A photographer's practical guide to jpeg

Stemming from the question on the last post about sizes of pdf books, I thought it was worth writing about JPEG images: sizes, resolutions, compression etc. This is based on a little technical reading and a lot of practical experimentation. Technically, I may be a little off in some areas but this will be good enough for the masses.

The aim of a JPEG image is to preserve as much detail as possible while compressing the file size as much as possible. By setting the quality/compression (these 2 are the reverse of one another - high compression gives low quality) value you are determining the trade-off between quality and file size. However,there is a general misconception as to how this works.

For highly detailed images (think lots of tree branches and small leaves) there is automatically less compression as the JPEG tries to keep the detail. If your image is highly detailed then you can actually use lower quality setting and still get an excellent image. In the other direction, large expanses of nothing compress very well. If you shoot a picture of a white wall even the highest quality setting will give a small file. JPEG is clever like that. The tricky stuff comes when there is a mixture of detail and even tones (e.g. trees against a blue sky). The even part compresses well, the detail is kept in the main but at the edges JPEG gets confused, throwing up those nasty artefacts. This is where judicious use of the quality level is needed. I'll come onto advice on values later.

The next part is the resolution or pixel size of an image. If you are printing, you'll want high quality files with lots of pixels. Typically 240-300ppi for the given print size. That means the total pixel size will be the paper size multiplied by 240 or 300 (e.g. if I print a 10"x8" at 300ppi, that'll be 3000x2400 pixels). Nice and easy.
The tricky part is on-screen display. To display on the majority of monitors, you only need an image about 1000pixels wide or 600 pixels tall. Maybe a bit more to cover larger displays. If your software works as a size & resolution then the size multiplied by resolution shouldn't be more than about 1000x600pixels (e.g. if I have a 10"x8" page for on-screen display, I need a resolution of only 100ppi to give 1000x800pixels). This is important for pdf generation, as pdf is designed to work in physical size and resolution. To cover most monitors today, you don't need more than 100ppi resolution (nor page size more than about 12"x8").

So what about recommendations for compression values? JPEG generation generally has one of three ways of setting quality: a high/medium/low scale; a JPEG compression factor (typically a number up to 100) or another numerical scle of some sort (e.g. Photoshop's 1-12).

For printing, go high. Photoshop 9 or 10 (11 and 12 really are pointless and don't practically give better results), JPEG factor 90 or 95. NOTE: JPEG compression factors are NOT percentages. the maximum value (from the JPEG standard) is 95 - any scale that goes to 100 is distracting you.

For on-screen display, you only need a medium level. Photoshop 7 or 8 (sometimes lower but go careful); JPEG factor 70-75. This will yield a files size about half that of the higher qualities and still give excellent on-screen display. Use the lower values unless there is a big mix of detail and even tones.

A final note on setting resolution versus compression, especially in pdf generation. The biggest win in file size (small being good) is a lower resolution. 300ppi has 9 times the pixels of 100ppi. To get that kind of compression with quality you need to go down to about 20-25 quality factor, which is barely recognisable. You could probably go down to 90ppi and give good display quality even on large monitors (a factor 11 smaller than 300ppi).

As for pdf books for the web (aimed squarely at the SoFoBoMo crowd) - aim for file size that gives around 50-60kB per page or 100-150kB per image overall (non-image pages generally take very little space in pdf). If you've gt 40 images in 55 pages (typical SoFoBoMo fare), that's 2.7-5.8MB total filesize.