# How to determine infinity focus distance?

Discussion in 'Digital Cameras' started by My View, Dec 31, 2004.

1. ### My ViewGuest

How can I determine at what distance the Canon 300D kit 18-55 lense (or any
lens for that matter) starts to focus at infinity.
I am setting up a table of hyperfocal distances and I want to ignore
distances beyond which the lens will be focusing at infinity.
Is it 63 metres or 200metres or 1 kilometre??? Is there an equation that
will calculate this?
regards
PeterH

My View, Dec 31, 2004

2. ### Dave MartindaleGuest

First, you should understand that there's a contradiction in your
question. The lens "focuses at infinity" (meaning it gives the sharpest
image of things a long way away" only when it is actually set to the
infinity position, not somewhere nearer.

Now consider depth of field. Given a particular criterion of
acceptable sharpness, usually expressed in terms of the size of the
"circle of confusion", the depth of field is defined to be the range of
subject distances that are acceptably sharp. This doesn't mean that
the whole range of depth is equally sharp; it's not. And the near and
far limits of the depth of field are, by definition, right at the
minimum limit of acceptable sharpness.

The hyperfocal distance is the lens focus distance that makes the far
limit of depth of field equal to infinity. In other words, everything
from some near limit all the way out to infinity is acceptably sharp,
and this is the setting that maximizes the range of depth that is so.
Thus, it's useful when you absolutely need this. But it guarantees that
objects at infinity are just acceptably sharp, not good and sharp. You
*will* get sharper images of distant objects if you set the lens closer
to the infinity mark than at the hyperfocal distance.
Having said all that, there is a formula that will calculate hyperfocal
distance, which should be found in any good lens or photography
reference book. But it depends on focal length *and* aperture *and*
your chosen circle of confusion size. You'd normally pick some fixed
value for CoC, then calculate a 2D table of hyperfocal distance as a
function of focal length and aperture setting.

Dave

Dave Martindale, Jan 1, 2005

3. ### DonGuest

Don, Jan 1, 2005
4. ### Bart van der WolfGuest

SNIP
Or on the web as a small Windows application at:
<http://www.bobatkins.com/photography/technical/depth_of_field_calc.html>,
which directly gives the "point blur diameter at infinity" as you move
the slider through the distance setting. That also allows to set the
CoC to one value and play with the other settings for the amount of
infinity defocus, while giving a warning if the aperture setting
causes the diffraction circle to exceed.

Bart

Bart van der Wolf, Jan 1, 2005
5. ### Bart van der WolfGuest

SNIP
Yes, it is good to stress that there is only an infinitesimal narrow
plane of optimal focus, and the rest is 'acceptable enough' for a
given output magnification. The standard CoC recommendation is a
somewhat arbitrary figure, usually chosen as giving an 'acceptable'
blur at a given output size (often something like 5x7 or 8x10 inch at
standard viewing distance).

However, with Digital imaging on a sensor, we do have a restriction
that was not present in film. The sensor pitch will pose a physical
limit to what can be resolved. One could adopt a very high standard
CoC of what's ultimately acceptable/achievable by choosing a CoC of
twice the sensor pitch. As long as the out-of-focus detail is smaller
than pitch x 2, it will look as good as something in perfect focus.

So by setting the CoC to 0.0148 for the OP's 300D (7.4 micron pitch),
the maximum resolution (assuming proper AA-filtering) range is
achieved, without any sort of magnification restriction other than
inherent sensor resolution (assuming the lens outresolves the sensor).
Hyperfocal calculations will then exactly give the maximum,
uncompromised, range of best attainable focus.

Bart

Bart van der Wolf, Jan 1, 2005