Thread: f-stop

After years of photography I understand that a lens on a camera set at f-16 means that the diameter of the aperture is 1/16th the distance from the aperture to the "film plane" so my question is:
If I mount a 50mm lens from an old 35mm camera on my E-PL1 with an adapter and it becomes a 100mm lens does it also become f-32 when set at f-16? Is my Zuiko 90mm f-2 now a 180mm f-4 or is it a 180mm f-2 or does it fall somewhere in between? Is it now a 180mm because it is farther from the "film plane" or is it that the sensor is smaller and the same amount of light still gets through?
I know it is technical and doesn't really matter, but it's driving me crazy because I don't want to misinform people who ask me about my new camera set-up. Thanks for any help you can provide.

Hal

The lens does not change; a 50mm at f16 stays that way no matter what camera it is on. When you use it on your E-PL1 it is still a 50 mm lens, but it covers a field (angle of view) equivalent to a 100mm lens on a 35mm camera because the micro 4/3 sensor is half the size. I hope this is clear. So, as applied to your 90mm f2 lens, it stays the same, but acts equivalent to a 180 f2 on a 35 mm camera when you use it on your E-PL1.

Originally Posted by halmanas

After years of photography I understand that a lens on a camera set at f-16 means that the diameter of the aperture is 1/16th the distance from the aperture to the "film plane"...

No, not quite right on that one. The f-stop is a relative measure of the aperture diameter to the lens focal length, NOT to the distance to the film plane. The distance of a lens to the film plane is constant for a given camera mount for all lenses that fit on that camera. So on a 50mm lens at f/16, your aperture diameter would be 3.125mm. The distance to the film plane (i.e. registration distance) for all MFT cameras is 20mm, and it's constant for all MFT lenses.

As stated above, nothing on the lens changes. What changes in reference to a FF camera is the field of view (FOV) covered by your cropped sensor, and the apparent depth of field (DOF) imaged by the sensor. So a 50/2 lens adapted to an MFT camera images an equivalent FOV of a 100mm lens on a FF sensor, and produces the DOF equivalent to an 50/4 lens on a FF sensor. The amount of light hitting the sensor is the same (specifically the same units of light per sensor area at a given aperture) between all sensors.