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

Started Jul 24, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 63,558
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. This is the whole point of using f numbers rather than apertures in mm -- a small camera such as 35mm and a large camera such as 10x8 inch, using the same film, need the same exposure time at the same f number.

The intensity of the light is the same.

Likewise for different focal lengths on the same camera (ignoring lens faults such as vignetting).

The difference with digital is that the design of sensors varies greatly, so you are not using "the same film".

The performance of sensors varies rather less than the performance of film. Of a generation, the efficiency if sensors is within a fraction of a stop. Over 10 years or so, it has only improved by a stop or so. Shadow noise is better, that's improved about two stops.

Film works on almost exactly the same principle as digital, and no-one in film days that using the same exposure on 110 (Four Thirds) and 135 (full frame) would give the same result.

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