Resolution and File Size

Introduction

Pixels have no set size, they just expand or contract to fill the space available. Below the same image is shown twice, both times at the same image or document size. But each copy is made up of a different number of pixels which just expand in size to fit the image size. So while both 'image sizes' may be the same, the resolution of each is clearly different.

Figure 1 This image is made up of 1200 x 800 pixels and looks to the eye like a scene might look in reality.

The aim is to have enough pixels, which when resized to fit the image size, are smaller than the monitor or printer dots. The image above has a pixel size smaller than the monitor dots and so looks good. The picture below has too few pixels and so have resized to the point where the pixels are larger than the monitor dots and the image looks either bad, or even useless as in the 12 x 8 pixels image.

Figure 2 Although blown up to the same size as the previous image, this image is made up of only 120 x 80 pixels and now we are starting to see the actual pixels, which is called 'pixelation.'

Below are the same images as before but resized so that the pixel resolution is 72  dpi to suit the monitor. Since the pixels are now set to the same resolution, it is the image, or document size, that shrinks or expands to fit. Both images now have the same resolution, it is now the image size of each that is clearly different.

Figure 3 Here the same two images are now set to the same resolution. While both look good now, what differs is the size at which the image is displayed. The smaller image only has enough pixels to display well at that size. 

This means that when talking about the working file size of an image it is not enough to just state the resolution or image size by itself. Both resolution and image size need to be used together.

Resolution

So often people speak about a 300 dpi image as being a quality image. But what do they mean by a '300 dpi' image? This is rather like someone asking you how far is it from Abuja to Lagos and you answer '70 kilometers per hour'. They would look at you funny. That is because you only told them half of the answer. Yes it may be 70 kilometers per hour, but you need to tell them for how many hours.

This is the same as someone saying, 'Oh, just give me a 300 dpi image'. What they have told you is that they want each inch of your image to have 300 dots, but they haven't told you how many inches they want the image to be. It is the number of pixels in an image that determine file size; resolution by itself is not enough.

Figure 4 This is one inch of the image below. DPI means 'dots per inch' but you need to know how many inches to know the file size of an image. Just giving the resolution is not good enough.

Figure 5 Above is the full image. The image is clearly more than just one inch square. So to understand the file size you need to know both the resolution (300 dpi) and the reproduction size (8 inches high by 12 inches wide, for example).

So when an editor or a designer is asking you for an image and they say, "just send me a 300 dpi image", you need to ask, "300 dpi at what dimensions?" In other words, at what height and breadth?

File Size

There is actually an easier way to ask the editor what he really wants and that has to do with file size. This is much the same way that if someone asked you the distance between Abuja and Lagos it would be easier to answer, '700 kilometers'.

Figure 6 An A4 grayscale image at 300 dpi is calculated below at 8.6 MB in size. In reality it is closer to 8.7 MB simply because the dimensions of an A4 page are larger than 8 x 12 inches.

To see how the file size is calculated, take the image above as an example and assume that it is A4 at 300  dpi. To simplify it let's assume that is 8 inches  x 12 inches. 

File size is determined by the number of pixels so the aim is to find out how many pixels the image contains. Since the resolution is 300 dpi, it means that each inch is 300 pixels across. As the image is 8 inches wide that means there are 2,400 pixels in width (8 x 300). Likewise, as the image is 12 inches long, the image is 3,600 pixels in length (12 x 300). To get the total we multiply the breadth by the length and the answer is 8,640,000 pixels (2400 x 3600).

We now know how many pixels are in the image. So how do we get to file size. Well this is a grayscale image and we know from the section on image pixels that for a grayscale image each pixel is 8 bits or 1 byte in file size. So in total the A4 image of 8,640,000 pixels has a file size of 8,640,000 bytes or about 8.2 MB (this is because there are 1024 bytes in a kilobyte and 1024 kilobytes in a MB).

So it would be a whole lot easier if your editor asking you for an image big enough to fit the size he wants to print it at asked you for an 8 MB grayscale image. Or if he wanted it in RGB colour he would multiply that figure by three and ask you for a 24 MB image.

NOTE:  While it is simplest to have just one unit of measurement, such as everything measured in either inches or centimetres, it is still common in some countries to use a mix of the two. Often paper sizes are measured in millimetres by using 'A' sizes, but image resolution is still quoted as 'Dots per Inch'. This is inconvenient as it requires measurements to be converted at times, but you will just have to be put up with this until full metrification becomes standard.

Image Size Panel

It is not necessary to calculate the file size exactly every time. Photoshop has an Image Size panel that can be used as a calculator. The panel can not be accessed without an image being opened, so create a new image first (Ctrl+N) choosing the colour mode needed (RGB, CMYK, Grayscale, etc).  Make sure the 'Constrain Proportions' box is unticked to allow width and height to be separate, and that the 'Resample Image' box is ticked to allow the resolution to be separate as well. Now figures can be entered into the Document Size (highlighted) with the file size appearing at the top of the palette (indicated).

Figure 7 The image size palette provides an accurate way of calculating the size of a digital image you have opened in Photoshop.

File Size 'Rule of Thumb'

You aren't always in front of a computer when you need to calculate image size though. So it is really helpful to have a rule of thumb that guides you. Just think of an A4 as 24 Mb for an RGB colour, 8 bit image at 300dpi. The beauty of the number 24 is that it is so divisible, and therefore easy to calculate file sizes. Take a piece of A4 paper and write '24 on it.  Fold it in half and write 12, fold again and write 6, then fold once more and write 3. Almost all the images you use will fit one of these sizes. So the next time you are asked what file size is need for a quarter page RGB photo, fold an A4 twice to give you 6 Mb.  

