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

Started Jul 24, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,683
Whoops again!

D Cox wrote:

Great Bustard wrote:

Wheatfield7 wrote:

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Um, it is absolutely correct.

It is perfectly correct.

It is not.

The illuminance is the same, and therefore the exposure time needed is the same (if the sensors match in sensitivity).

The same exposure time will result in 16x as much light falling on a FF sensor at 50mm f/2 than a 4x sensor at 12.5mm f/2 for a given scene luminance.

Illuminance determines both exposure time and noise levels, for a given sensor.

Totally wrong.

(Different sensors may have different noise levels at the same exposure. Technology keeps on improving.)

It is a gross misunderstanding to believe that the reason larger sensors are less noisy for a given exposure is because they are more efficient than smaller sensors.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

This is correct, assuming the "amount of light" is measured in lux (i.e. lumens per square meter), which is the appropriate unit.

The exposure (density of light falling on the sensor) is measured in lux · seconds, or, equivalently for a given luminosity function, photons / mm². The total amount of light falling on the sensor is measured in lumen · seconds, or, equivalently for a given luminosity function, photons.

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