'Crop factor' is incorrect and misleading terminology, '35mm equivalent focal length' is more accurate as what you are really asking is; what is the new angle of view of my lens when on this camera?
In photography, the 35mm equivalent focal length is a measure that indicates the angle of view of a particular combination of a camera lens and film or sensor size. The term is useful because most photographers experienced with interchangeable lenses are most familiar with the 35mm film format so this gives a common ground for everything to refer to. All crop factors are based on 35mm full frame as a standard 1:1 reference point.
On any 35mm film camera, a 28mm lens is a wide-angle lens, and a 200mm lens is a long-focus lens. However, now that digital cameras have mostly replaced 35mm cameras, there is no uniform relation between the focal length of a lens and the angle of view, since the size of the camera sensor also determines angle of view, and sensor size is not as standardized as film size was. The 35mm equivalent focal length of a particular lens–sensor combination is the focal length that one would need for a 35mm film camera to obtain the same angle of view.
35mm equivalent focal lengths are calculated by multiplying the actual focal length of the lens by the crop factor of the sensor. Typical crop factors are 1.5x for Nikon APS-C ("DX") format, 1.6x for Canon APS-C format, 2x for Micro Four Thirds format, crop factors of 5x to 6x for compact digital cameras and even higher crop factors for built-in cameras of mobile devices like cell phones or tablets.
Everything is based on 35mm being the 'Standard' 1:1 reference point:
|Lens Size||Film/Sensor Size||Divide By|
Using a 80mm Hasselblad (6x6) lens as an example: 80 ÷ 1.6 = 50mm. So an 80mm Hassy lens is going to give a similar angle of view to a 50mm lens when used on a 35mm full frame camera.
Moving down in size from 35mm needs additional math. If the customer does not have a full frame camera say an APS-C or MFT instead, use the chart below:
|Lens Size||Film/Sensor Size||Multiply By|
|35mm FF||4/3 - MFT||2|
So an 80mm Hassy lens is going to give a similar angle of view to a 205mm lens when on a APS-C camera. 80 * 1.6 = 128 * 1.6 = 204.8 ~ 205mm
x = √ (a1*a2) / (b1*b2) [x = √ (60*45) / (36*24)]
a1 & a2 are larger format sensor size dimensions
b1 & b2 are smaller format sensor size dimensions
Normal lens size, and Equivalent sizes are rough
A. 4x5 in, normal lens 150mm, crop factor 0.29
B. 6x9 cm, normal lens 105mm, crop factor 0.40
C. 2 1/4 x 3 1/4 in, normal lens 100mm, crop factor 0.43
D. 6x7 cm, normal lens 85mm, crop factor 0.50
E. 6x6 cm, normal lens 80mm, crop factor 0.53
F. 6x4.5 cm, normal lens 75mm, crop factor 0.57
G. 35mm and FX digital, normal lens 43mm, crop factor 1.00
H. DSLR DX (APC), normal lens 28, crop factor 1.6
I. Large compact (e.g., G10), normal lens 9.5mm, crop factor 4.52
J. Regular compact, normal lens 7.18mm, crop factor 5.6
K. Mobile phone camera (e.g., iPhone), normal lens 4.5mm, crop factor 9.5