The specific answer to the core of your Title Question, "the term for the distance", is: Infinity. Infinity is the (imagined) subject distance in front of the optical center of the lens that corresponds to an in-focus image on the sensor when it is spaced behind the lens at the nominal focal length. The engraved "focal length" which appears somewhere on the lens housing is a hypothetical specification of the nominal distance from the lens to the sensor when (imagined) subjects at infinity appear "in-focus" on the sensor. For a simple axi-symmetric double convex lens, the measuring reference point is the optical center (also called the geometric center) of the lens. To focus images of real subjects closer than infinity, the lens must be moved away from the sensor, towards the subject. In this situation, the focus (not focal) distance is always longer than that number engraved on the barrel (the focal distance). Thus, the nominal focal length is a convenient label to characterize the focus properties of the lens assembly. For compound lens assemblies there is no easily found reference point. The reference point is the center of a hypothetical single element with the same focal length. In this case the technique to determine the reference point is very complicated. It is up to the reader to investigate further. In answer your second question regarding: “stuff at this distance will be in focus”, the term is "subject to lens distance". Given f = focal length, u = subject to lens distance, and v = sensor to lens distance, the following formula represents the relationship: 1/f = ( 1/u ) + ( 1/v ). At infinity, 1/u approaches zero.

Surgery such as LASIK can worsen HOAs in the eye — particularly, spherical aberrations. Procedures like keratoplasty (corneal transplants) can also increase the risk of developing spherical aberration.

Spherical aberration is one of many varieties of a higher-order aberration (HOA) in the eye. HOAs are subtle optical imperfections in the eye that are more complex than refractive errors such as astigmatism, nearsightedness (myopia) and farsightedness (hyperopia).

Yearly comprehensive eye exams are vital for your sight and overall eye health. Vision screenings are especially important for those with spherical aberration and other HOAs, as these can be associated with cataracts, tear film disorders and various corneal problems.

An optical aberration is a loss of clarity of images formed by the eye, and aberrations can occur in eyeglass lenses or other optical devices (such as binoculars or telescopes) as well. It is normal for all eyes to have some degree of aberration, but correction is rarely required. However, in severe cases causing lower quality vision, you may need special treatment.

As you place your chin on the chin rest of the aberrometer, you will look inside and focus on a point of light. During this time, your pupil is measured, as this is where a wavefront will pass through your eye.

Sphericalaberration in a lens

So if you focus on an object 3 meters away with a focal length of 18mm and aperture of f/11, everything from 1m to infinity will be in focus. However, if you focus on the same subject with the same aperture with a focal length of 135mm, the near focus limit is 2.9m and the far focus limit is 3.1m - the depth of field is only 20cm deep, in other words.

Dysfunctional tear syndrome dry eye disease and associated tear film disorders — new strategies for diagnosis and treatment. Current Opinion in Ophthalmology. January 2017.

The crystalline lens of the eye, which provides part of the eye’s light focusing power, changes as a natural part of aging — this can also contribute to spherical aberration.

Sphericalaberration example

Aberrometry measures how light passes through the crystalline lens and cornea, which are the light-focusing parts of the eye. The distortions that result as light passes through the eye during this exam are called aberrations.

Focal length is the distance between the lens and the sensor when the subject is in focus, not the distance to the subject.

The remaining 15% of imperfections in vision are due to higher-order aberrations like spherical aberration, coma aberration and trefoil aberration.

Sphericalaberration in mirrors

The focal length of a lens is defined by the thin lens equation, and it can be interpreted as a measure of the inverse strength of the lens. If you make a lens's optical surfaces more strongly curved, or if you increase its index of refraction, it gets stronger, and the focal length goes down. When you change do and di so as to maintain focus, the focal length f normally stays constant; this is what justifies interpreting it as a fixed property of the lens. (As pointed out in a comment, some lenses do contain moving parts that allow them to automatically change their focal length, but this is a side issue.)

Sphericalaberration formula

Higher-order aberrations are often called “irregular astigmatism” and are associated with flaws in how the eye refracts light rays as they travel through the vision system. Coma and trefoil aberrations are also considered HOAs.

A map of this projection is created, defining the shape and severity of the distortions (aberrations) that occur in your eye.

