Not every case of vision correction would see additional benefits from using aspheric lenses. A study in the Journal of the American Academy of Optometry investigated whether aspheric design soft contact lenses reduce ocular aberrations, resulting in better visual acuity and subjective appreciation of clinical performance than spherical soft contact lenses. They found that with the brands of lenses tested (Biomedics 55 Evolution and CooperVision), the fitting of aspheric design soft contact lenses does not result in superior visual acuity, aberration control, or subjective appreciation compared with equivalent spherical design soft contact lenses.

where s (s’ ) is the object (image) position with respect to the lens, customarily designated by a negative (positive) value, and f is the focal length of the optical system (cf. Fig. 1). The distance from the object to the front lens is called working distance, while the distance from the rear lens to the sensor is called back focal distance. Henceforth, we will be presenting some useful concepts and formulas based on this simplified model, unless otherwise stated.

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Focallength

Spherical aberration is the distortion of vision because our eye is a curved sphere and not flat. When light enters our cornea, it gets bent and curved because our cornea is a curved surface, leading to an image that is seemingly curved entering our eye, even if it is not. This happens when the outer parts of a lens do not bring light rays into the same focus as the central part. Images formed by the lens at large apertures make them blurry but get sharper at smaller apertures. This is also referred to as the pinhole effect.

Converginglens

The F-number affects the optics depth of field (DoF), that is the range between the nearest and farthest location where an object is acceptably in focus. Depth of field is quite a misleading concept because physically there is one and only one plane in object space that is conjugate to the sensor plane. However, being mindful of diffraction, aberration and pixel size, we can define an “acceptable focusing distance” from the image conjugate plane, based on subjective criteria. For example, for a given lens, the acceptable focusing distance for a precision gauging application requiring a very sharp image is smaller than for a coarse visual inspection application.

Different mechanical mounting systems are used to connect a lens to a camera, ensuring both good focus and image stability. The mount is defined by the mechanical depth of the mechanics (flange focal distance), along with its diameter and thread pitch (if present). It’s important that the lens flange focal distance and the camera mount flange distance are exactly the same, or focusing issues may arise. The presence of a threaded mechanism allows some adjustment to the back focal distance if needed. For example, in the Opto Engineering® PCHI series lenses, the back focal adjustment is needed to adjust the focus for a different field of view.

where p is the sensor pixel size (in microns), M is the lens magnification and k is a dimensionless parameter that depends on the application (reasonable values are 0.008 for measurement applications and 0.015 for defect inspection). For example, taking p = 5.5 µm and k = 0.015, a lens with 0.25X mag and WF/# = 8 has an approximate DoF = 10.5 mm.

Aspheric lenses can correct optical defects and provide sharper vision in the dark and at dusk because light can penetrate and meet at one focal point instead of at multiple points. Although spheric lenses have brilliant imaging properties, aspheric surfaces perform better in severe ametropia because they provide much more precise light guidance. This is especially apparent in the marginal areas of the lenses because the light is directed to the focal point of the visual aid from those areas. This means if you have severe ametropia and have been prescribed to use aspheric lenses, you may find that your vision is wider, clearer, and brighter. These lenses can be helpful for those with low levels of astigmatism during sports or work with computers.

Some eye care patients prefer to opt for aspheric lenses after trying them out because they feel more comfortable, most likely related to improved visual acuity resulting in less eye strain. There is a noticeable difference in how tired their eyes are after a long day at work in front of their screens. This is not a universal experience, however, for all wearers. Each person’s eyes differ substantially, so it’s essential to visit your eye care professional and understand your specific needs better.

focallength是什么

This leads to a lens designed to correct your vision ideally at each point of the contact lens — instead of a -3.00 throughout the whole lens, like typical contact lens designs. For example, a -3.00 right in the center with a -2.93 outside and a -2.87 at the periphery. The aim is that each point in space gets corrected accurately through your lenses to help ease how hard your eyes need to work, especially with a varied modern lifestyle of walking while constantly looking at phone screens.

Manufacturers have developed more aspheric soft contact lenses designed to correct the eye’s spherical aberration and improve vision clarity for contact lens wearers over their spheric lens counterparts.

A certain number of parameters must be considered when choosing optics, depending on the area that must be imaged (field of view), the thickness of the object or features of interest (depth of field), the lens to object distance (working distance), the intensity of light, the optics type (telecentric/entocentric/pericentric), etc.

For common optical systems, in thin lens approximation, the focal length is the distance over which collimated rays coming from infinity converge to a point on the optical axis.

Typical F-numbers are F/1.0, F/1.4, F/2, F/2.8, F/4, F/5.6, F/8, F/11, F/16, F/22, etc. Every increment in the F-number (smaller aperture) reduces incoming light by a factor of 2. The given definition of F-number applies to fixed focal length lenses where the object is located ‘at infinity’ (i.e. a distance much greater than its focal length). For macro and telecentric lenses where objects are at a closer distance, instead, the working F/# (wF/#)is used. This is defined as:

Mxx-mounts are different types of camera mounts defined by their diameter (e.g. M72, M42), thread pitch (e.g. 1 mm, 0.75 mm) and flange focal distance. They are a common alternative to the F-mount for larger sensors.

Macro and telecentric lenses are designed to work at a distance comparable to their focal length (finite conjugates), while fixed focal length lenses are designed to image objects located at a much greater distance than their focal length (infinite conjugates). It is thus convenient to classify the first group by their magnification, which makes it easier to choose the proper lens given the sensor and object size, and the latter by their focal length.

