Is my thinking about equivalence right?

Started 5 months ago | Questions thread
prsc Regular Member • Posts: 246
Re: Is my thinking about equivalence right?

alfn wrote:

Mark Ransom wrote:

RobBobW wrote:

- the argument about total light is pointless as what is important is light density. Yes FF will bring in 4 times the light, but FF also has 4 times the surface area of sensor to illuminate, so it is a wash. Faster lenses bring in more light per unit area, period.

This is simply false. The reason apertures are measured by F-stops is because this equalizes the light density per unit area between lenses of different characteristics. A lens at F/1.2 will produce the same light density, no matter the focal length of the lens or the size of the sensor behind it. This means that a sensor with 4x the area really will collect 4x the light, when measured over the whole image, as long as the F-stops/T-stops of the lenses are the same.

The reason this matters is the nature of noise. The majority of noise in today's cameras is from photon shot noise, which is a property of the light itself and not of the lens or sensor or any other camera electronics. The only way to reduce shot noise is to collect more light. Whether you do this with a larger sensor, a larger aperture, or a slower shutter speed is immaterial.

That depends on how you are measuring the "noise". The signal to noise ratio (SNR) for each pixel remains the same for example, all else being equal. As does the SNR per unit area of the image.

Do you mean that when a MFT sensor and 4x (let's assume this to be exact make calculations easier) bigger FF sensor have the same pixel size, in both systems each pixel and each mm^2 has the same SNR? That would be correct, but comparing same area doesn't make sense when the claim is that the difference in area is what makes the difference!

It is true that the SNR of the entire image increases statistically but how does that translate to visible "noise"?

What do you mean by visible noise? What is non-visible noise?

As for the majority of "noise" in today's cameras being from shot noise, that depends entirely on the intensity of the light being recorded. Shot noise is virtually irrelevant in very bright areas of an image.

No it does not depend on that. No matter the intensity the majority of noise still comes from shot noise in usual shooting conditions. Apart from a few sensors that still have poor dynamic range, as that is the result of other that shot noise ruining the dark areas in low ISO shots.

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