A type of chromatic aberration, chromatic focal shift describes how different wavelengths focus along different longitudinal positions (along the optical axis). The goal of most imaging lens designs is to have all desired wavelengths focus on the same plane (where the sensor is located). It is physically impossible to get a singular focus plane over a wide spectral range. However, it is possible to come close. If the wavelengths are focused closer to the same plane, fewer issues will be observed in the image.

Astigmatism is a function of field angles. To summarize, astigmatic aberration occurs when a lens must perform over a wide field, but the performance in the direction of the field is reduced compared to the performance orthogonal to the field (either sagittal or tangential, respectfully). If one looks at a series of bars that are half horizontal (tangential) and half vertical (sagittal), the bars in one direction will be in focus, but the bars in the other direction will be out of focus (shown in Figure 2a and 2b). This is caused by the fact that rays that are away from the center of the object, do not pass through rotationally symmetric surfaces like the on-axis rays do (Figure 3). To correct this, two things must occur: lens designs must be symmetric about the aperture and field rays must have low incident angles. Keeping a design symmetric leads to forms that are like a double gauss lens. Note that symmetric designs prevent the use of telephoto or reverse telephoto designs, which can cause long focal length designs to be large and short focal length designs to have small back focal lengths. Reducing the angles of incidence, much like for spherical aberration, requires higher index glasses and additional elements, leading to an increase in lens size, weight, and cost. The simplified definition used here intentionally combines the effects of astigmatism and coma for ease of understanding.

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In 2012, as they entered their 25th performance season, Toronto Mass Choir released '25', a CD featuring highlights from their recording career and four previously unreleased songs from their early years. That same year, TMC was also awarded the Lifetime Achievement Award from the Gospel Music Association of Canada.

After twelve years as a high school music teacher, Burke became a Professor in the music department at York University in 2005 and established the York University Gospel Choir. She now serves as the Chair of the music department. Her music career includes working as a guest speaker, classical piano teacher, adjudicator, and clinician offering gospel music workshops at venues including the University of Western Ontario, the Ontario Institute for Studies in Education, the Toronto Mendelssohn Choir, the Metropolitan Separate School Board and the Ontario Mennonite Music Camp. In addition to her busy schedule, Burke frequently conducts gospel music workshops abroad and as an artist-in-residence at other universities and festivals.

Spherical aberration

It is important for the well-being of the choir that members remain accountable to their families and churches all the while recognizing that our touring schedule often takes place on the weekends including some Sunday mornings.

Feb 4, 2020 — 4 Core Components of a Microscope Objective Lens · Meniscus and hemispherical lenses: A meniscus lens contains two spherical curved surfaces, ...

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Nov 8, 2016 — Depth of focus relates to the placement of the focus plane on the film/sensor behind the lens whereas depth of field relates to the distribution ...

In 1988, Burke co-founded the Toronto Mass Choir's diversified group of singers and musicians and continues to serve as its Artistic Director. She runs the choir with her husband, Oswald Burke, who is the choir's Business Manager and Executive Producer. Along with her devoted choral staff, Burke develops the choir's touring schedules and choral arrangements. Since its inception, she has also written many original songs for the choir's recordings.

Figure 6 shows a chromatic focal shift curve. Since this is an example of an achromatic lens design, two wavelengths are focused on the same plane. The y-axis shows changing wavelength from short to long (blue to red in the visible spectrum). The vertical black line represents a plane that could be the sensor location, and the x-axis shows the distance away from that location. The blue curved line shows the relative location of the best focus as a function of wavelength. The curve verifies that this design is achromatic since even if moved slightly to the left or right, the black line intersects the blue curve at only two points/wavelengths.

 Gospel Music label Micah Communications Inc released the choir's debut album, "Great Is Thy Glory," on vinyl and cassette in 1988. It was later re-released in the Compact Disc format in April 1990 under the new title 'The Early Years.' TMC's second live recording, God is our Hope, was released in 1994, followed in April 1998 by its Juno-nominated album, Follow Him.

