[LargeFormat] Hyperfocal distance

[Robert Lawrence]

Diane Maher wrote:

> Is there an easy explanation for hyperfocal distance for LF? Does it
> depend on what lens you're using?

There is an easy mathematical formula but the explanation and
understanding is a different issue!  The normally accepted explanation
is that the hyperfocal distance is the nearest distance of the object
that you could theoretically focus on that will extend the acceptable
sharpness of the final print to infinity.

Technically the hyperfocal distance (H) is equal to the focal length of
the lens multiplied by itself (ie squared) and divided by the (F-stop
number in use multiplied by the acceptable circle of confusion (at the
film size)). Wow!

Then the near distance that is sharp is (H x U)/(H+U) where U is the
object distance and the far distance is (H xU)/(H-U) and the Depth of
Field is the difference of the two last calculations. At 5 x 4 the
traditional value for circle of confusion is 1/250th inch or one tenth
of a millimetre (0.1mm) which is reckoned to be the limit of what the
human eye can detect on a 6 x 7.5inch print viewed at 10 inches.

Not to get too bogged down in the maths if your far limit is infinity
then the Hyperfocal distance approximates to 1500 multiplied by the
physical size of the lens aperture diameter (which is focal length
divided by F stop number). Best illustrated by an example: 150mm lens at
f16; physical size of diameter = 9.375mm; 9.375mm*1500 = 14062.5mm or
roughly 14 metres. A similar calculation at f32 gives 7 metres, which is
good because we would expect depth of field to increase as we stop down
or decrease the focal length of our lens.

What does all this mean IN THE FIELD with LF?  Virtually nothing. Clives
original question was a touch mischievious, if you properly use your
movements, focus on the far and use front tilt to get the near into
focus, ie. place your plane of focus from your near object through to
the far (usually along the ground) most people do this by inspection on
the ground glass. If you have tall objects in the near field then DOF
does, however, become important as in a front tilt situation the cone of
acceptable sharp focus starts off below ground level - front rise, in
addition to the front tilt, may help here.

However, you don't need to know what the piston does in a car to be able
to drive from A to B; but an understanding helps to make you a better
driver.

Hope that sheds some light on a very confusing (no pun intended) subject
(if not don't worry about it - just keep taking pictures!)

Robert Lawrence.