefore represent 256 different states. 2 to the power of 8 = 256.

Bit Depth

All images have three channels of colour (red green & blue) recorded onto the sensor when the shutter is released.

In an 8-bit image each channel records 256 levels of brightness levels. When talking about 8-bit imagery, we really should be referred to it as 24-bit, given that each channel records 8bits of information separately.

The higher the bit rate the more information a single pixel can record. A 16-bit image can handle 65,536 levels of information per channel, so a 16-bit image should really be called a 48bit image. A 48-bit image is capable of recording billions of colours.

The key advantage when using high bit depth is when you apply adjustments to you images in post-production. In an 8bit image you only have 256 levels of data, so when data is stretched or compressed it can lead to posterization. Posterization is abrupt shifts in colour or brightness in an image. The more bits available per channel the more realistic and fine the image.

Resolution

Resolution is how many pixels horizontally and vertically in an image. A 15 mega pixel camera has a resolution of 4752 X 3165.

To calculate the amount of pixels of a chosen resolution, you simply multiply the width by the height.

Example: A resolution of 4752 X 3165 = 15,054,336 – rounded to the nearest million = 15 mega pixels.

A mega pixel is one million pixels.

Resolution is what distinguishes cameras. The more mega pixels you have the higher the resolution. Of course resolution really only comes into play when you are printing. Depending on what size you want to print your images, those extra mega pixels might be of no use to you.

Print Resolution

Resolution also refers to how many pixels per inch (PPI) you have when you print on paper. A common resolution standard for the printing industry is 300 pixels per inch.

It is important to note here that print resolution only comes into effect when you are printing. It is pointless adjusting the resolution in Photoshop, or whatever software you are using until you know what size you are going to print your image.

DPI

DPI is a term that originated in the printing industry. It refers to how fine ink is laid down on the printing paper. In the printer driver of you particular printer you can adjust the dpi settings before printing. This doesn’t affect the size of the print, only the quality, amount of ink used and the printing speed.

Re-sampling

All image software allow you to re-size imagery, but it is important to understand that by simply re-sizing a small resolution image to a higher resolution wont give you a better quality image, just the opposite will happen. When re-sizing an image to meet printing specifications of 300 dpi, all you are doing is stretching the image. This will inevitably lead to loss of quality in the image.

To get the maximum size you can print an image from your cameras native resolution without re-sampling, simply divide 300 (PPI) into the width and height of the resolution.

4752 X 3168 = 15,054,336 (15mp)

4752 / 300 = 15.84in

3168 / 300 = 10.56in

15.84in X 10.56in is the max size you can print a 15MP image without re-sampling.

The best way to insure high quality prints is to start with a high resolution and use the formula mentioned above.

Ideally you want to print super sharp prints at 300 PPI so they even look good through a magnifier.

You can calculate this using the formula:

Long print dimensions in inches = 4 X (square root of megapixels)

Long print dimensions in centimeters = 10 X (square root of megapixels)

Example:

The square root of 15 (MP) is 3.872 – 4 X 3.872 = 15.49 inch

The square root of 15 (MP) is 3.872 – 10 X 3.872 = 38.72 cm

This formula will give you the biggest you can print an image when viewed through a magnifier without loosing sharpness. 

The resolution issue is also one of scale and viewing distance. Given that most of us don’t go around with a magnifier in our back pocket checking the quality of prints, it is safe enough to print images at a resolution that is sharp enough viewed at arms length. 

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