f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
D Cox Forum Pro • Posts: 20,609
Re: f-number equivalent between m43 and FF

olliess wrote:

D Cox wrote:

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.

I thought we went through all this a few weeks (months?) ago. Defining the "signal" in an image to be the "differences between pixels" is deeply problematic.

No it is not. What is a photographic image but the differences between the pixels? If they are all the same, there is no image, just an exposure level. (There may be noise indicating the random variations in pixel values.)

A single photoreceptor doesn't give a signal from an exposure: it gives a single number. A signal is a series or array of values (analogue or digital), not one single value. A single dot or dash is not a Morse code message -- the signal is a sequence of dots and dashes.

The uncertainty of a single pixel measurement comes up as noise when you look at the whole image.

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.)

If you compare ("view") all the capture images at the same size then of course it's relevant.

If you view the area common to all the sensors at the same magnification, the extra parts of the bigger sensor are irrelevant. They will in practice have the same noise as the part you use for comparison.

I'm thinking about the effect of putting an APS-C lens on a FF sensor here - the camera may or may not crop to suit. You can apply the same crop to a lens that does cover the whole FF.

Sure, if you want to compare the output from a tiny phone sensor at its "true" (captured) size against the output from a MF sensor, then the phone sensor will probably be better. You'll have a very high quality, very tiny image, but comparing images this way for photographic purposes is kind of silly.

I would expect the phone sensor to be noisier. If it has smaller pixels, it will have higher resolution. But we can't easily compare phone sensor with others. What you can do, if you have the cameras, is put the very same 50mm lens (or 35mm or whatever) on an M4/3 camera, an APS-C camera and a FF camera, and compare the sensors. If you use the same lens and the same exposure then you can compare the sensor noise.

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