How to upsize an image for Alamy and other stock photo libraries

Alamy ask for files that are 48 Mbytes in size, RGB, 8-bits per pixel, and jpeg compressed using quality 10 or above in Photoshop. This needs some translation.

The 48 Mbyte file size is for an uncompressed photo. The image has to be RGB (rather than say, CMYK which has a reduced colour space).

Remember that 8-bits is one byte. Each pixel has one byte for the Red, one byte for the Green, and one byte for the Blue colour channels. So, we have 3 bytes per pixel.

A typical 8 Mpixel camera will give an uncompressed file size of 8 x 3 = 24 Mbytes. Alamy require 48 Mbytes, or equivalently 48/3 = 16 Mpixels.

The source of possible confusion with Alamy is that they ask for a 48 Mbyte file as a jpeg image.

Jpeg is a wonderful technology if used right, it reduces file sizes by throwing out information that is the least easily perceived in a photo (it is a lossy compression technique). You can control the amount of information that is lost, but you can very easily overdo it by throwing out information that you can see, leaving behind ugly artifacts.

The file size for a jpeg image is highly dependent on the image content. File sizes can be reduced without artifacts or any loss in perceived image quality if the compression setting is not too severe – hence Alamy ask for jpegs saved at quality 10, 11, or 12 (the highest in Photoshop).

So how do you upsize to meet the Alamy requirement of 48 Mbytes?

The answer is simple, upsize your photo to 16 Mpixels.
Figure 1 shows how to take a 9.5 Mpixel photo (from a Canon G7) to 16 Mpixels. You need to upsize by 130%.

How did I get that 130% figure?

The CCD sensor in a digital camera is about the size of a finger nail, for example the Canon G7 has a sensor chip in it that is 7.18 by 5.32 mm. This is divided up into 9.5 million little square pixels just as if the chip was tiled. A pixel is effectively a unit of area. In terms of area you need to upsize by:

Area upsize factor = (16 Mpixels) / (Camera Mpixels)

For the Canon G7 the Area upsize factor is: 16 / 9.5 = 1.68

Software that can upsize images (eg Photoshop, SizeFixer, etc) use a "linear upsize factor". This is a percentage increase along the width (or height) of the chip, rather than the increase in area. Both sides of the rectangular chip are upsized by the same amount.

So we need to determine the correct Linear upsize factor to apply in order to meet the requirements of Alamy:

Linear upsize factor = SquareRoot[ Area upsize factor ]

                             = SquareRoot[ (16 Mpixels) / (Camera Mpixels) ]        Eq. 1

For the Canon G7 the linear upsize factor for Alamy is:

     Sqrt[ 16 / 9.5 ] = Sqrt[ 1.68 ] = 1.30

And since most software expresses this as a percentage we can do the same, ie 130%.

You can use Equation 1 to compute the upsizing you need for Alamy (or any other Stock Photo Library). In Table 1 I have given a list of values for typical Mpixel sizes.

Tim Atherton
FixerLabs

Table 1. Example Linear upsize factor for Alamy.