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?

RobBobW 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.

Mark, you just said the exact same thing as I did. A given unit area of film or sensor does not care how much film or sensor is around it in order to record the amount of light hitting it. Light intensity/density is what is important. Otherwise we would be using different exposures with different sized sensors. A person can use a hand held light meter reading to determine the correct exposure regardless of the film or sensor format being used. Yes more total light hits the larger piece of film for a given exposure, but that is only because more total light is needed to achieve the required light density per unit area.

I think where you lost me is when you said "it is a wash".  FF lenses don't automatically bring in 4x the light, but the larger sensor is able to capture more of what is provided.  To get equivalence you need to light up the smaller sensor 4x brighter, which requires an F-stop 2 stops lower/faster.

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.

But this is also a function of the size of the individual light sensor pixels. The more pixels per unit area, be it from sensor size or sensor resolution, the more noise, all other things being equal.

Did you read the link I provided?  Pixel size matters a lot less than you think it does.  Because in the end noise per pixel doesn't matter as much as noise per unit area of the picture.

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