The Recent F/stop Controversy

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Re: The Recent F/stop Controversy
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kiwi2 wrote:

Michael Fryd wrote:

Yes. modern cameras are labeled in f/stops, and actual mm. Yet we routinely suggest that people think in terms of effective f/stops, and effective focal lengths.

I am suggesting that instead of "effective f/stop" we use the more straightforward "aperture diameter."

Here is a shot taken with a 6 mm aperture...

6 mm aperture

Here is a shot taken with a 150 mm aperture in the same light with the same camera with the same shutter speed and ISO...

150 mm aperture

Which one put more photons onto the sensor..?

The first, because it gathered light from a way, way, way larger portion of the scene. For example, let's say that the first photo was taken at 24mm f/4 (24mm / 4 = 6mm) and the second taken at 1200mm f/8 (1200mm / 8 = 150mm) both with the same exposure time and ISO setting (for a given scene, relative aperture, and exposure time, the ISO setting affects the lightness of the photo, not the amount of light that made it up).

The 24mm f/4 photo is gathering light from 2500x as much of the scene as the 1200mm f/8 photo. However the 1200mm f/8 photo has an aperture with 625x more area. Thus, 2500 / 625 = 4x (2 stops) more light is being projected on the sensor for 24mm f/4 as 1200mm f/8, assuming the same average scene luminance and exposure time. Clearly, it's easier to simply say f/4 is two stops brighter than f/8, but the above is *why* f/4 is two stops brighter than f/8 (more scenarios explained in detail here).

All that said, I wish to make it clear that I am *not* an advocate of replacing relative apertures (e.g. f/4) with effective apertures (e.g. 6mm). I'm absolutely fine with the system as is

However, when comparing light gathering across formats, it is the effective aperture that is relevant, not the relative aperture. Specifically, for a given scene, exposure time, and effective aperture diameter (*not* relative aperture), the same total amount of light is projected on the sensor (the DOFs will also be the same if the photos are taken from the same position). For example, with regards to *both* light gathering and DOF (diffraction as well), f/4 on mFT is equivalent to f/8 on FF because the aperture diameters are the same for a given [diagonal] angle of view.

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tex
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