f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
D Cox Forum Pro • Posts: 20,049
Re: That "explanation" is so wrong...

Great Bustard wrote:

D Cox wrote:

goshigoo wrote:

Just read the thread about the new 15 f/1.7; I see people saying f/1.4 is f/1.4 regardless of format

I do agree with this statement on the meaning of f-number, but it is confusing since it is the format + f-number which determines the amount of light captured by a picture, which accounts for the final Signal-to-noise ratio of a picture

The final SNR depends (assuming technology of the same generation) on the amount of light falling on each pixel. This determines the uncertainty of the measurement of light by that individual photodetector. The more photons, the more certain the measurement.

The signal in a photographic image is the differences between pixels. The uncertainty (error bars) for each pixel gives a random, or noise, component when comparing the pixels.

The total area of the sensor is irrelevant. If you put the same lens on various sizes of sensor, the noise level in the part of the image that they all record will be identical. (Assuming they are the same generation of technology.)

we should really comparing 25mm f/1.4 @ m43 vs 50mm f/2.8 @ FF
as they both yield the same View of field, DoF, Signal-to-noise ratio of the final picture and the lens size should be comparable too

Should I think this way?

Not really.

...that I find it hard to believe something like that could be posted on this forum. Let us now debunk your false "explanation" with a simple experiment anyone can do.

Take a photo of a scene from a particular position, with a particular focal length, f-ratio, and shutter speed. Then take another photo of the same scene from the same position with twice the focal length and the same f-ratio and shutter speed. Crop the first photo to the same framing as the second and display the photos at the same size. Which is more noisy?

At the same enlargement, they will have the same noise.

If the noise is not "white" but has an amplitude that increases with frequency, then increasing the enlargement of either shot will make the noise more visible. This partly depends on the spatial frequency response of the human eye.

So whichever one you enlarge more is likely to appear more noisy. But that is a difference in your print size, not a difference in the sensors.

For example, take a photo of a scene at 50mm f/5.6 1/200 ISO 1600 and another photo of the same scene from the same position at 100mm f/5.6 1/200 ISO 1600. Crop the 50mm photo to the same framing as the 100mm photo. Display both photos at the same size.

The sensors are the same efficiency (indeed, they are the same sensor), the pixels are the same size, the exposures are the same, and the same amount of light falls on each pixel.

And the noise level is the same. You are mixing up lenses and angles of view with sensor noise. They are different variables.

So, no, the noise in the photo does not depend on the amount of light falling on each pixel (for a given sensor efficiency), but rather on the total amount of light that made up the photo, and the cropped photo, in this example, is made with 1/4 the total light as the uncropped photo, and thus it will have twice the noise.

The total light falling on the sensor is the same in both your cases. Yet you suggest that one has more noise than the other.

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