The focal length and the focus plane coincide only when the object is placed at an infinite distance, indeed beams from a point on the object can be considered as parallel. When instead the distance from the object is ‘short’ (rule of thumb: <10x Focal length), we are in macro mode and the focus plane is placed further away from the optical system compared to the focal length.

Focallength formula

The main features of most optical systems can be calculated with a few parameters, provided that some approximation is accepted. The paraxial approximation requires that only rays entering the optical system at small angles with respect to the optical axis are taken into account. The thin lens approximation requires the lens thickness to be considerably smaller than the radii of curvature of the lens surfaces: it is thus possible to ignore optical effects due to the real thickness of the lenses and to simplify ray-tracing calculations. Furthermore, assuming that both object and image space are in the same medium (e.g. air), we get the following fundamental equation:

Every optical system is characterized by an aperture stop, that determines the amount of light that passes through it. For a given aperture diameter d and focal length f we can calculate the optics F-number:

Since fixed focal length lenses also follow the previous equation, it is possible to calculate the required focal length given the magnification and working distance, or the required working distance given the sensor size, field of view and focal length, etc. (some examples are given at the end of this section). For macro and telecentric lenses instead, the working distance and magnification are typically fixed.

The basic purpose of a lens of any kind is to collect the light scattered by an object and recreate an image of the object on a light-sensitive ‘sensor’ (usually CCD or CMOS based).

FOV tofocallength

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Focaldistance vsfocallength

This is why a spacer kit is supplied with Opto Engineering® telecentric lenses including instructions on how to tune the back focal length at the optimal value.

Focallength camera

F-mount is a bayonet-style mount originally developed by Nikon for its 35 mm format cameras and is still found in most of its digital SLR cameras. It is commonly used with bigger sensors, e.g. full-frame or line-scan cameras. Lenses can be easily swapped out thanks to the bayonet mount, but no back focal adjustment is possible.

Many cameras are found not to respect the industrial standard for C-mount (17.52 mm), which defines the flange-to-detector distance (flange focal length). Besides all the issues involved with mechanical inaccuracy, many manufacturers don’t take into the due account the thickness of the detector’s protection glass which, no matter how thin, is still part of the actual flange to detector distance.

CooperVision as a manufacturer, mapped out the “average” cornea, knowing that we have spherical aberration-caused visual distortions inherent to the eye itself, and designed a contact lens to cancel out the spherical aberrations that are naturally present. The power profile of the contact lens changes gradually from the central cornea to the peripheral cornea to match the bend and curvature change of the average corneal shape.

CS-mount is a less popular and 5 mm shorter version of the C-mount, with a flange focal distance of 12.526 mm. A CS-mount camera presents various issues when used together with C-mount optics, especially if the latter is designed to work at a precise back focal distance.

Each camera mount is more commonly used with certain camera sensor formats. The most typical sensor formats are listed below. It is important to remember that these are not absolute values – i.e. two cameras listed with the same sensor format may differ substantially from one another in terms of aspect ratio (even if they have the same sensor diagonal). For example, the Sony Pregius IMX250 sensor is listed as 2/3” and has an active area of 8.45 mm x 7.07 mm. The CMOSIS CMV2000 sensor is also listed as 2/3” format but has an active area of 11.26 mm x 5.98 mm.

Updated October 17, 2024 – Professional eye care practitioners have been using contact lenses to aid vision problems for a long time. Typically, the contact lenses worn for everyday issues like myopia have a spherical shape. However, the usage of aspheric lenses has been around for quite some time as well, and their purpose is to increase the depth of focus of the eye: more commonly in the context of multifocal lenses.

focallength中文

Another study listed as an article in the Contact Lens Spectrum found no evidence that aspheric lenses optically correct astigmatism. However, one aspheric lens called the Biomedics Premier effectively corrected the spherical aberration that the contact lens power induces and the spherical aberration of the average human eye.

For optical systems used in machine vision, in which rays reflected from a faraway object are focused onto the sensor plane, the focal length can be also seen as a measure of how much area is imaged on the sensor (Field of View): the longer the focal length, the smaller the FoV and vice versa (this is not completely true for some particular optical systems, e.g. in astronomy and microscopy).

Spheric and aspheric designs differ in the curvature of a contact lens. Aspheric lenses showcase varying curvatures across their surface that include a flatter area, unlike traditional soft lenses with a spherical design. Curvature varies significantly towards the edges with aspheric lenses than in a spherical lens, which has a gradual curvature. Aspheric lenses are also generally more flexible than spherical lenses, where the value of the base curvature is the same at any point on the lens.

C-mount is the most common optics mount in the industrial market. It is defined by a flange focal distance of 17.526 mm, a diameter of 1” (25.4 mm) with 32 threads per inch.

The focal length is a typical characteristic of an optical system. It is a measure of how strongly the system converges or diverges rays of light. If collimated rays converge to a physical point, the lens is said to be positive (convex), whereas if rays diverge the focus point is virtual and the lens is said to be negative (concave). All optics used in machine vision applications are overall positive, i.e. they focus incoming light onto the sensor plane. CCTV lenses are commonly identified by their focal length, expressed in millimeters (12mm, 25mm, 35mm, etc.).

A common F-number value is F/8 since smaller apertures could give rise to diffraction limitations, while lenses with larger apertures are more affected by optical aberrations and distortion. A rough estimate of the field depth of telecentric and macro lenses (or fixed focal length lenses used in macro configuration) is given by the following formula: