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johnvanr wrote:
The MFT light also has to cover a smaller sensor, so each pixel still gets the same amount of light.
First, I believe that I made a mistake in my first post where I calculated that the 600mm f6.3 and 400mm f6.3 are different by 1.3 stops of light. The difference should be greater, in favour of the FF system because of a greater FoV at 600mm vs the mFT at 400 mm. Not only the diameter of the entrance pupil of the 600mm lens is larger (95.2mm) but also it draws more light than the 400mm (equivalent to 800mm on FF) from the extended FoV.
Comparing non-equivalent systems can be less straightforward than dealing with equivalent systems.
Back to your comment "The MFT light also has to cover a smaller sensor, so each pixel still gets the same amount of light."
I wish I knew what you mean - sorry about this. N photons that hit an mFT or FF sensor should produce the same signal from the sensor, assuming the same quantum efficiency of the sensors.
In general, the concept of equivalence is well-developed in the literature. For example, a detailed paper on "Equivalence theory for cross-format photographic image quality comparisons" was published by D. Andrew Rowlands in Optical Engineering 2018, 57(11), 110801
www.spiedigitallibrary.org/journals/Optical-Engineering/volume-57/issue-11/110801/Equivalence-theory-for-cross-format-photographic-image-quality-comparisons/10.1117/1.OE.57.11.110801.pdf
I am going to copy-paste some of the relevant paragraphs (note that R is the crop factor, e.g. R = 2 between mFT and FF):
"Equivalent photos can be produced by different formats provided an “equivalent” combination of focal length f, f-number N, and ISO setting S is used on each respective format:
• equivalent focal lengths are related through R;
• equivalent f-numbers are related through R; and
• equivalent ISO settings are related through R2 (R squared)."
My example: mFT 20mm f1.4 and FF 40mm f2.8 will produce equivalent photos when the cameras are set to the same SS.
The FF ISO is going to be 4 x mFT ISO, e.g. 6400 (FF) vs. 1600 (mFT).
Another quote: "Equivalent photos are defined as photos taken using cameras based on different sensor
formats that have the following characteristics:
(1) the same perspective;
(2) the same framing (or AFoV);
(3) the same display dimensions;
(4) the same DoF;
(5) the same shutter speed; and
(6) the same lightness."
"Equivalence theory enables photographers to calculate the equivalent focal lengths, f-numbers, and ISO settings required on different camera formats in order to produce equivalent photos, i.e., photos with the same appearance characteristics including perspective, framing (AFoV), DoF, and shutter speed."
"the same entrance pupil diameter is required on each format in order to produce equivalent photos and that equivalent photos are produced using the same amount of light."
Importantly, this paper demonstrates that the dynamic range of cameras that have sensors of different sizes is practically the same when the camera systems capture equivalent images. Note that this is at the equivalent ISO, such as for example mFT ISO 1600 and FF ISO 6400. One can easily see that this observation is true by looking at the Photographic Dynamic Range (PDR) values that are readily available from photonstophotos. It is often said that noise and dynamic range are the two sides of the same coin. Indeed, equivalent photos must display the same amount of noise and the same signal-to-noise ratio.
Finally, it is true that a FF and mFT camera systems that have equivalent focal lengths (e.g., FF 50mm and mFT 25mm) and set to the same SS, the same aperture f-number (e.g. both at f2.8), and the same ISO gain (e.g. both at ISO 200) and used from the same spot at the same distance to a subject should produce correctly exposed images that have the same framing. These images are not going to be EQUIVALENT because the mFT image should have a more shallow DoF and, most importantly, the signal-to-noise ratio that is 1/4th (25%) of that for the FF system, because under the above conditions the mFT sensor received only 1/4 (25%) of the number of photons that hit the FF sensor. This is a natural consequence of the surface area of the entrance pupil of the FF lens at 50mm f2.8 being four times larger than that of the mFT lens at 25mm f2.8.
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