Figure 8 The illustration above shows a quick way of calculating the file size of an RGB image needed for printing an A4, A5, A6 and A7 image.

A grayscale is one third of an RGB so write 8,4,2,1 instead. For a CMYK image write 32,16,8,4 as it has one extra channel. If the image is 16 bit then double the figures. In this way, wherever you are you can impress your editors or clients by a speedy calculation of what file size they need.

NOTE: The figures used above are not exact, but they are easily divisable round numbers that allow you to quickly calculate the rough image file size required.

Image Interpolation

If you need to have a larger file size than was provided by the digital camera or scanner, then Photoshop can create the extra pixels. But this does come at a cost. On the right is an image that was one tenth the image size of that on the left. It was then interpolated 10-fold (1,000 %) to bring it up to the same dimensions. While there is nothing to stop you resizing an image this way, be aware that Photoshop has no reference to what the original unscanned image looked like and therefore has to 'invent' the extra pixels. An increase of several percent will not be noticed, but the more the resizing, the softer the image will become.

Figure 9 The image on the left is the original quality. The one on the right was reduced to a 10th the size and then interpolated up in Photoshop to the same size. You can see how soft the image becomes as Photoshop simply invents pixels and blurs all edges.

The best solution is to do the scan again at the correct size. But if rescanning is not possible then there are some different interpolation methods to help with the problem. Genuine Fractals is one such program that allows significant interpolation without significant loss of quality.

File Size and Compression

At this stage it needs to be mentioned that an image can have several types of file size:

The Camera Raw file has a size, but this will be dealt with later.

The Image converted from a Camera Raw file, or scanned from film, has a different file size.

You may then crop the image to reduce the file size, or use interpolation to increase the file size.

For the sake of simplicity, let us just deal with the finished image after you have shot it, converted it, and done any cropping. This image, when opened in Photoshop, has an "open" or "working" file size, as seen in the Image Size Panel section above. However, when you come to save the finished image you will have to decide about file compression.  Usually you will be save the image either as a TIFF file, which can have compression or none at all, or as a JPEG file, which will always have compression.  If the image has been saved with compression then you notice that the save image has a much smaller file size than what it was when it was open.  Think of this as "saved" or "compressed" file size.

The problem comes when you look at an image that has been stored on your hard disk and you need to decide what the "open" or "working" file size is as it is the "open" file size that counts when you have to send an image to a client.  If the file is a JPEG then you know already that it is compressed, but if it is a TIFF then you may not know if compression was used.

As an example:

Figure 10 The image details as seen in Windows 7

Viewing the image details in the file browser it can be seen that the file size is listed as 1,178Kb.  As this is a JPEG file it means that this is the compressed size, so the all important "open" file size must be a lot bigger.

Figure 11 The image 'properties' as seen in Windows 7

Looking at the file 'properties' will show that the image has dimentions of 2336 x 3504 pixels.

NOTE:  The way that the above information is displayed will vary between Mac and PC computers.

Figure 12 The image details as seen in an image browser

Usually this information can also be viewed in Bridge, or which-ever image browser software you are using.  Again the dimensions are given as 2336 x 3504 pixels, and that it is a 24 bit RGB image. The total number of pixels is 2,336 x 3,504 = 8,185,344 pixels.  Now if this was a grayscale image that would mean that the image is 8,185,344 bytes in size, as each pixel equals 1 byte.  However, as the image is a 24-bit RGB file, meaning that each colour channel is 8 bits, and therefore 1 byte each, then the RGB file size is 8,185,344 x 3 = 24,556,032 bytes.

24,556,032 bytes / 1024 = 23,980.5 kilo bytes

23,980.5 kilo bytes / 1024 = 23.418 mega bytes

By now you will have probably concluded that this is a little painful, to put it mildly!

Figure 13 The image opened in Photoshop

A much easier way is to simply open the image in Photoshop...

Figure 14 The "open" file size is given at the top of the panel

...then open the Image Size panel (Image > Image Size on the menu bar).  At the top is shown the "open" file size, 23.4Mb in this example.

The main point here is that there is a clear difference in file size between the "compressed" size on disk of just over 1Mb, and the "open" or "working" file size of over 23Mb.  So unless you like the idea of torturing yourself with a lot of needless calculation then just open the image in Photoshop and let it do the work.

NOTE:  If you have a lot of images that you need to inspect for their working file sizes, then there is JavaScript that can run through a whole folder of files and create a spreadsheet that lists the compressed and open file sizes for you.

What to Deliver

Always check to see if the client has a set list of standards, and always follow that.  Usually that will be images saved at 300 dpi, unless they are only intended for the web where the common resolution is 72 dpi.

File size requirements, that is "open" file size requirements, will vary from client to client, but expect that the minimum size to be 24 Mb for RGB images.  However, this can be lower for newspapers and will almost always be higher for picture libraries.

RULE #1:  ALWAYS follow the supplied set standards.  Clients will often reject work that does not meets the required standards, with a resulting loss of income for you.

RULE #2:  Don't get caught in the trap of supplying the images at different working file sizes to fit the layout requirements of a particular job.  That is the responsablity of the designer, inless of course they are willing to pay you extra for doing the work.

NOTE:  Get in the habit of saying "open file size" or "working file size", and "saved file size" or "compressed file size", and not just using the vague term "file size".  This can save a lot of confussion when dealing with clients.