MTF

1.常见的MTF曲线)

(下图中底部的那张图。横坐标：Spatial frequency空间频率；纵坐标：MTF 值)

这颗摄像头的COMS 传感器信息：

resolution:4160*3120

chip size:6.1(H)mm*4.524(V)mm

unit cell size:1.12um*1.12um

曲线上的一个点的MTF 值是用来反映在此点对应的空间频率下的成像清晰度表现，值越大越清晰。

整条曲线就是成像清晰度随空间频率（何为空间频率？也可简单理解为采样频率，如这颗sensor 的水平方向4160个像数点，空间采样频率为：4160TVL/ph）的一个变化趋势。

2.清晰度的含义

1)sharpness

清晰度=对比度（反差）+分辨率

单位：

lp/mm ，LW/PH ，TVL/ph...

The LW/PH means line width per picture height. It means how many lines can be fill in your picture.
For example, If your resolution is 4160 x 3120, then the max line width (LW) is 4160. This means 
each pixel can represent one line.
The lp/mm means line pair per mini-meter. The concept is the same as LW/PH just the different
unit. First we convert the Lp and LW. LW means line and Line pairs means a pair of black line and 
white line. 

lp/mm（主要用于胶片）转换lw/ph（主要用于数码，CCD，CMOS）的方法是先乘以2，再乘以传感器高度的毫米数。比如对于24×36mm的全画幅传感器，30lp/mm=30×2×24lw/ph=1440lw/ph

如下有来至imatest 的官方解释：

Film camera lens tests used line pairs per millimeter (lp/mm). This worked well for comparing lenses because most 35mm film cameras have the same 24x36 mm picture size. But digital sensor sizes varies widely, from under 5 mm diagonal in camera phones to 43 mm diagonal for full-frame DSLRs— even larger for medium format backs. For this reason, Line widths per picture height (LW/PH) is recommended for measuring the total detail a camera can reproduce. LW/PH is equal to 2 * lp/mm * (picture height in mm).

Cycles/Pixel (C/P)	Shows how well pixels are utilized
Cycles/Distance (cycles/mm or cycles/inch)	Cycles per distance on the sensor. Pixel spacing must be entered. Used for comparing resolution in the old days of standard film formats (e.g., 24x36mm for 35mm film).
Line Widths/Picture Height (LW/PH)	Measures overall image sharpness. Line Widths is traditional for TV. Note that 1 Cycle = 1 Line Pair (LP) = 2 Line Widths (LW).
Line Pairs/Picture Height (LP/PH)	Measures overall image sharpness. Used by dpreview.com. Equal to (LW/PH)/2.

2）常见MTF曲线，MTF50P，MTF20P

From Imatest description:

– Sharpness results are indicated by MTF50P and MTF20P for each ROI and for the weighted mean over 
the image. (MTFnnP is the spatial frequencywhere MTF falls tonn% of its peak value.) MTF50P and 
MTF20P are the same as MTF50 and MTF20 in the absence of strong software sharpening. Sharpening 
boosts MTF50 and MTF20 values considerably, but perceived image quality may not be improved; it may 
even be degraded due to the "halos" near edges.  MTF50P and MTF20P show less increase and are more 
representative perceived sharpness


Different with 20,50???

MTF50: Spatial frequency where MTF is 50% of the low (0) frequency MTF

MTF20: Spatial frequency where MTF is 20% of the low (0) frequency MTF

Why 20P,50P???

– MTF20P is closer to "vanishing resolution"-- to the spatial frequency where you no longer see 
detail, while MTF50P best represents perceived sharpness

Metrics unit???
– MTF50P and MTF20P are presented in LW/PH spatial frequency units, which indicates how much total 
detail an image can reproduce. LW/PH units facilitate comparisons between cameras with different 
resolutions (pixel counts)

3) MTF的三大区域：

MTF曲线不同区域对应的画面内容特征。

硬边界区：

在空间频率（TVL/ph）比较低的区域（就是线对比较粗），高楼大厦、砖块、高反差色块之类界线明显的部分。这部分的MTF曲线称为“硬边界区”。

纹理区：

当空间频率增加一些的时候（线对变细），对应的是画面中有质感的部分，比如人的皮肤、毛发、衣服褶皱、以及缓慢的明暗过渡。这部分MTF称为“纹理区”。通常的画面中这一区域会占据相当大的比例。

分辨率能力区：

最后是“分辨率能力区”，它对应的是画面中微小的细节部分，比如头发的末梢、报纸上的文字等。下面那三副画面分别是三个区域比较突出的画面。

3.意义

– The MTF Edge profiles are given for the center and part way out ROIs as

well as for high(10:1) and low(2:1) contrast edges, which give an indication
of the edge enhancement tuning
– In general, in the spatial domain, a steep, sharp edge without too much
overshoot “halo” (5-25%) is preferred while in the frequency domain, high
MTF50P or MTF20P values below the Nyquist frequency with
oversharpening under 20-25% is a good target.Absolute numbers depend
on the camera type, application, and desired enlargement
– Too much energy above the Nyquist frequency can indicate there might be
aliasing effects such as color moiré

overshoot  应予以控制，过高的overshoot 会导致白边。

4. 曲线上的Nyquist Frq:奈奎斯特频率

CCD除了在奈奎斯特极限后会产生伪像，还有另外一个产生伪像的本事。

于是我们看到，CCD的MTF曲线在归0之后，奇迹般地“触底反弹”，反弹的曲线就是CCD“胡来”搞出的莫须有图案。这样，CCD的MTF曲线就有两个伪像区，一个是奈奎斯特极限后因失真产生的伪像，另一个是CCD到达分辨率极限后自己“创造”的伪像。我们在实际画面中看到的伪像，就是这两个伪像区“齐心协力”的结果。

在目前像点数并不高的情况下，还有另外一种方法减弱伪像，就是在CCD前加一块低通滤色片（Low-Pass Filter）。

Refference:

http://www.imatest.com/docs/sharpening/

理解摄像机.pdf