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

Started Jul 24, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 63,506
Re: Whoops!

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

Illuminance determines both exposure time and noise levels, for a given sensor. (Different sensors may have different noise levels at the same exposure. Technology keeps on improving.)

For a given sensor the noise level is determined by the total number of photons that it collects and counts. Of course, for a given sensor, that is in turn determined by the exposure, so long as you always use all of the sensor. However, since Equivalence is about comparison, it doesn't make much sense to talk about 'a given sensor'.  The minimum for a comparison is a given pair of sensors, and then it becomes clear that the important factor is total light (exposure integrated over the whole sensor), not exposure.

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.

Lux doesn't measure 'amount of light'. It measures light flux. Lumens measures 'power of light'. Lumen seconds measures amount (energy) of light. So, the amount of light collected by a lens depends on the size of the light collector put behind it.

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