Is my thinking about equivalence right?

Started 3 months ago | Questions thread
Mark Ransom
Mark Ransom Veteran Member • Posts: 7,669
Re: Is my thinking about equivalence right?
31

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.

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Pentax K-7 Pentax K-01 Olympus E-M5 II Pentax smc DA 15mm F4 ED AL Limited Pentax smc DA 18-135mm F3.5-5.6ED AL [IF] DC WR +6 more
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