The release of their seventh album in 2010, A Christmas Gift, marked the first Contemporary Gospel Choir Christmas CD release in Canada and showcases TMC at its best. A Christmas Gift also won 2 Covenant Music awards, including 'Seasonal Album of the Year.

No. The Toronto Mass Choir is a multi-denominational choir with members from several different evangelical Christian denominations.

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In November 2005, the choir broke new ground, using a 20-piece orchestra on The Live Experience, which also garnered a Covenant Award in 2006 for Traditional Gospel Album of the Year. A Shai Award, the gospel music people's choice prize for Urban/Soul album of the year, followed in 2007.

The Toronto Mass Choir has a busy touring schedule which is has been primarily in Ontario, Quebec, Eastern Canada and on a limited basis, Western Canada, the U.S., Europe and the Caribbean. The choir has also travelled to 10 different countries including Italy, Romania, England and Poland. Schedules of these ministry opportunities including rehearsals are given out about 3-4 months in advance so members are able to plan their lives around them. While TMC does not offer services for weddings the choir’s engagements do include church services and other large-scale events including concerts, community events, banquets, television appearances and other special events such as jazz festivals and benefit concerts.

The choir rehearses on average about twice a month on a Friday evening and sings out on average 1-2 times a month for a total of about 15-20 engagements a year. The choir travels by bus to engagements more than an hour away at no cost to themselves except incidental expenses such as a meal en route. Members are expected to secure their own transportation for engagements in and around the GTA.

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Karen Burke, a graduate of McMaster University, the Royal Conservatory of Music, and a Professor of music at York University, is the Artistic Director of this diversified group of singers and musicians along with her husband, Oswald Burke, the choir's business manager, and Executive Producer.

Upon graduation from McMaster, Burke accepted a position as a church music director. She and her husband Oswald Burke co-founded Micah Communications Inc., a Gospel music production, live events, and publishing company.

Light of different wavelengths focuses at different points since the refractive index of glass varies based on the wavelength of light. Lenses using longer wavelengths of light have relatively longer focal lengths than lenses using shorter wavelengths. Because the dispersion of a glass determines the refractive power of the glass at different wavelengths, chromatic aberration can be removed by designing an imaging lens that contains both positive and negative lenses made using glasses with different dispersions. This is shown in Figure 5, which compares a singlet to an achromatic doublet lens. A downside to such a design is the increase in the number of lens elements used. To reduce the aberration, lower index lenses (having higher abbe numbers) must be used. As mentioned before, higher index lenses are needed to correct spherical and astigmatic aberrations; if corrections for spherical, astigmatic, and chromatic aberrations must be done, additional lens elements are needed. Additionally, the most desirable glasses for color correction often have properties that make them more expensive and difficult to manufacture. Minimizing chromatic aberration by using monochromatic light has considerable savings in cost and complexity.

Spherical aberration refers to rays focusing at different distances depending on where they interact with the lens and is a function of aperture size. To describe spherical aberration, the incident angle of light must be known. This angle occurs where light rays strike the curved surface of a lens and is the angle between the ray and the surface. The steeper the incident angle, the more the light will be refracted (Figure 1). Figure 1 shows that as the parallel rays in object space collide with the lens, the incident angle increases the farther up they hit on the lens’s surface. Image quality from lenses with large apertures (small f/#s) are more likely to suffer from spherical aberration, because of this larger angle of incidence. Lenses that suffer from spherical aberration can be improved by increasing the f/# by closing the iris, but there is a limit to how much this improves image quality. Closing the iris too much causes diffraction to limit performance sooner (see diffraction limit in The Airy Disk and Diffraction Limit). Optical designs that include high index glass or additional elements are used to correct spherical aberration in a fast (small f/#) lens; these designs reduce the amount of refraction at each surface and, with it, the amount of spherical aberration. However, this increases the size, weight, and cost of the lens assembly.

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February 2002 saw the release of Instrument of Praise. That year the Gospel Music Association Canada (GMAC) awarded TMC a Covenant Award for Best Urban/Contemporary Gospel Album of the Year. In 2003, they picked up a Juno Award for Contemporary Christian/Gospel Album of the Year, the Vibe Award for Urban/Soul Album of the Year, and the Urban Music Association of Canada Award for Gospel Recording of the Year.