If you’ve noticed any changes in your vision, do not hesitate to contact your eye doctor. And if you’re concerned about spherical aberration or any other type of HOA, ask about a wavefront exam — based on your needs, you may be a good candidate.

Aberrations can be examined and diagnosed through wavefront technology (aberrometry), using an instrument called an aberrometer.

The effect of spherical aberration on visual performance and refractive state for stimuli and tasks typical of night viewing. Journal of Optometry. September 2018.

An aberration (which literally means a deviation from the usual or natural) of the eye occurs when there is an abnormality on the surface of the cornea. As mentioned, some degree of aberration is natural in every eye, though there are several factors that can contribute to the development and/or severity of aberration.

I was under the impression that the “focal length” of a lens is the distance at which stuff appears in-focus. (E.g., perhaps I set the camera so that objects 3 meters away appear sharp, and anything nearer or further than that is blurry.) But everything I've read seems to suggest that focal length is actually a slightly odd way of describing the field of view of the lens, and actually nothing to do with focus at all. (?)

Sphericalaberration correction

The term for the distance to the subject in focus is the focus distance and is measured from the image plane (sensor/film plane). The distance from the lens to the subject is called the working distance which can be significantly less within the context of macro photography. The zone which appears in focus either side (front and back) of the subject is the depth of field. This varies with the aperture - depth of field increases as the aperture gets smaller (f-number gets larger). All else being equal, depth of field is greater at f/4 than at f/2.

It is normal to have some degree of aberration — as no eye is perfect — but if it is unusually high, vision can be noticeably affected.

In particular, spherical aberration can alter the focusing ability of the eye, especially in dark or dimly lit environments (contrast sensitivity).

Higher-order aberrations such as spherical aberration are so complex that they cannot be improved with ordinary eyeglasses or soft contact lenses. However, they can be treated with rigid contact lenses.

Generically, in optics, this is called the object distance. In photography it can also be referred to as the focal distance.

everything I've read seems to suggest that focal length is actually a slightly odd way of describing the field of view of the lens

What issphericalaberration in Physics

Sphericalaberration and chromatic aberration

Every vision prescription includes specific numbers that accompany a “sphere” and “cylinder.” These numbers are lower-order aberrations. About 85% of lower-order aberrations contribute to vision imperfections such as nearsightedness, farsightedness and astigmatism.

A beam of light is shined into your eye and is then reflected back by your retina. The wavefront of this light is captured and measured by the aberrometer.

The map produced by an aberrometry is often referred to as an “optical fingerprint,” as it captures the unique ridges and valleys of the eye — because, like a fingerprint, no two eyes are the same.

A possible source of confusion is that in many cases when you're doing photography, do is much greater than di. Under these conditions, di is approximately the same as f. Therefore some people may be under the impression that the focal length is defined as the distance from lens to sensor. But in reality, when you change the focus on your camera, di changes while f stays the same.

If spherical aberration (or another type of HOA) begins distorting your vision, your ophthalmologist will recommend the best treatment for your specific case.

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Sphericalaberration photography

So what's the correct term for “stuff at this distance will be in focus” then? (I.e., the thing you change with the focus ring.) If I want stuff 3 meters away to appear sharp, what parameter have I set to 3 meters?

Spherical aberration is not a threat to eye health, but cases that impair vision should be assessed and treated as recommended by your eye doctor.

Sources used at the time of publication are provided in the Notes and References section. These may include peer-reviewed scientific research, data from government health authorities, and other trusted sources of information.

This article was written or reviewed by an All About Vision expert contributor, a partner medical reviewer or a member of our medical advisory board.

In the eye, spherical aberration tends to increase with pupil size, and is therefore more noticeable in low light conditions than in bright sunlight. Visually, spherical aberration causes blur, loss of contrast and halos around lights.

Spherical aberration is a specific type of optical aberration. It occurs when light rays passing through the periphery of the cornea and lens come to a focus at a slightly different location than light rays passing through the center of the cornea and lens. This results in decreased image quality. While spherical aberration cannot be corrected with ordinary eyeglasses or soft contact lenses, rigid contact lenses or other more advanced options may be recommended, on a case-by-case basis.

People who have large pupils may experience more severe symptoms of spherical aberration — even more so in poor lighting when pupils widen (or dilate) to compensate for low light.