mtf群体

Saggital: related to resolution for periodic patterns with the period parallel to a line through the center of the image.

In 2006 and 2007, after her film debut in Casanova (2005) as Sister Beatrice, Cohan starred in the sequel to Van Wilder (2002), Van Wilder: The Rise of Taj, as Charlotte Higginson. Her next role was in the 2008 film Float. In February 2010, she was cast in Death Race 2[13] and was the lead in the 2016 supernatural-horror The Boy.[14] Also in 2016, she appeared as Martha Wayne, Batman's deceased mother, in Batman v Superman: Dawn of Justice and played Leila Steinberg in the 2017 biographical drama All Eyez on Me.

Spatial Frequency is defined as one divided by the period of a repeated structure. A picture would probably be helpful here.

LensMTF

As you may have surmised by now, periodic patterns away from the center of the image have an MTF that depends on their orientation. If you orient a periodic pattern of parallel lines so one of the lines passes through the center of the image, the MTF of that pattern will be determined by the Saggital MTF curve, which is the one labeled S. MTF also varies with the distance from the axis of symmetry (optical axis) of the system. That’s why there are separate curves for 0.00 deg (the optical axis or center of the image), 14 deg and 20 deg. This particular lens was analyzed by tracing rays through the lens that made a 14 degree angle with the optical axis and another set at 20 degrees. In this lens, things don’t get much worse between 14 and 20 degrees, but they are different. Other lenses will behave differently, depending on the aberrations of the optical system. But that’s another topic.

Note that this curve is different for every wavelength and f/# (or NA). However, the only thing that changes is the scale of the abcissa (x-axis). The cutoff frequency is given by

modulation transfer function中文

In early 2018, in the midst of her contract negotiations with AMC regarding The Walking Dead, Cohan was cast as CIA operative Francesca "Frankie" Trowbridge in the ABC action comedy-drama Whiskey Cavalier.[24] Whiskey Cavalier was picked up to series in May 2018 and premiered on February 24, 2019.[25] ABC did not renew the series for a second season.[26] In October 2019, it was reported that Cohan would return to The Walking Dead as a series regular in the series' eleventh and final season.[27] Following the conclusion of the series, Cohan starred alongside Jeffrey Dean Morgan in the spinoff series The Walking Dead: Dead City, with both also serving as executive producers.[28] In July 2023, the series was renewed for a second season.[29]

From this equation, you can see that if min = 0, modulation will be 1.0, and if max = min, the contrast will be 0.0. When the modulation is near one, the contrast is very high, and the image looks sharp. Conversely, when the modulation is near zero, it is so blurred that it is difficult or impossible to see details.

mtf光学

Some types of lenses, such as telescopes and microscopes, commonly reach the diffraction limit. Others, for example camera lenses, rarely approach the diffraction limit. A camera lens might have an MTF plot like the one below:

Another way to get a picture of spatial frequency is to think in terms of dots per inch (dpi). Computers went through a revolution when Hewlett Packard introduced its first laser printer that produced 300 dpi. A few years later 600 dpi became the norm and now printers put out so many dpi that we don’t care anymore. The same thing is happening in cell phones. My first cell phone had a 1.75″ display with 320 x 240 resolution. That works out to 230 dpi. Now you can get a cell phone with over 400 dpi, so the picture is much sharper.

One final pointer: We are often asked, “What MTF is good enough?” The answer is, of course, “it depends…” But a good rule of thumb is that when the MTF drops below 0.2 (20%), it is probably unacceptable. Beyond that, lenses are rarely as good as the ideal model in the computer. How much difference depends on the manufacturer and the tightness of tolerances. Tolerances are discussed on another page.

mtf是什么意思

Tangential: related to resolution for periodic patterns with the period tangent to a circle centered on the optical axis.

In 2007, Cohan garnered her first major supporting role, which was also described as a season lead, when she was cast for season three of Supernatural as Bela Talbot, a thief who procures valuable supernatural objects and sells them to rich and powerful people in the supernatural world. The character appeared in six episodes of the series. Her final appearance was in the penultimate episode of season three titled "Time Is on My Side".[15]

Cohan played the recurring character Rose, a 560-year-old vampire, in The Vampire Diaries. In 2011, she joined the television series Chuck in a recurring role playing Vivian McArthur Volkoff, a "charming, sophisticated socialite" from the United Kingdom, and the daughter of primary villain, Alexei Volkoff. She also guest starred on shows such as Archer, Cold Case, CSI: NY, Life, and Modern Family.

