Is my thinking about equivalence right?

Started 3 months ago | Questions thread
OP Muster Mark Contributing Member • Posts: 588
Re: Is my thinking about equivalence right?

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.

Only if illumination is even, which it is not. Now, it gathers MORE light for sure, but what inspired this post was that vingetting affects total light to the degree that 4x turns into more like 2.8x depending on lenses being considered. This might seem like technical hairsplitting, but to dig into technicalities was the point of the post.

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.

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Cheers,
-Ian

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