The September 2007 release of Going Home, a full Caribbean gospel album, showcased the choir's soulful style of reggae, calypso, ska, and new sounds with the fusion of jazz and reggae.

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The Toronto Mass Choir (TMC) continues to be on the cutting edge of the Canadian gospel music scene bringing its distinct gospel sound to the world stage. Now 40 members strong plus a five-piece band, this not-for-profit choir consists of adult members from various local church denominations across the Greater Toronto Area.

TMC's travels abroad have taken them to several countries, including the United States of America, the United Kingdom, Poland, Hungary, Romania, Italy, Trinidad, and Barbados.

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TMC's music incorporates contemporary gospel, traditional gospel, and Caribbean music influences. In particular, TMC's combination of gospel lyrics with various musical styles has resulted in a unique style in Canada.

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Born in Brantford, Ontario, Karen Burke entered McMaster University's Honours music program in 1979, where she developed an interest in choral conducting. During her time there, Burke received the Louise E. Bettger Scholarship recognizing excellence in choral work, the Humanities Medal for Special Achievement, and the Hamilton Alumnae Branch Award for the outstanding graduating female student upon completion of her Bachelor of Music Degree in 1983. Burke also has an A.R.C.T. Diploma in Piano Teaching and a Masters in Education.

"It has been essential for us to showcase Canadian gospel songwriters, share culture and provide an environment for music to be created and recorded," Burke said of the choir's twelve recordings.

The blue, green, and red dots represent wavelengths associated with common 470nm, 520nm, and 630nm (blue, green, and red) LEDs. Notice the green dot focuses to the left of the sensor plane, while the red and blue dots focus to the right; this is the most balanced position of focus of the lens system if all the wavelengths or white light (which encompasses all wavelengths) are used. This design displays non-ideal image quality, as none of the wavelengths are truly in focus. If only one wavelength is used, the performance will improve since balancing effects used for the other wavelengths are eliminated. While this example demonstrates that red and blue can be balanced, this is not always true. Most lens designs are achromatic, but for very small pixels, this can be an issue.

Field curvature (Figure 4) is the aberration that describes the amount in which the image plane curves due to the curvature in the lens design. This aberration is caused by the sum of the focal lengths of the lens elements in the system (multiplied by the refractive indices) not equaling zero. If the sum is positive (typical for an imaging lens), the image plane will have a concave curvature. Since curving the image plane is almost never an option for a machine vision lens, the optical designer must insert negative powered corrective elements to reduce this sum. This makes lenses longer and forces a negative lens to be close to the image plane, reducing the lens’s back focal length.

TMC has also been a special guest at countless church services, festivals, award shows, and venues such as The Ottawa Bluesfest, Quebec's La Festival de Musique Sacree de Saint-Roch, the Toronto Jazz Festival, the Canadian National Exhibition, the Empire Club of Canada, Roy Thompson Hall with the Toronto Symphony Orchestra, Hamilton Place with the Hamilton Philharmonic Orchestra and at the Gospel Music Workshop of America conventions in Washington, DC, Atlanta and Detroit. The choir has also appeared with well-known Gospel singers such as Tramaine Hawkins, Wintley Phipps, and Take 6, Canadian recording giants such as Ben Heppner and Jane Bunnett, and jazz legend Jon Hendricks.Â

Shown at the same scale as Figure 6, Figure 7 shows an apochromatic lens. An apochromatic lens is designed to focus three wavelengths on the same plane. While this is a far more complicated design, it allows for superior balancing across the wavelength spectrum. As shown, all three LED colors can be brought to focus on the same sensor plane allowing for superior image quality. Apochromatic lens designs have high performance, but low versatility and work well over a smaller range of magnifications and WDs. Additionally, these are often high-cost designs due to additional elements made of expensive materials. Many high-end, high-magnification objectives (such as microscope objectives) are apochromatic.

Toronto Mass Choir continues its busy concert schedule, appearing on many radio and television broadcasts for stations such as CTS, CBC Radio One, CITY-TV, and CBC TV. It has also been the focus of several television documentaries.