Understanding JPEGs

[two_third]There is a lot of confusion with clients in the area of understanding JPEGs, digital file sizes and resizing of images – what file size is required for what reproduction in different mediums. Digital photography has exploded in the last two to three years, now accepted as the norm for the capturing of all sorts of professional photography.

Not many years ago most clients had little or no idea what to do with a disc full of images, now they are often expected to not only be able to handle those images but understand all the processes related to both photography right through to general printing.

Digital camera technology is moving on as fast as computer technology moved 10 years ago. No sooner have you bought the latest digital camera with x number of mega pixels than it is superseded by the next camera with even more mega pixels, so how much is enough?

The answer to this question is long and complicated but taking a simplified view it is possible to produce a large image even from a small file. The detail will be less than a large file but it is possible. So how does mega pixels relate to file sizes? Very roughly, a three million mega pixel camera will produce an RGB (Red , Green ,Blue file) of about 7.5mb, a 6 million mega pixel camera about 17 MB and the latest professional cameras from the likes of cannon and Nikon are now heading towards 12-16 mega pixels (2007, to Dte file sizes continue to grow year on year)  – that’s getting up to file sizes of 35-48 MB as big as some of the digital backs on the market for medium format cameras.

Many clients phone with their requirements stating they require a 300 dpi file. This is 300 dots per inch. Unfortunately this is only half the information. Any file size can be set to 300 dpi, a scale often requested by litho printers for reproduction. The results would be a very small reproduction from a small file varying to much larger for the big files.

To find out what file size you ideally need you need to know the reproduction size you wish to finish up with. Lets take for example a small image in a brochure, about a quarter of the A4 page, about 5 x 3 inches (125 mm x 75 mm). At 300 dpi an ideal file size would be 3.74mb. A full A4 reproduction would ideally require a file size of 25 MB and a full double spread would ideally need a 50 MB file at 300 dpi.

When you start to look at posters or display material if we carried on at 300 dpi a file size of 890 MB would be needed for a 6 feet by 4 feet exhibition poster – which is ridiculous as most peoples computers could not even handle files of that size let alone have any medium to transport them – one on a DVD?

Now you can very quickly see that the file sizes are climbing very rapidly beyond the capacity of even the best digital cameras available today, so what’s going on? Well firstly in a lot of cases printers are asking for files at 300 dpi when they end up printing at 150 dpi. They deliberately ask for larger files so they have better quality material than they need giving them a safety margin.

Next we often have designers working on large panels like our 6 x 4 display panel on artwork at half or even quarter size – hence the file sizes required are greatly reduced. These artworks are then blown up at the stage of making the plates for the printing or if they are going via a large format inkjet printer these have their own internal computers that extrapolate the information to larger sizes. They fill in dots between the dots your file size produces with dots of a similar colour, it sounds a crude technique but works wonderfully when you take into account a lot of the inkjet technologies actually work by bleeding the dots into each other.

Extrapolation of file sizes is a wonderful magic cure which often most people aren’t even aware is happening to their files somewhere along the reproduction process. Take any file size and blow it up beyond it’s capacity and it is possible to eventually see the pixels that make up the picture, probably on close inspection of an image reproduced at 72 dpi the degradation in quality would start to become apparent. But extrapolate that file back up to a 300 dpi file and it would be difficult to see anything more than a slight softening of the detail.

Below is a few examples of what we have talked about here that may help illustrate the point. But don’t get carried away the next time you receive an image via email and you look at the file size and it’s only 1-2 MB. We have another factor coming into play here and that’s jpeg’s!

Understanding JPEGsThis is the area of the photograph we will be looking at closely.

Jpeg’s are a format that saves a file at a compacted rate. A normal tiff file records every pixel at what colour it is but a jpeg will record each series of dots of the same colour as an equation, so cutting down on the file size. The more you compact a jpeg file the more detail that will be lost, but the smaller the file will become for transit. Below is another set of examples of the different effects of compacting jpeg files.

So lessons to be learned with digital imaging – always aim for a file size big enough for the end results you want to use it for. If that’s not possible or if you aren’t sure get the biggest file size offered by the photographer, as a rule of thumb anything bigger than 5 mega pixel is going to be sufficient for most peoples purposes. Though if you are aiming for display material reproduction it might be worth getting your designers to have a chat with your selected photographer so that everyone knows who’s doing what. Some designers are not as up to speed with digital imaging file sizes as you might think.

Lastly the smallest file sizes are actually needed for where all this stuff started – the computer! If you are producing a web image it’s worth remembering most screens are displaying images at around 72 dpi so if you want a picture to appear at 4 x 3 inches on your screen you can go down to file sizes as small as a quarter of a MB. Usually it’s best to aim here for a file size slightly bigger, say at 100dpi for the same size to help make those screen images look super crisp.

Understanding JPEGsOriginal definition of a section of a handheld 6 mega pixel file at 300 dpi.

Understanding JPEGsThe same file reduced to 72 dpi showing the quality loss of the image when the file size is reduced and information thrown away. Viewing the whole image one would notice the jagged edges. If you looked very closely like this!

Understanding JPEGsThe 72 dpi file above resized back up to 300 dpi by extrapolation As you can see the image is smoothed out and only looses a little definition over the original file. Looking at the whole file below you would not notice more than a slight softening of the image compared to the original.
Understanding JPEGsThis is an extreme example of extrapolation – resizing images slightly bigger from an original would have even less impact than this. Next, comparison of JPEG compression!This is the area of the picture we will be looking at closely next. Original definition of a section of a handheld 6 mega pixel file at 300 dpi.The same file reduced to 72 dpi showing the quality loss of the image when the file size is reduced and information thrown away. Viewing the whole image one would notice the jagged edges. If you looked very closely like this!The 72 dpi file above resized back up to 300 dpi by extrapolation As you can see the image is smoothed out and only looses a little definition over the original file. Looking at the whole file below you would not notice more than a slight softening of the image compared to the original..

This is an extreme example of extrapolation – resizing images slightly bigger from an original would have even less impact than this.

The whole extrapolated image that the above detail was taken from.

Here you can see the comparison of the effect of compressing a jpeg image to it’s smallest possible file size. The top image is an uncompressed section of the original image, below is the same image compressed to a jpeg quality of 1. Where 12 is the least compression and 1 is the maximum compression possible on a jpeg.

You will note the format tends to pixelate patches of colour in an attempt to reduce the information saved and therefore the file size.

Jpeg is often described as a “Lossy” file compression type in that each time you open a file and make changes then re-save, it will loose a little more of the original detail. So a point worth making is that once a photographer has supplied you with a jpeg image it is worth either going back each time to the original disc to supply others with the best original you can or try to save the image as few times as possible.

Understanding JPEGs

If you must make changes to an image it is better to save the original to a non lossy format at the earliest opportunity like a tiff or Adobe PSD format and only save back to a jpeg when you need to transfer an image electronically.