Zone focusing consists of choosing your aperture and point of focus so that everything within a desired zone or range of distances will be acceptably in focus.
I'll take my original conclusion and put it here as it is the "simple" result of the long winded descriptions.
A look through a series of relevant tables and a bit of playing will show you what is liable to work.
- If you set the lens to f/8 and focus at the point where the bike will (hopefully) be, you'll normally do OK. For the example lens and camera situation below you really want to use f/11 or higher.
eg At 10m away you get 5-223 m in focus ((see table and text below).
At 4m away you get about 3m to 6.5m.
At 3m away you get about 2.3 to 4.2m and realise that you REALLY should be using f/11 or f/16 and standing back !!!.
IF you are able to use f22 (unlikely for an action shot) then you get focus from minimum to infinity at all distances over 4 metres.
Now the long version:
"Focus" is not absolute - what is done is to decide that objects are in focus when the viewer can not decide whether a "dot" on the image is perfectly in focus or is really up to a preselected distance across due to defocusing. This acceptable maximum degree of defocusing is named "the circle of confusion". Typical CofC sizes are as large as about 0.03mm on a high sensor site size full frame 35mm sensor down to about 0.004mm on a very small sensor point & shoot camera.
Read what Wikipedia - circle of confusion says then report back ...
At this stage you could also skim through Wikipedia - depth of field just to make sure we are agreed on what is meant when we talk about in or out of focus and depth of field.
This is also extremely good DOF - demystifying the confusion - ignore the pun in the title.
SO - we are agreed re what is meant re "in focus" and depth of field.
Now you want to preset you camera so that everything in a desired range is acceptably sharp. To do this you have three parameters available to you - namely, aperture, focal length and actual centre of focus point.
The actual range achieved will vary with the focal length of the lens used.
You can use the formulas from the above DOF article to determine the camera settings, but this is quite hard work.
You can use any number of of online calculators - but you don't want to have to carry an internet to a bike race.
So, an excellent alternative is an old fashioned printed table.
However, the table will vary with lens, focal length and camera used OR circle of confusion chosen. Fortunately, DOFMASTER have supplied this very nice Depth of field calculator that calculates a printable table for the focal length and camera (or circle of confusion size) of your choice. (Turn on background when printing to get the table to look correct.)
As an example the table below is for a D700 (or other similarly high sensor cell size camera) with a circle of confusion of 0.03mm (large) and a 50mm lens.
If your bike will be about 5 to 10 metres away and you are happy with a 3 metre to 15 metre depth of focus then the 5 metre line has a 2.99m to 15.2m entry at f/11.
Or, if you focus at 10 metre and use f/8, what is the focus zone?
The table's 10m line and f/8 column say 5.11 to 223 metres.
The 223 is not a typo with a missing decimal point. Look at 10m,f/11 and above and you'll see that you reach infinity at the upper bound. At f/8 you are almost there.
This suggests another way to use the table. If you choose the "parfocal distance" such that yur zone has the outer bound at infinity then you can read off the focus point and minimum focus distance for a given aperture. Or given any two of aperture, focus point and minimum focus distance you can read off the third. eg For 5m to infinity at f/11, where is my focus point, the table indicates a 15 metre focus point.
Few people are going to want to use the formulas involved, but these show what is involved in producing the table and elated calculations. (From above ref).