At 06:30 PM 7/13/99 +0100, you wrote:
>Warren Kato wrote:
>. > ...I think
>>that there's no easy solution to my depth of field query and that I'll have
>>to print out a table for each of my wide angles...
>
>The only 'rule of thumb' solution that occurs to me is to remember that
>d-o-f extends a third of the way in front of a focussed-upon subject and two
>thirds behind it, so a speedy way to spread the available d-o-f where it
>will do the most good is simply to focus on something a third of the way
>into the area you want to be sharp. This is the method I am used to using
>with my XA, 35RC and modern p&s cameras that have no d-o-f scale. In the
>case of an oncoming train (round a bend so that you can see all of it),
>press the shutter release as the first third of the train reaches your point
>of focus.
>
>Regards,
>Keith Berry (Birmingham, England)
>keith_r.k.berry@xxxxxxxxxx
It's a wrong rule of thumb, at least in many situations. For family snaps
(3m, f/5.6, 35mm lens) it actually works. It is best at 1/3 the hyperfocal
distance. At less than 1/5 halfway is more accurate, although in these cases
you usually have to place the plane of best focus on the center of interest.
At more than 1/2 hyperfocal, the rear DOF starts growing rapidly, and,
obviously,
becomes infinite at hyperfocal.
Here is a rule for hyperfocal distance that really works:
For a 50mm lens divide 120m (or 400ft) by the f #. Calculations need only
be approximate. For other focal lengths multiply the distance above by
2 for every factor of 1.4. For example at f/8, it is 15m (50ft). For a
70mm lens 30m, a 100mm -> 60m, a 135mm -> 120m, a 35mm -> 8m. The area
from half the hyperfocal distance to infinity should have decent focus.
For small prints you loosen up a stop or so.
A fellow named Merklinger has some interesting ideas, and his own method,
on his web page, which you can find easily with a search engine.
For an oncoming train, just setting the lens at infinity should work for
just about anything that doesn't get you killed.
Paul Farrar
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