To calculate the equivalent (in FF terms) focal length of a lens on crop, multiply by the crop factor (1.6). To calculate the equivalent aperture (in FF terms) of a lens on crop, again multiply by the crop factor (1.6). This holds true for both DoF and the total quantity of light delivered to the sensor (the measured aperture eg f2.8 is a unit of brightness, total light is a combination of brightness and area).Great explanation. In this scenario, I could choose between two options:
Conversely, due to the larger surface area of a larger sensor (FF is 1.6x horizontally and 1.6x vertically bigger than crop), it can be used at 2.56x as high an ISO as crop to deliver the same S/N ratio. This is of course based on both sized sensors using the same tech - which rarely is the case.
So shooting a scene at 50mm, f2.8 and ISO 2,500 on crop is the equivalent of 80mm, f4.5 and ISO 6,400 on FF. Shutter speed would of course remain the same.
In other words, if you switch to a larger sensor, feel free to stop down the lens by 1.3 stops and boost the ISO by 1.3 stops should you want to retain the larger DoF you had with crop.
Canon 70D + 50mm F1.4 = U.S. $ 1550.
Canon 6D + 85mm F1.8 = U.S. $ 2400.
I seem to have some advantages 70D over 6D except high ISO.