调制传递函数

MTF

Based on these definitions, we can state that MTF is a plot of contrast versus spatial frequency. Experience tells us that smaller details are harder to see. In terms of MTF, this is expressed by saying that the modulation decreases as spatial frequency increases. Somewhat surprisingly, this is true even for lenses that are perfect. Such lenses are called “diffraction limited” because a physical phenomenon called diffraction keeps them from resolving infinitesimal detail. A plot of MTF for a perfect lens is shown below.

MTF stands for Modulation Transfer Function. It is a function that calculates the sharpness or fuzziness of an image. It is important to understand several terms before we can fully describe MTF.

So how does that relate to spatial frequency? Actually, dpi is spacial frequency! Let’s change the units to make this clear. To make one cycle, we need two pixels so one can be on and the other off. This means that 400 dpi = 200 cycles per inch. Since optics is done in metric, we still need to convert to millimeters. There are roughly 25 mm in 1 inch, so we just divide cycles per inch by 25. Thus 400 dpi = 200 cy/in = 8 cy/mm.

Lauren Cohan (born January 7, 1982)[1] is an American actress known for her role as Maggie Rhee in the AMC post-apocalyptic horror television series The Walking Dead (2011–2018; 2020–2022), a role she reprises in The Walking Dead: Dead City (2023–present). Her other notable TV roles include Bela Talbot in the dark fantasy drama Supernatural (2007–2008), Rose in the supernatural teen drama The Vampire Diaries (2010–2012), Vivian McArthur Volkoff in the action comedy Chuck (2011), and Francesca "Frankie" Trowbridge in the action comedy-drama Whiskey Cavalier (2019). Her film appearances include the comedy Van Wilder: The Rise of Taj (2006), the psychological thriller horror The Boy (2016), the biographical drama All Eyez on Me (2017), and the action thriller Mile 22 (2018).

In the square wave pictured above, the spatial frequency would be 1.0 divided by the distance shown. For example, if one period is 10mm, the spatial frequency would be 0.1 cycles/mm, sometimes written 0.1cy/mm. It is important to remember that spatial frequency increases as the period gets smaller.

Modulation transfer function

Upon her mother's remarriage, Lauren's surname was legally changed to that of her stepfather, Cohan.[3] She converted to Judaism at the age of five.[9]

She has younger half-siblings through her mother and father's remarriages.[10][11] Cohan graduated from King Alfred's College, Winchester (now the University of Winchester)[12] with a bachelor of arts in drama and English literature before touring with a theatre company she co-founded at the university.

Note that there are now five curves on the plot rather than just one and also that the maximum spatial frequency is only 100 cy/mm rather than 840. The blue curve (labeled TS 0.00 deg) is for the center of the image. Before discussing the green and red curves, we need two more definitions.

Modulation: Assume the above square wave plot is the amount of light in an image, and the peaks represent bright portions and the valleys represent dark portions. Let’s call the amount of light in the peaks “max” and the amount in the valleys “min”. Then the modulation, or contrast, is given by:

In April 2011, Cohan was cast in her most notable role as Maggie Greene (later Maggie Rhee)[17] on The Walking Dead,[18] an AMC post-apocalyptic horror television series based on the comic book series of the same name. She first appeared in season two as a recurring character, but she became a series regular at the start of season three.[19] The drama was the highest-rated series in cable television history in 2015.[20] At the conclusion of the eighth season of The Walking Dead, Cohan completed her contract obligations as a series regular. It was later reported Cohan had not reached an agreement to sign on for season nine as a main cast member due to a pay dispute, as she demanded an increased salary closer to the salaries of her male co-stars Andrew Lincoln and Norman Reedus. AMC refused and Cohan began making herself available for TV pilots.[21] Cohan eventually reached an agreement to appear in five episodes in the first half of season nine of The Walking Dead[22] and departed the series in 2018.[23]

Cohan starred in a pilot for The CW called Heavenly (2011) in which she played the lead character, Lily, an attorney who teams up with an angel; however, it was not picked up as a series.[16]

Cohan was born Lauren Storholm in Cherry Hill, New Jersey, to an American father and Scottish mother.[2][3] She was raised in New Jersey and spent a year in Georgia.[4][5][6][7] When she was thirteen, her family moved to her mother's native UK,[8] settling in Surrey.[3]