Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
JeanPierre Martel Veteran Member • Posts: 3,003
Re: Equivalent focal length for MFT lenses

draleks wrote:

If I have an MFT 300 mm f/6.7 lens, and a full format 600mm f/6.7 lens that is of a worse manufacturing quality and just happens to be twice as soft as the MFT lens. Then, on equal ISO values, those two lenses will be an exact match. Right?

A native 300mm m4/3 lens has exactly the same narrow angle of view as a 600mm lens on a full frame camera.

A F/6,7 lens is always a F/6,7 lens. Whatever that lens is for a compact camera, a m4/3 camera ou a full frame camera. All three will bring the same number of photons per square mm on their respective sensor. So pictures taken of the same object under the same lightning conditions will produce the same bright (or dark) picture on the compact, the m4/3 or the full frame camera when shot at the same aperture (F/6,7) and same ISO settings.

What makes a difference is when a lens made for a full frame camera is used on a m4/3 camera (with an adapter). The angle of view doesn't change. The number of photons per mm2 on the m4/3 sensor doesn't change, but since that lens is made to draw a much bigger image (for a bigger senser) more light is lost inside the camera around the m4/3 sensor. In other words, light bounce back and forth inside the camera body. The consequence is that the image looks soft, specially on very pale objects and immediately around it (a halo effect).

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