Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 55,144
Re: Let's correct it, then.

D Cox wrote:

Great Bustard wrote:

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

It does.

The number of photons arriving on each square mm is not affected by the size of the sensor.

But the total amount of light reaching the sensor is affected by its size, and total light, not light intensity, controls signal to noise ratio.

In other words, exposure is largely irrelevant. Total light is what matters, and total light is controlled by aperture diameter (for a given field of view), not f-stop.

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