Focal length(f)

Focal length is the length between len and sensor element,we can change the magnify multiples of the lens through changing the Focal length of lens,and thus change the size of image.

When the subject is far away from the lens,we can use following formula to calculate:The magnify multiples of lens≈Focal length/Object distance.If increasing the focal length of the len,the magnify multiples is increased,and the distant view can be zoom in,the scope of the image will be small and the detail of the distant view will be more clear;If decreasing the focal length of the len,the magnify multiples is decreased,the scope the image is enlarged and larger view can be seen.In practical engineering,we often use horizontal vision field corner to reflect the capturing range of the image.The bigger focal length(f) is,the smaller vision field corner is,and the image range forms in the photosensitive element is smaller;otherwise,the smaller focal length(f) is,the bigger vision field corner is,and the image range forms in the photosensitive element is bigger.

Aperture（F）

Aperture is installed in the back of the lens,the bigger the aperture is,the bigger light quantity passed through the lens is  and the better definition the image is;the smaller the aperture is,the smaller light quantity passed through the lens is and the lower definition the image is.Normally,we use F(Luminous Flux)to indicate.F=Focal length(f)/Clear aperture.In camera technical indicators,we can always find parameter such as 6mm/F1.4,it represents the focal length of lens is 6mm,and the luminous Flux is 1.4,we can easily calculate the clear aperture is equal to 4.29mm.

With the same focal length(f),the lower value of F,the bigger aperture is,the bigger Luminous Flux arrived at CCD sensor and the better quality the len is.Distortion:The straight line out of principal axis in the subject flat surface will transform into curve throush optical system imaging,as a result,the imaging error of the optical system is called distortion.The distortion of aberration only affects the geometrical shape of the image,it will not affects the definition of the image.

Depth of Field(DOF)

In the photographic field space,the photographic field located in the front and back of focusing content flat surface within a certan distance can still get relatively clear imaging.The above depth between photographic field which located in the front and back of focusing content flat surface and get relatively clear imaging,that is,the depth scope of photographic field space to get relatively clear image in the actual image flat surface is called depth of field.