### focus at infinity ?

greenspun.com : LUSENET : Large format photography : One Thread

Hi all,

What is infinity ? I never think about it but it sounds weird. If i have 210 mm lens i need to make an distance between the film and lens 210 mm, yes?

10x

Martin

-- Martin Kapostas (martin.kapostas@asdo.com), March 16, 2001

For all practical focusing purposes you can take the sun, moon and stars, or even the distant horizon, as being at infinty.
Theoretically, it's the other side of the universe and keep going.
Infinity is useful as a mathematical construct for calculating things like hyperfocal distance or other optical tom-foolery, usually using the identity 1 divided by zero.

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 16, 2001.

God is infinite ;-)

-- Martin Kapostas (martin.kapostas@asdo.com), March 16, 2001.

For all practical photographic purposes, an object is at infinity if, with your 210mm lens adjusted to 210mm from the film, with the lens wide open, it is sharp.

-- Garry Edwards (bestsnapper@freenetname.co.uk), March 16, 2001.

210 mm is the distance from the nodal emerging point to the film plane when the lens is focused at infinity

The lens has some width. Obviously one must mention the part from which the distance to the the film is measured. Genarally, you consider the flange of the lens, resulting in the measurement of the flange to film distance (which is never the focal of the lens but may be close to it).

A teacher in optic said "the distance of infinity depends upon the optical system that you are using". The infinity for an optical system is a distance at which, moving noticeably toward direction or away, you may not observe a shift in the focus with the cosnidered system (noticeably needs a further developppement). It may be at 1 meter for an interferometer ... at least 10 km for a 10 inches aperture SC reflector. Perhaps at 200 yards for a 210 mm lens.

-- Dominique Cesari (cesarigd@club-internet.fr), March 16, 2001.

Martin:

In photography, infinity is finite. Infinity is the point beyond which increases in the lens-to-subject distance will not require a change in the lens-to-film distance. The lens-to-subject distance is measured from a lens's front nodal point, and the lens-to-film distance is measured from a lens's rear nodal point. Theoretically, when a lens is focused on a point at or beyond infinity, the lens-to- film distance will equal the lens's focal length.

There are a number of factors that affect the finite distance to infinity. Some of these factors are: lens focal length, system resolution, and acceptable or unacceptable sharpness. An old rule of thumb goes something like this: take the lens's focal length in millimeters and call that feet (210mm focal length is called 210 feet); multiply that by a factor of 2, 3, or 4 depending on the ultimate resolution needed, and you have the finite distance in feet to the point called infinity. Like most rules of thumb, this one is only very approximate, and it leaves a lot of room for interpretation and modification. Dominique's suggested distance to infinity for a 210 lens would fall right in the middle of this rule of thumb.

-- Ken Burns (kenburns@twave.net), March 16, 2001.

Take a look at the formula that relates the distance to the subject (p), the distance to the image (q), and the focal length of the lens (f)..............

1/p + 1/q = 1/f

As p increases toward infinity, 1/p goes to zero, and 1/q = 1/f. So, for an object at infinity, the focal length equals the distance from the lens to the image (and hopefully, the film).

Of course, this formula is based on a simple thin lens, and not a complex multi-element design. The basic idea is the same, though.

-- Kevin Bourque (skygzr@aol.com), March 16, 2001.

Infinity is a cup of a particular beverage which never empties, no matter how much you drink.

-- Ivor Nigel Grate (ingrate69@hotmail.com), March 22, 2001.

A light wave is said to be 'from a source at infinity' when it's wavefront is perfectly planar and not curved. Equivalently, collimated light in which all the rays are parallel (e.g. from a laser) is also said to be from infinity.

'Focussed' at infinity really means that the film has been placed so that a parallel beam of light is be concentrated into a single spot. In simple, so-called 'thin' lenses that means that the lens itself will be one focal length away from the film, but photographic lenses are more complex and it is the rear 'nodal plane' which ends up one focal length from the film. Unfortunately, the position of the nodal plane with respect to the actual glass of the lens can vary widely, and you need to calculate or measure it for any particular lens.

In practice, for any given optical system you can define an object distance beyond which slightly divergent light rays cannot be distinguished from truly parallel ones. The actual distance for any given lens is obviously related to its angular descrimination and hence depends on the aperture setting. For photographic lenses a rough rule of thumb is 20-30 focal lengths.

-- Struan Gray (struan.gray@sljus.lu.se), March 22, 2001.