What is “infinity focus”?

Question

I have been reading a bit about astrophotography and the term infinity focus keeps coming up. What is infinity focus and how do you set the camera to infinity focus?

Answer 1

There are two ways "infinity focus" could be taken — "an infinite amount of focus, so everything possible is in focus", or "focused on something infinitely far away". In photography, is specifically the second of these — infinity focus means your lens is focused so an infinitely-distant object would be sharp.

When an object is infinitely far away, the rays of light coming from it are, in theory, parallel. A lens set at infinity focus is set so that theoretical object would be in focus.

In practice, all stars are close enough to infinitely far away to count, and in fact for most purposes for photography, everything beyond a hundred feet or so (depending on focal length) is as well.

Note that almost all modern autofocus lenses can focus past infinity. This allows for much wider tolerance in manufacturing, and makes it possible to allow for changes due to temperature. Older manual-focus lenses usually are made so turning the focus ring all the way puts it at infinity.

Some macro lenses (or macro configurations) do not allow infinity focus, and this is also the case with some situations where you're adapting one lens type to another mount.

For earth-bound photography, if you wanted that first thing — an infinite amount of focus — the hyperfocal distance is often more useful than infinity focus, as this maximizes practical depth of field. If you're looking for infinite depth of field, unfortunately, you can't really have it (not with traditional optics), but this and a small aperture will do in many cases.

Answer 2

Infinity focus is when the camera is set to focus on things at an infinite distance from the sensor plane.

This is a property of the lens rather then the camera. If you have a DSLR or SLD then most - but not all - lenses can focus at infinity. If you lens has a focus distance scale, then the infinity will either be at the focus limit at the wide-end or close to it. In the latter case, be careful as you can focus beyond infinity on some lenses to accommodate for dilatation of the lens and possibly other phenomena such as to focus infrared light.

If you have a fixed lens camera with manual focus, then simply increase the focus distance until you cannot anymore. Some cameras have an aptly named mode called infinity focus which does this for you automatically. Otherwise, you simply have to focus on the furthest object possible, probably one that is very very far such as a star or planet.

Answer 3

Imagine light rays spreading out from a point and hitting the top and bottom of a lens, forming a triangle. Now the further away the point the longer the triangle.

When a lens is set to focus at a certain distance it bends light rays at a certain angle into a point. Infinity focus means the lens brings parallel rays into focus. Now going back to the triangle example if your point is far enough away the sides of the triangle are effectively parallel so infinity focus will bring them into focus.

The actual setting which gives infinity focus changes with temperature, the precise distance from the lens to the sensor etc. so most lenses are designed to go a little way past infinity (i.e. they would bring converging rays into focus). For this reason you can't simply turn the focus ring as far as it will go, to achieve infinity focus you need to either use live view or focus bracket your shots.

Answer 4

In astrophotography, the process of getting proper infinity focus can be done via a few methods:

1. Optical device with direct visual measurement: a knife-edge focus or ronchi screen will allow the user to approach perfect focus on a point source of light. These devices are usually attached in place of the camera body for focus measurement. Magnifying viewfinders can also be used with some success although most focus screens are simply too dark to show good star images.
2. Optical device with sensor measurement: usually these are a mask placed in front of the lens to provide a diffraction spike or other visual cue that infinite focus has been reached. The Hartman mask features multiple holes giving images that converge as focus is adjusted. The Bahtinov mask features a grating that will show diffraction spikes that move to indicate focus movement.
3. Software measurement of image data: FWHM measurement will provide a number value of the relative size of a star image on the sensor surface. Lower numbers indicate focus. This is effectively the same as LiveView but with math.

Note that the above methods can all work for dedicated digital imaging devices and DSLRs in an astrophoto setup.

Comments as noted above regarding temperature sensitivity depend on the materials in the optical train. The contraction rate of metals vary and can be adjusted. Even the glass in the device can change with temperature. Some astrophotographers map the temperature characteristics of their equipment and can predict focus position offsets.

Answer 5

My question was asked because it dealt with astrophotography. After asking the question I came across a site the was mentioned by @JohnCavan in another question called Catching the Light that has a wealth of information. One of those pages deals with focusing. @Smigol mentioned a few of them and here is the link for 17 other methods of focusing that may be useful to others who come across this question.