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Amazing lens, incredible sharpness

Started Aug 20, 2013 | User reviews thread
Great Bustard Forum Pro • Posts: 45,641
Well, what do you mean by 1.8?
5

tinternaut wrote:

In terms of DoF, yes it's a 3.6 equivalent...

I'm glad you agree.

...but in terms of light gathering (i.e. the shutter speed it enables one to achieve at a given aperture for correct exposure)...

Anders explained it nicely in his post above, but I'll take a stab at it, too. The exposure is the amount of light per area that falls on the sensor, not the amount of light gathered. So, f/1.8 on mFT results in 4x as much light per area as f/3.6 on FF, but a FF sensor has 4x the area, so the two neatly cancel each other out, and the light gathering of f/1.8 on mFT is the same as the light gathering as f/3.6 on FF.

...surely 1.8 is 1.8.

Well, 75mm f/1.8 has an aperture (entrance pupil) diameter of 75mm / 1.8 = 42mm, and 150mm f/3.6 also has an aperture diameter of 150mm / 3.6 = 42mm.  So, f/1.8 on mFT is equivalent to f/3.6 on FF, where by "equivalent to", I mean results in the same diagonal angle of view, the same DOF for a given perspective, framing, and display size, and projects the same total amount of light on the sensor for a given shutter speed, resulting in the same noise for equally efficient sensors.

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