Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
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slimandy Forum Pro • Posts: 16,263
Re: Equivalent focal length for MFT lenses

JeanPierre Martel wrote:

slimandy wrote:

JeanPierre Martel wrote:
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.

I think this is where people get confused about equivalents. You are right, the actual aperture does not change, BUT NEITHER DOES THE ACTUAL FOCAL LENGTH. What we are talking about when comparing to FF is the equivalent field of view and depth of field that results form the smaller sensor size. A 300mm f6.7 lens is always a 300mm f/6.7 lens, but the equivalent FF lens would be 600mm f/13.

Let me disagree. At the same ISO setting, the image shot by a 300mm m4/3 lens on a m4/3 camera will be exactly the same as the one taken by a 600mm FF lens on a FF camera when both apertures are set to F/6,7.

No it won't. The FF image will have shallower DOF.

Lets assume those two combinations and the subject 20 feet away.

On a Panasonic DMC-G1, 300mm @ f6.7  total DOF 0.26 feet

On a Nikon D700,  600mm @ f6.7 total DOF 0.12 feet.

Figures from DOF Master.

Of course, the diameter of the opening of the diaphragm at F/6,7 will be wider for the FF lens, compared to one on the m4/3 lens. So are their sensors. The end result is that the number of photons per square mm will be the same at F/6,7 and consequently, the image will be as dark or as pale on the FF as it is on a m4/3 camera.

So the exposure will be that same.

However, it is true that if we put a m4/3 lens on a FF camera (using duck tape to hold it), a 300mm m4/3 lens at F/6.7 will draw a small image on the FF sensor and consequently, the average number of photons per square mm on the whole FF sensor will be lower and will then be equivalent to a 600mm FF lens at F/13.

But who on Earth would put a m4/3 lens on a FF camera?

No-one. But why over complicate it? Just look at the actual figures (not theoretical) above.

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